Diagnosing Grundig 7028

Hi: I am looking for some help diagnosing my grundig 7028. It was working....then it doesn't. I am new, but I am a quick learner? If there is a person who could mentor me a little I would be grateful. G.

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Hi Glenn,

It looks like this must be the unit that has your Perpetuum-Ebner turntable in it.

Just a quick pointer on how to start a new thread like this:  Always go to the model page for the specific radio you have a question about.  In this case, that is here, but you can also find it by a search from the home page.

To answer your question, looking at the schematic, it appears that the power to the turntable goes from the power cord through the main power switch for the radio chassis (but not through the fuse for the radio) and then to the turntable via two pins of the speaker connector.  Check the main power switch with an ohmmeter to make sure it closes properly.  If that switch is OK, then check the power wiring for the turntable going through the speaker socket and a special 0.5 amp fuse for the turntable.

Since you mention that the whole unit is dead, also check the fuse for the radio chassis.

Have you done any work on the radio chassis yet, such as replacement of electrolytic capacitors?  The have a high failure rate at this age, and if one of them shorts, it could blow the radio chassis fuse.

Tom

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Tom:

I have replaced the tuner fuse.   That is the first thing i checked.   It had no effect.

I will check the switch and look for the special fuse for the turntable.  That should tell me if the power supply is bad or not.  

I have not did any work on the caps.   I was able to get it  to fire up, knowing I was probably on borrowed time.  I suspect that I did blow a cap during my turntable witch hunt.  

Before I get too deep, I am new to AC circuits.  So, do you have an recommendations for tools, and books that i can get up to speed.  

I know I need to get a 1000Ohm resistor to drain capacitors for safety.    I understand what the components do like  resistor vs. capacitor etc., but typically not in a AC setting.  

I have a Heathkit TC-2.   I tested 3 tubes 2 EL95s and one other.   When i put the tubes(Two EL95 6DL5s (I do have a valid chart) )  the needle does not deflect to either good nor bad.   I am new to this technology, so for this type of tube I don't know if that is normal.  However, One of them shows a glow briefly where the heater pins are connected, the other nothing.    

I am trying to figure out if that (the tubes) are inline to prevent power to go to the turntable. In reading the schematic, (which is more like guessing)  it looks like at some point power is diverted to the turntable and tuner, but i am not clear if the EL95s are prior or post that split.

First thing first, I am going to have to get the chassis out into the open so that I can access the components.

g.

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Glenn,

On tester, the tube filaments sould light, and except in fairly rare circumstances, the meter needle should deflect.  Do you have confidence that the tube tester is working properly? 

In any case, the tubes should not play any role in whether power reaches the turntable.  Check the main power switch next.

This is a fairly complex radio for a beginner.  To get some knowledge of radio circuitry and radio servicing, I'd recommend the following books, which do a good job of making the technology accessible without too much abstract theory or math:

Elements of Radio by Marcus and Marcus (best place to start)

Elements of Radio Servicing by Marcus and Levy

You can find these as used books online, and you might even find a downloadable copy online.

Tom

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Tom:

Yes I realize that I picked a fairly complex radio.  The encouraging part was that it was working.     So, for me getting to a root cause will be great and an excellent learning experience.   I also, have options if things get too complex for my skills.  

I have inquired to a local shop to check to see if my tube checker is accurate and working properly.  

So, if you are willing to point me to some common component problems, I can figure it out.   Just want to avoid getting hurt in the process.    

Thanks for the problem solving so far.  I really appreciate it.

g.

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A good first exercise would be to use an ohmmeter to check wiring and switch continuity from the power cord to both the turntable motor and the primary of the power transformer on the radio chassis.  You can do this without powering up the radio.  In general, once the radio has sat without power for several hours, it is safe to work on, even if you have not taken any explicit step to discharge the electrolytic capacitors in the power supply.  If you want to be very certain, however, just use a simple wire to momentarily short between the two terminals of any large cylindrical electrolytic capacitors (do you know how to identify them?).  Then they will definitely be discharged.

Basically what you're looking for is to show that the circuit is complete, starting from one prong of the power cord, through the switch, through the turntable motor fuse, through the turntable motor, and back to the other prong of the power cord. 

Likewise, you should see a similarly complete circuit through the switch, the radio chassis fuse, the voltage selector switch, and through the primary winding of the main power transformer back to the other prong of the power cord.

Are you familiar with how to do continuity checking with an ohmmeter?

Try and see if you can understand how the wiring you're observing on the radio chassis and turntable relate to what you see on the schematic.

Do you have the U.S. Grundig-Majestic version of this radio, or do you have the German version?  The language of the labeling on the front panel should make clear which you have.  The Sams schematic available here at RMorg for the U.S. version is a little easier to read than the schematic available for the German version.  The Sams schematic also appears to include more explicit information about how the turntable power is wired.

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Is there any tip or tricks I should know on how to get the chassis out? 

 

g.

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Typically it's necessary to remove the knobs from the front, and then there are four or more screws holding the chassis from the bottom.  Speaker wires may need to be temporarily removed (desoldered or unplugged), the antenna unplugged, and the tuning eye (if present) may need to detached from the front panel if it is not mounted directly to the chassis.

These are just general guidelines for typical German radios.  Yours may be a little different.

Someone with specific experience on this model may be able to provide more tips.

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Tom: I have successfully removed the chassis from the cabinet. However, I had to disconnect the turntable and will not be able to reconnect unless I put it in close proximity to the cabinet. So, at this point I can only diagnose the tuner, until I can wire up the turntable. Ok so here is what I have done. I put the ohmmeter on the connector to the main power. I perceive this as testing the tuner circuit. I originally got a a resistance reading, then I didn't. I powered it up, and actually for a few moments it was working then not. I did then a stupid thing. With the power on I reconnected the multimeter and got a spark. I blew my variac fuse, and a spark took a bit of metal out. I went on continue to receive open circuits. G G

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You were on the right track using your multimeter to take a resistance reading across the power cord.  When everything is proper, you should get a resistance of somewhere in the range of 1 to 20 ohms with the power switch turned on, and infinite resistance with it off.

Check to see if your multimeter still works properly.  If you leave it on the resitance setting when votlage is applied, the meter might be damaged, but most likely not.  When voltage is applied, the meter needs to be set to read AC volts.

The power cord, power switch, radio chassis fuse, and the primary winding of the power transformer form a simple loop circuit.  Use your ohmmeter to check this element by element and you will find which one is intermittent or open circuit.

Once you get to the point that the radio powers up properly, you'll want to replace the electrolytic capacitors and many of the paper capacitors.

Since you're a beginner at this, be very careful.  Keep your hands out of the circuitry when power is applied.  If you want to probe live circuitry, a good approach for a beginner is to use alligator clips to connect your meter probes to the points you want to measure with the power off, and the apply power without having to hold any probes in the live circuitry with your hands.

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I think I found the smoking gun. I need to replace a 20 microfarad 40 volt non polar capacitor that sits on top of the coil labeled T2. How do I begin to match up a modern cap to one of those? G

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Hi Glenn,

That particular capacitor is part of the speaker system crossover network.  If that capacitor is open, the midrange speakers won't have any sound.  If that capacitor is shorted, the midrange speakers will carry bass content along with the woofer.

You can replace it with two 10 uF 50 V non-polarized capacitors in parallel from your local Radio Shack.  You can likely also find a 20 uF 50 V non-polarized electrolytic from one of the many electronics mail order places like Digi-Key or Newark.

Note, however, that that particular capacitor failing wouldn't be responsible for blown fuses or other power supply problems, so you may have some additional problems to troubleshoot.

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Tom:

It is becoming apparent that i havent found the FM radio loop in the schematic..  Could you help me figure out a fundamental loop  for the FM?  

In addition, if that capacitor feeds the speakers, wouldn't it be in play for the turntable?  Since that is connected through a plug to power all of them?

Living the schematic dream,

glenn

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Glenn,

Although the speaker wires and the turntable power happen to go through a common connector, their circuits are independent, so the speaker crossover capacitor will play no role in whether power is applied to the turntable.

To get started with your troubleshooting in a concrete fashion, let's take a look at some specific parts of the power supply circuitry.  From what you've said above, it sounds like the complete unit is totally dead right now -- there is no evidence of anything really being powered up when you plug it in and turn it on.

Please get the first schematic page from the model page (which is the left hand side of the Sams schematic, and says page "5" at the bottom).

The first thing we want to check is the circuitry carrying power from the power cord through the power switch and fuse to the primary winding of the main power transformer.  If you look at the middle of this schematic page, you see the following:

-  the power cord has an interlock connector

-  there are two RF chokes L34 and L35 (basically for noise suppression)

-  there is a fuse M2

-  there is the main power switch M18

-  there is the voltage selection switch (marked with two positions "220 VAC" and "110 VAC")

-  there is the primary winding of the main power transformer T1, with three connections labeled "BLK," "GRN," and "WHT."

-  there are wires U and V going to the speaker socket (carrying the power to the turntable motor)

By tracing the route of the wires yourself, and by looking at the pictures in some of the other Photofact pages you can download from the model page, try to find every one of the components mentioned above, and then start by testing them individually:

-  The interlock should show a low resistance from the plug prongs of the power cord through the contacts of the interlock.  Resistance should be less than 1 ohm.  

-  Both chokes should have very low resistance, probably less than 1 ohm

-  The fuse, in its holder, should have very low resistance, less than 1 ohm.  This is a high failure rate area; the contacts on the prongs of the fuse holder are often oxidized and the fuse makes poor contact.  If you see high resistance, first check that the fuse is good, and then try scraping the fuse holder contacts until you see a reliable reading of less than 1 ohm between the connections of the fuse holder.

-  The main power switch should show less than 1 ohm across it when it is turned on, and infinite resistance when it is turned off.  This is another high failure rate component, and may be failing to turn on.  If the switch doesn't work, you can bridge a wire across it for now, so the radio is always turned on (temporary fix).

-  Check that the voltage selector switch is set to the 110 VAC position, and that there is less than 1 ohm of resistance across that pair of contacts.

-  Check the resistance of the primary winding of the main power transformer.  When the voltage selector switch is set to 110 VAC, you are using the part of the winding that is between the black and green wires.  The Sams schematic tells you the resistance between these two wires should be 9 ohms.  Confirm that this is correct (it doesn't have to be exact; it might read somewhere in the 8 - 12 ohm range).  This can also be a point of failure.  If the primary winding of the main power transformer is open (infinite resistance), you will have to replace the transformer.

If each of the above tests good, then try testing the entire circuit loop, starting from one prong of the power cord plug and measuring its resistance to the other prong.  If the power switch is on, this should show basically the resistance of the primary winding of the power transformer, or around 9 ohms.  When the power switch is off, it should show infinite resistance.

If the individual component checks seem OK, but the total circuit check is open, something's wrong in the wiring.  You should be able to figure it out by observation, or by measuring resistance step-by-step through the circuit.  For example, measure from one prong of the power cord through the following in sequence, and see where the circuit goes open:  interlock connector, L34, fuse, voltage selector switch, transformer winding, power switch, and back to the other prong of the power cord.  If there's a problem, you will find it.

Using the principles explained above, you can then follow the power from wires U and V through the speaker connector to the turntable.

There will be more to address after these first test are done, but this is a good place to start.

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BTW, I really appreciate the effort you are taking with me to uncover this mystery.  I hope this is appropriate for the musesum.     I find it really interesting.   My goal is restore power and then replace the caps (i hope).    I hope this is helpful for other members of radiomuseum, I am by no means a technologist, but am having fun.

(I have marked my responses with [gp]

Glenn,

Although the speaker wires and the turntable power happen to go through a common connector, their circuits are independent, so the speaker crossover capacitor will play no role in whether power is applied to the turntable.

To get started with your troubleshooting in a concrete fashion, let's take a look at some specific parts of the power supply circuitry.  From what you've said above, it sounds like the complete unit is totally dead right now -- there is no evidence of anything really being powered up when you plug it in and turn it on.

Please get the first schematic page from the model page (which is the left hand side of the Sams schematic, and says page "5" at the bottom).

The first thing we want to check is the circuitry carrying power from the power cord through the power switch and fuse to the primary winding of the main power transformer.  If you look at the middle of this schematic page, you see the following:

-  the power cord has an interlock connector  [gp] I have identified it. [Black Left top, Red right bottom] 

-  there are two RF chokes L34 and L35 (basically for noise suppression)  [gp] need to identify it

-  there is a fuse M2 [gp] Getting flow from interlock to behind fuse bracket.

-  there is the main power switch M18 [ small black device and shows resistance]

-  there is the voltage selection switch (marked with two positions "220 VAC" and "110 VAC") [gp] triple confirmed it is on 110 V.

-  there is the primary winding of the main power transformer T1, with three connections labeled "BLK," "GRN," and "WHT."   [gp, i tried to figure this one out,  there are multiple wires and could not find an exposed area for the probe, i even tried removing the two black caps"]  

 

-  there are wires U and V going to the speaker socket (carrying the power to the turntable motor)

By tracing the route of the wires yourself, and by looking at the pictures in some of the other Photofact pages you can download from the model page, try to find every one of the components mentioned above, and then start by testing them individually:  [gp I have been using the schematics mentioned]

-  The interlock should show a low resistance from the plug prongs of the power cord through the contacts of the interlock.  Resistance should be less than 1 ohm.  [gp will check]

-  Both chokes should have very low resistance, probably less than 1 ohm [gp will check]

-  The fuse, in its holder, should have very low resistance, less than 1 ohm.  This is a high failure rate area; the contacts on the prongs of the fuse holder are often oxidized and the fuse makes poor contact.  If you see high resistance, first check that the fuse is good, and then try scraping the fuse holder contacts until you see a reliable reading of less than 1 ohm between the connections of the fuse holder. [gp fuse holder looks very clean, will check the resistance.]

-  The main power switch should show less than 1 ohm across it when it is turned on, and infinite resistance when it is turned off.  This is another high failure rate component, and may be failing to turn on.  If the switch doesn't work, you can bridge a wire across it for now, so the radio is always turned on (temporary fix).  [gp will check that]

-  Check that the voltage selector switch is set to the 110 VAC position, [gp 3x] and that there is less than 1 ohm of resistance across that pair of contacts. [gp will do]

-  Check the resistance of the primary winding of the main power transformer.  When the voltage selector switch is set to 110 VAC, you are using the part of the winding that is between the black and green wires. [gp I will check that out, i removed the lids but found it virtually impossible to find an exposed connection without exerting force] The Sams schematic tells you the resistance between these two wires should be 9 ohms.  Confirm that this is correct (it doesn't have to be exact; it might read somewhere in the 8 - 12 ohm range).  This can also be a point of failure.  If the primary winding of the main power transformer is open (infinite resistance), you will have to replace the transformer. [gp the transformer is T1]  

[gp can you clarify what is meant by having the power switch on?, does that mean live electricity or that you have clicked on a selector button like FM]

If each of the above tests good, then try testing the entire circuit loop, starting from one prong of the power cord plug and measuring its resistance to the other prong.  If the power switch is on, this should show basically the resistance of the primary winding of the power transformer, or around 9 ohms.  When the power switch is off, it should show infinite resistance.

If the individual component checks seem OK, but the total circuit check is open, something's wrong in the wiring.  You should be able to figure it out by observation, or by measuring resistance step-by-step through the circuit.  For example, measure from one prong of the power cord through the following in sequence, and see where the circuit goes open:  interlock connector, L34, fuse, voltage selector switch, transformer winding, power switch, and back to the other prong of the power cord.  If there's a problem, you will find it.

Using the principles explained above, you can then follow the power from wires U and V through the speaker connector to the turntable.

There will be more to address after these first test are done, but this is a good place to start.

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Hi Glenn,

A few clarifications:

When you press the "off" button, the power switch is off.  When any other button is pressed, the power switch is on.

To check the transformer primary winding, locate the green and black wires coming out of it.  Then follow these wires to where they are soldered to something.  That's where you can probe them and measure the resistance of the winding.  You should also be able to see that they are connected as shown on the schematic diagram.

For all of the tests discussed in the last few posts, the unit should be unplugged.  No power applied.

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With a newer multimeter as a back up, i got an infinite reading on the m18 switch. Where or how do I replace that? G

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If you're mechanically inclined, try disassembling the switch and see if you can fix it.  Sometimes you can; sometimes you can't.

If that route isn't going to be successful, I suggest simply bridging the switch by soldering a wire across it for now.  That will mean the radio is always on when it is plugged in, and you can troubleshoot the rest of the radio.

If you get the rest of the radio working, and you would like a working power switch, you'll have to get creative.  It might not be possible to find a modern switch that is a direct replacement (unlike most other components that have modern replacements that are easy to substitute).  You can advertise to see if someone has a good one from a junker they can sell you.  There might possibly be someone making a replica or substitute switch that you can buy.  You can install a modern switch somewhere on the cabinet wall or back, bypassing the original switch.  Or you can leave the internal switch bridged and use a power strip with a switch to turn the radio on and off.

I wouldn't worry about implementing a final fix for the power switch until you get the rest of the radio (and phonograph) working.

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Tom:

Just to make sure we are talking apples to apples, I am referring to the component labeled M18.   it is a small two terminal box like item on the underside of the bread board.  it doesn't appear to be mechancial to my eye.

Thanks.

 

g.

 

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Yes, that's it.  And it is mechanical in its operating principal.  The bushbutton mechanism will have some kind of tab or lever that flips the switch.  Check to see that the pushbutton mechanical assembly is doing what it should.  If it is, then the problem is internal to the switch.  As I mentioned above, sometimes you can repair the internal mechanism of the switch; sometimes it is really not possible.

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I've repaired power swiches by epoxy on a small microswitch. Also repaired mains/battery switches operated by  cable cord  socket that pushes on to chassis plug.

This Schnieder had no schematic, no tube list, tubes missing, speaker missing, tuning cord broken and power switch broken. Maybe reading the "journey" on it may give you ideas.

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Hi:

Thanks Michael, I will take a closer look.  

I did some more checking last night on the transformer.

I put my red on the black wires just behind the fuse, leading into the T1 transformer.    I then looked at the output of wires from the T1.   I traced the blue to ground, and checked resistance.  I got nothing.  I traced the brown output towards the face of the stereo.  I got nothing as well.   The yellow I traced to the coupling and got 0 ohms as well.  

I also tested the leads across the "microswitch" and it tested 0 ohms.    

So, my next step is to bridge across the microswitch by removinng it out of the circuit, and see if the chassis powers up.  I am guessing that i can just connect the two to cut the microswitch out of the picture.

That will at least remove the question of the switch being defective or not.  

If it is the transformer, which seems a likely culprit.  What are my options for repairing it?  Buying a new one? Getting the transformer rebuilt?  

g.

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"nothing" is ambigious.

An OFF switch should read infinity (open circuit), a kind of "nothing".

An ON switch should read 0 Ohms (or very close), a different kind of "nothing".

A transformer winding can be a fraction of an Ohm (winding for powering valve/tube heaters, or winding for a 3 Ohm to 16 Ohm loud speaker) or 50 Ohms  (high power primary) to 1000 Ohms (very miniature transformer). The Primary to anode/plate of output transformer normally 100 to 500 Ohms (the 2000 Ohms or 4000 Ohms is AC load transformed from the 3Ohm to 16 Ohm speaker, never the DC resistance). The HT Secondary might be 100 (large) to 500 Ohms (small) depending on power rating.

Check any fuses and voltage selector switches. From the main switch mains wiring.

0 ohms means a connection

1 at left and blank not 000000 on rest of DMM usually means infinity or open circuit or more than 1M ohm on really cheap meter or more tnan 20 M ohm on more expensive.

If your fingers are sweaty and pressing on probes you can measure as low as 100K Ohm. Very dry skin and loose touch can be over 2M Ohm.

Some models only switch one of the mains wires and others have switches in both wires.

A DMM / DVM can get confused by inductance of Transformers and take a while to stabalise reading. Very high inductance chokes may need a PSU and ammeter or an analogue meter! Switching the DMM/DVM from Auto to manual and selecting the correct resistance range is a big help.

 

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Glenn,

Your switch may be OK.  As both Michael and I have pointed out above, it should read close to zero ohms (less than 1 ohm) when it is turned on, and infinite resistance when OFF.  Since you measured zero ohms across your switch, it was probably turned on, and that means it is fine. 

On the transformer, as mentioned above, you want to measure the resistance between the black and green wires, and that resistance should be around 9 ohms, as indicated on the Sams schematic.  Measuring resistance between the black wire and other wires will in most cases give an infinite resistance reading, and that is fine.

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All:

I realize that "nothing"  is ambiguous.  I was originally going to provide some photos to validate my transformer testing.  But they are quite dark, so you can't see what I wanted to share.

What I did, was find at the back of the fuse a collections of wires both black and brown bundled to go into the T1 transformer.  I placed my multimeter's live (red) to that connection.  

There is another bundle of wires which travel to the T2 power supply, or somewhere else.  

I made sure there wasn't heavy oxidation that would block readings when I checked the connections.

From the output of wires from the T1  transformer I traced Brown which goes toward the Ferrite antenna and Volume pot.  I have switched to a new analog multimeter, as I became suspicious of my original multimeter, especially when measuring AC Volts.   When I touched the black(MM)  to the brown wire, i received no needle movement, no response, no ohms.    

Secondly, i traced the ground (blue wire), and received no response, no needle twitch, no ohms detected.

Thirdly, I traced the yellow wire, to the power coupling (suspecting it goes to the power the turntable) and also received no response, needle twitch, no ohms.  

What I cannot recall, is whether i had the M18 microswitch in off or on.   I will have to make a matrix to check my combinations.

There is still a chance for me to mess up as some of the wire colors have become oxidized and the area is quite small.  

What I am going to do next is to take the red lead and place it on the interconnect.  That way i know it will ultimately be in the "correct" position to power a check.  Then return to my matrix with the FM switch pushed to On, and then Off.  

At that point, I understand that I would not be able to eliminate the switch, but my method should I be able to determine if those two items in conjunction pull any ohms.  (Perhaps this is your point, that 0 might be a valid answer)

After that, I think I should bridge (by eliminating the microswitch and connecting the two wires together to see if the unit then powers up).   If so, it eliminates the T1 being the open circuit.  

If nothing happens, I can then contemplate replacing/repairing the T1.

If i have missed anything or there is another option in this test to consider I am all ears.

Thanks for all the help.

g.

 

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I bridged the m18 switch and the chassis fired up. I also found a replacement switch if I dare. While I have the chassis out I will now try to replace the caps. I expect to replace the electrolytic ones. I know the turntable cap that needs to be replaced. When replacing vintage caps with modern versions what are some tips. If there is some literature please let me know. I understand all paper caps should go, mica can stay. Any other tips. G

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My recommendation with this radio is, any work you do further, take pictures(with your smartphone?) and move forward on the restoration. There are two power resistors, one around the power tube and one in the FM tuner, that I would immediately replace with sand resistors. I would also, replace the selenium rectifier with a silicon bridge, and replace the electrolytics and paper capacitors. I would love to see your step by step progress, in pictures, on this forum

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I will upload pictures starting with this post.   My first challenge is this capacitor.  

2x50 micro fahrad.

I believe this is the one you have identified as the one around the power tube?  

It is in a can,  but before I remove it.  What kind of replacement am I looking for?

I have to downgrade my pictures.  I will have them up this evening.

 

g.

 

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...that is your main power supply filter capacitor and one of the very first things you should replace.  It consists of two 50µF electrolytic capacitors in one can.  Typical marking on those will be something like '50+50µF  350/385V-  Minuspol am Gehäuse'.  What this means:

'50+50µF'; two 50µF capacitors

'350/385V-': maximum rated voltage (first number is 'working' voltage, that is, normal operating voltage; second number is a 'surge' voltage rating (the highest voltage it can withstand for a short time, such as a voltage 'surge' at turn-on).  The following 'dash' represents DC voltage.)

'Minuspol am Gehäuse': means 'negative pole on the case', in other words, the metal can is the common negative connection for the two electrolytics inside. 

...There will either be two terminals underneath representing the two separate positive connections (remember, most electrolytics are polarized, and it's important to connect them correctly) or two red wires coming out (more likely the terminals). 

...You need to replace this can with two separate 50µF (47µF will work) electrolytics; use 450V.  (You can always use a higher voltage rating than the original, but NEVER lower.)  You can leave the old can in place for appearance, but you need to remove all wires from the two positive connections (do one at a time) and connect them to the positive leads of the new caps, then connect the negative leads together to one of the old can connections. 

 

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Hi:

I found this can cap as well.   Does this work?  

 

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My soldering iron's tip is quite chunky.   Does anyone have a suggestion for a better soldering iron, or use of silicon adhesive when dealing with dense areas of the chassis?

g.

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...The cap you referenced should be fine.  You may need to get the clamp to go with it because the original is probably mounted to the chassis with metal tabs and the new one won't fit (unless that's the reason you're asking about the silicone adhesive).  Also, that cap has a separate negative terminal (not connected to the can) that will need to be connected to the chassis (where one of the original mounting tabs is probably soldered). 

,,,What do you have for a soldering iron that has such a 'chunky' tip?  That might actually be OK for the soldering work you will probably have to do directly on the chassis, such as removing that old electrolytic can.  For the smaller work, especially where you're in a tube chassis without a printed circuit board, I'd suggest a Weller SP-23 'Marksman' (they still make some version of it).  It's a 25W 'pencil' iron with replacable tips (there are a few different types; I'd suggest a tip with at least one flat side) and it's fine for normal under-the-chassis work (it's also OK for the older PC boards with larger traces and pads if used carefully, but on anything newer I'd look for something smaller). 

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Hi:

Since i have the chassis out of the cabinet, it is quite difficult for me to test for sound output.  I noticed that the chassis has external speaker terminals.  

I am assuming that is a 3-4 ohm output.    Any ideas for what type of speaker to connect to it?

 

g.

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I use a pair of double ended croc clip cables and the original speaker usually.  Or it there is a field coil etc more leads.

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Hi!

Thanks for the quick response.    I am going to have to get an external speaker, so I was wondering what the  specs would be.     I am thinking a single 3-4 ohm 5 x 7 speaker.   This is just so that i can hear output while i test the chassis and replacing old caps.

what is a good speaker choice?

 

g.

 

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...those typically spec'ed a 5 ohm external speaker.  I'm not sure how they came up with that figure, but I doubt a 4 ohm (or even an 8 ohm) speaker is going to hurt anything.  Your 5x7 will probably be fine for testing purposes...

 

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Were you able to bypass the capacitors?

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Omer:

Not sure what you mean by bypassing the capacitor.   I was able to figure out that the switch on the chassis was broken.   I have bypassed that switch and the unit came back to life.  

I am now in the process of replacing my power capacitors.  I have removed the first can, which was highly relunctant to leave.    

I am now in the process of figuring out which way to go on replacing the can.

I have inquired at hayseedham to see if they could build me a cap with the same specs and connector.   I have also looked into to cutting the can and replacing the innards with a modern cap.  But during the removal process the three tabs that mounted the cap got mangled.  

I have tried to find replacement ones, but there aren't any out there.    

My best point of view is to see what the heyseedham folks come back with.  

I am open to suggestions.  Would love to save the can, but I am not so married with looking the same.  i just don't want to have to modify the chassis.   Newer cap mounts require screw holes to be drilled.  

 

g.

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Hi!

I am in the process of ordering capacitors.   I noticed on the SAMS schematic that voltage is missing from a lot of the capacitors.   Is there an default voltage for these, or do i have to find them individually and get the rated voltage.

 

g.

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...are you talking about the voltage ratings?  Most of the paper capacitors you can replace with 630V film caps (but note that C86 is rated at 1000V; you should stick with 1000V or higher for that one and I'd advise the same for C85).  Also note that this is a fairly early Sams and it is somewhat inconsistent with the cap values; note, for example, that you have C81 shown as '220mmf' (220pF), but C85 is shown as '.00047' (that's 470pF) and C86 is shown as '.00082' (that's 820pF). 

...You also have three smaller electrolytics in there (C2, C3 and C4).  You should also change those out.  C2 is shown as 4µF but it's on a B+ line for the audio and eye tubes that carries about 200V, so I'd use a 4.7µF 450V for that one.  C3 is a filter for the FM detector and shown as 10µF.  Even though the schematic does not show a lot of voltage on that capacitor, the original is probably going to have a '70/80V' rating on it, so use a 10µF 100V for that one.  C4 is the cathode bypass capacitor for the output tubes and is shown as 50µF,  You can put a 47µF 25V there.  (A bad C3 is going to cause audio problems on FM, and a bad C4 will cause low volume overall.)

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I have worked out most of capacitor problems.   I am going to move forward with a non-OEM can for the 2x50mf to move the project forward.  I will semi-secure it with hopes that I can redo the OEM can and replace it.    

After ordering the capacitors, I will probably go dark for while.    Swapping out each cap to replace should be a challenge.  

At this point, I would like to thank all who have helped me overcome my just in time learning style.  

Thanks again.

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Does C5 have to be non-polar?

 

what is meant by C14 having .5-4?   is that the tolerance? or is it a four section cap?

And is C17 really 4700mF@630V?  

 

g

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Hi All! 

I am back from my 4 hour hiatus.  Is there a clever way to filter out the mica capacitors from the paper ones using the SAMS schematic?     I would like to only replace the paper ones and the electrolytic ones.  

Any secrets, or do I have to examine them one by one.

 

g.

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The Mica are flat rectangular and paper tubular, though there are paper caps in similar rectangular cases, but they are not common and may not be faulty. There are also paper caps in large rectangular cans (0.1uF to 10uF depending on voltage), but mica caps are never physically much larger than 1cm x 2cm x 0.5cm. They can be "naked", wax coated or encapsulated in a moulded case.

Paper caps are not always faulty. Also Electrolytic capacitors can be reformed. I don't replace capacitors with out testing and considering circuit voltage as leaky paper caps may be fine at low voltage and low impedance.  It's rare that I haven't reformed Electrolytics. Of course I have to replace most paper types. Often I leave the chassis / earth end connected on decoupling parts and tuck the small modern part behind it. ONLY replace one at a time to avoid wiring errors!

See Replacing old Capacitors

The article by Ciardiello also shows photos of Mica and Paper types.

Also on the schematic usually paper types are 1nf (= 0.001uF or 1000pF or 1000uuF) to 2uF. Mica types are typically 5pF to 1000pF (large).

I recently found a 1950s technical book which discussed paper capacitors. They were rated according to encapsulation with life estimates. The best hermetic metal cap types had a 20 year rating and the cheapest waxed card case only 2 to 3 year rating. The manufacturers knew when they were fitting rubbish. Non-metal (often military) cased tropicalised perhaps 10 years. My own experience is that the crumbly Hunts brand, soft waxed card and black "tar like"  case are usually useless. EMI (Marconi / HMV) at certain periods must have been using tropicalised parts as though card they often are not sticky wax but a hard sheen and often low enough leakage to be left in place except on Anode to Grid coupling. I've found that either all the metal can tubular paper caps are OK or all bad in a particular radio.

The paper (in early 1950s UK anyway) was not Wax or Oil impregnated but actually Petroleum jelly, i.e. Vaseline. The plates of course Aluminium foil. There are two leakage modes:

1) The paper is a poor choice and becomes leaky, perhaps due to contamination reacting.

2) The sealing isn't good enough and the paper absorbs water. Water does gradually penetrate wax and Petroleum jelly.

Though of course modern candle (paraffin wax) is related to Vaseline as Vaseline is to paraffin (Kerosene). You can make the softer wax used to "lock" cores or coat card capacitors (for replica cases on plastic film types) by blending Petroleum Jelly BP with cheap candles. Vaseline is just a more expensive branded product often with added colour and scent.

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So, what i heard is that I need to inspect each capacitor and determine its type.   There isn't a tell in the schematic numbers or the way that farads are represented that indicate whether it is electolyic,mica etc.

I am trying to gather a list of problematic capacitors and replace them while I have the chassis out.   If there is a good starting list of capacitors that immediately need replacing I would appreciate it and that would be my starting position for replacement.  

Since, I am new to classic radios and I picked a complicated one, I am trying to skim the surface and only deep dive when necessary or required.    After getting this up and running, I intend to get a simpler one to cut my teeth deep.    

g.

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Maybe I wrote too much, you can "guess" from schematic

Also on the schematic usually paper types are 1nf (= 0.001uF or 1000pF or 1000uuF) to 2uF. Mica types are typically 5pF to 1000pF (large).

But you need to visually examine all components anyway, not just capacitors. It's one of the basic repair skills.

 

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So, I have bought a modern equivalent to replace my grundig can, but it is too big. I have emptied out the original can. How do I wire in the 2 50s onto the two pole base? Are there any other options? G

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This is my first cap.  It is a dual 50mfd @ 600v.  

I used the original mount and drilled in holes for positive and negative leads.  

Any comments would be welcome.  The next step is to test the capacitor and install.

 

thanks.

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• My First Cap Build (51 KB)

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You are on your way!  One by one, replace the paper capacitors and electrolytic capacitors

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(note: Glenn and I have been discussing this via e-mail)

...I can now see why you are unable to reuse the original housing.  I don't know if you would have been any better off using 450V caps instead of those 600V caps in terms of size and getting them to fit inside the can.  But you definitely have the right idea.  The one thing I would add is putting some shrink tubing on that solder joint where you connected the black wire to the two negative wires.  Try to get the caps down as close to that base as you can and then use some RTV silicone to fill and secure the base.  That will seal everything up, keep the wires from moving, and be flexible for vibration.  Once that is set, strip one of your green wires, fold it down against the base, wrap it around the base of the lug a few times and solder it.  Do the same with the other green lead and the other lug.  Now, you should be able to fold those negative leads down close to the caps and bring them together, then tuck them into that area between the caps.  Wait until you actually remount this assembly before you connect the black wire; then you'll do the same thing you did with the two positive leads: strip it, fold it down close and connect it to one of the mounting lugs.  Once you have everything in place and hooked up, you might be able to find some large heat-shrink sleeving that you can slide over the caps and shrink down to cover them.  (This is only a suggestion for a little extra neatness and not really necessary for function.)

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