grundig: SO302U/S; Musikschrank
? grundig: SO302U/S; Musikschrank
I've been reading about replacing the selenium rectifier in these late 50's, early 60's Grundig stereo consoles. I would like some assistance on getting the right parts.
Based upon the following schematic, you can see the rectifier
B250 C150 S u. H.
What would be the right silicon diodes to order to replace the selenium rectifier? Is there information on this diagram that I am missing that should give me the information that I need? Perhaps the B250 C150 notation?
What would you guess the drop in voltage is that I need to compensate for with an additional resistor? With respect to the resistor, let's say that the right answer is a 10 Ohm resistor. How do you determine the minimum wattage on it? Is it sufficient to say that 12 Volts over that 10 Ohm resistor means I need at least a 1.2 Watt, 10 Ohm resistor?
Thank you for any help that you can provide. I have a follow up question on R64 in the photo as well. How do I know what watt resistor I need? I see the voltage and capacitance on the electrolytic capacitors, but not sure if I can get the number that I need to rplace those electrolytics.
craig.
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Proper values
Dear Craig,
the selenium rectifier can be replaced by almost any silicon bridge rated for 400 V or more. The original type, B250C150, was rated for 250 V rms at 150 milliamps. For ease of installation you can use large rectifiers, as the KBPC 610, 6 amps, or even the KBU 810, 8 amps, since they have a hole that allows to fix them to the chassis.
About the resistor, you should assume an average load current in the order of 100, 120 milliamps. This means that, to compensate for the lower drop across silicon diodes, about 20 volts, you need some 150 to 180 ohms. The nominal power required is about 2.5 watts. Always for ease of installation, you can use a 25W aluminum encased wirewound resistor, fixed to the chassis with a couple of screws.
The power rating of R64 usually is around 7 watts. A 10 watts wirewound should work fine. Capacitor values are 50, 50 and 8 uF. 450-volt types will work fine.
Emilio
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Can Capacitor Replacement?
Hi Emilio - Thanks for the detailed answer. I'm going to try completing the restoration without changing out the rectifier with a bridge in order to ensure that I fully understand all the points that you outlined.
With respect to my second question about the resistor R64, I've attached a picture of the can capacitor that has all 3 electrolytic capacitors inside:
My question is this. Do I need to replace this group of capacitors even though the can looks to be in great condition? I ask because I purchased the necessary replacement electrolytic caps and due to the layout, it's difficult to get everything organized correctly since the caps are quite a bit larger in aggregate than the original can cap. I'm just trying to make sure that I don't replace something that doesn't need replacing.
I'm just not sure what's best and want to ask before tearing anything apart.
Thanks!
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Capacitor test
Unplug all the valves (tubes) and power HT line directly from 30V with series mA meter and a voltmeter on HT. Connect at + of rectifier and chassis.
See the current gradually drop. If it stays high a capacitor is leaky. Leave for an hour or so.
Disconnect +30V and see voltage decay. If instant drop to zero, the capacitors are dried out.
Now if that was OK, insert valves/Tubes* repeat powered via mains and measure HT.
(* Assumimg any leaky Anode to grid compling capacitors and leaky screen grid to chassis/0V capacitors replaced).
Even on a 70 year old set I have rarely had to replace electrolytics. The dried out ones (no smoothing) or leaky ones generally looked bad. In two cases battery tube sets started motorboating a few days after "restoration" and replacement of main HT electrolytic was needed. In one case I used the 4.7uF 300V from a broken CFL lamp to replace capacitor on a 1945 piece of test gear.
Low HT usually means excessive leakage
Hum or motorboating usually means high ESR / Dried out Electrolytic.
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Capacitor replacement
Unfortunately the look of a capacitor is not useful to understand when it must be replaced. If the case is damaged, the capacitor must be replaced, but a shiny case does not mean that the component is efficient or bad. You must check the leakage currents and the capaciyance value of each section to decide if the component must be replaced. If you find a single section dried, you can simply add another capacitor in parallel with the dried one.
Please refer to:
Servicing old radios: useful tips
Emilio
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Resistor Questions
Hi Emilio,
Thanks for the feedback earlier on the electrolytic caps. I'm investigating finding those resistors that we discussed (selenium rectifier replacement and R64) and I'm having a hard time finding the 10 watt wirewound and a 25 watt aluminum encased wirewound. Do you know what online or local store would have these? I checked the ones listed in the forum post but didn't have any luck. Typically I buy from Antique Radio Supply or Radio Daze. Help?
Thanks.
craig,
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Wirewound resistor
Craig,
chassis-mount wirewound resistors can be obtained from distributors as Farnell, Mouser, RS and similar.
Mouser lists a lot of values. Here are two manufacturer codes from their site:
- RH025150R0FE02 150 ohms, 25W, 1%
- RH025220R0FE02 220 ohms, 25W, 1%
You can search for 'Power Resistors Wirewound Aluminum Housed' 25W, 1 or 5%. Even Ebay lists similar devices. Give a look at the ebay article 290660169865
Emilio
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Troubleshooting Low Voltage
Hi Emilio,
I've been stedily working on my console and am getting quite close to finished. Everything is coming together nicely and our help has been invaluable.
Here's where things are at:
1. When everything is together (chassis remounted in console and reconnected), the FM radio plays really well. The volume level is quite loud and there's no static. Even some AM channels come in well although I am working in a basement.
2. When I change to the record player, the turntable is "sluggish" and turns slower than expected. If I can get it turning at a faster speed, the volume level from the record player is way low for what I would expect.
3. When I remove the chassis and take voltage measurements (external speaker attached), I get a voltage from the transformer of 242, after bridge rectifier 280, after 25W resistor 266. Those all seem to be ok in my eyes, but would like your opinion.
4. The replaced can capacitor has 3 electrolytic caps that have voltages of 265 (C71), 214 (C70) and 210 (C69) - Seems that these should be 280V according to the schematics.
5. I took voltage readings from the ECL 86 and have the followning results: Pin 3 = 196 (220 on schematic), pin 6 = 187 (250 V) and pin 9 = 87 (108V)
I also rechecked and rechecked my capacitor replacements and had found a couple of mistakes, but corrected those a while ago.
I'm kind of at a loss at this point where to look and what measurements to take to try and determine what the problem is. Any guidance that you can provide is greatly appreciated!
Thanks
craig.
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Perhaps not the Radio?
IF AM & FM are fine, the problem may be the pickup cartridge has deteriorated. The Speed issue is likely old hardenend grease on the shafts and bearings that needs cleaned off (maybe WD40) and then put fresh grease after drying off the cleaning fluid (paraffin lamp oil may also soften and remove hardened grease).
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Radio/Turntable
Hi Michael,
Well, the voltages are sufficiently off so that I'm concerned about that part first. the turntable has worked fine before I replaced the rectifier and can capacitor so I suspect that I have something wrong with some wiring or a resistor or something so I need some help to debug this problem. I think that the slow turntable is a sympton, not the problem.
Thanks for the idea, though.
craig.
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Overload
Hi Craig,
maybe that you have an overload condition in your radio. The voltage read on pin 6 of the ECL86 is too low, maybe due to an improper bias of the output stages. You should find about 250 volts here in normal conditions. Try to check the cathode and the control grid voltages of the output stages. You should find about 6 volts from the cathode pin 7 to ground and must read 0 volt from pin 8, control grid, and ground.
Emilio
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Problems with PU-operation
Hi Craig!
I agree with Michael Watterson in Post 9 that incorrect speed of the turntable might be a mechanical problem as the complete mains power-supply (115V) for the changer is taken off before the primary winding of the mains transformer.
I had a look at the schematics and my personal suspects are the switch contacts 4k and 4l (muting of decoder an RF-stages during PU-operation). I recommend to double check the wiring around these contacts as they are also closely involved with the output stages.
Good luck with further repairs an all my best wishes for a happy new year
Harald Schlosser
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Maybe 20% low
The ECL86 voltage is maybe 20% low, I would check the cathode resistors (on both) and temporarily disconnect any electrolytic capacitor on Cathode (reduces audio gain, but removes any leakage). You can measure the resistor with power off and then volts on it with power on and that tells you current in the Pentode part of ECL86 (do on both)
Surely the record player motor is AC and not even fed from transformer?
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Low speed
Craig,
the voltage measured on the plate of the power pentode sections is definitely too low. The first step is to investigate on the grid bias of the power amplifiers. If the cathode resistor is still the original one, I suspect some leakages in the grid coupling capacitors C67 or C68. This can be evidenced measuring the voltage across the cathode resistor R63, where the plate and screen grid currents of both the power amplifiers flow. The resistor value is calculated for a typical drop of about 6 volts, that is the typical negative control grid bias of the two amplifiers. Any drop greater than some 7 volts indicates an increased zero-signal plate current in one of the amplifiers. In this case you should measure the DC voltage from the control grid, pin 8, of both amplifiers to ground. Any positive value should indicate 99% a leak in the corresponding coupling capacitor. The other option is a gassy tube.
About the record player speed, in your case I guess its motor is connected directly to the 115 AC mains. Any overload in the power supply transformer therefore should not affect its speed. Did you remove the platter from its shaft? Many turntables have a small steel ball between the top of the shaft and the platter itself, in order to reduce the friction. If you removed the platter the ball may be dropped away.
Emilio
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Turntable Problem
Hi Harald and Michael,
I'm coming around to your perspective. It is true that the power is coming before the transformer is reached so it is around 115V AC.
And when I turn on the turntable and let it "think" for a minute, it will eventually get rolling at the right speed, so I am leaning toward agreeing that it is a mechanical problem likely just a greasing issue.
@Emilio - I have not taken the platter apart but that may be required to get this working right...
I'll have to take this apart later to fully investigate and correct the problem.
Still working on the low voltage issue.
cheers,
craig.
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Some Progress
Hi All,
I spent quite a bit of time this evening re-reading all the advice and doing some testing. Here's a few items that I learned tonight:
1. The 10 kOhm wirewound resistor that I needed as a replacement for R64 is actually at 4.5kOhm. The schematics call for 8.2kOhm. Obviously, this seems wrong, but I was wondering what could have caused this (besides just a bad resistor) and would this be causing the voltage to C69 and C70 to be low (220V) while C71 is at 268V?
2. I took the following measurements on the 2 ECL86 tubes as follows:
Tube 1
PIN Number - Voltage
3 - 200V
6 - 188V
7 - 9.0V
8 - 8.8V
9 - 87V
Tube 2
PIN Number - Voltage
3 - 198V
6 - 249V
7 - 9.0V
8 - 0V
9 - 120V
According to Emilio's earlier comment, that pin 8 positive voltage on Tube 1 seems like a problem. What I did next is switched the tubes to see if the voltage transferred with the tube, indicating a bad tube, or the voltage stayed with the electronics, indicating a bad capacitor or resistor. Please correct me if that logic is wrong.
When switching the tubes and restarting the console, the voltages stayed with the tube, almost one-for-one. So it seems that Tube 1 is leaking.
Note that the "leaky" tube appears to be one of the originals from the early 60's because it's got the European numbering while the other tube is clearly a replacement from Sylvania with the US numbering of 6GW8. I would think that it's safest to buy 2 of these to have one on hand in case there's another problem.
3. I also checked about 20 resistors and only found 2 that didn't match within the tolerance. Interestingly, they both should be 2k and measured just over 1k. R65 and R66, connected to the speaker transformers, are the culprits. What would the impact be of this reduced resistor?
That's all that I have for now. Any other ideas are welcome!
craig.
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Problem at a solution
Well, the problem is almost at a solution. An old tube may be gassy if the radio was operated for a while with a leaky coupling capacitor, before replacing it. Probably you should observe an iridescent getter in the bad tube.
Moving to resistors, a drift in their values is possible for carbon composition or even for some carbon film types. Anyway R65 and R66 are stable wirewound types and the wrong value you read is probably due to a wrong test method. I guess that you are reading their in-circuit value. But, if we ignore the resistance of the two windings in series with them, the two resistors are connected in parallel, between C71 and C70. So you read a value equal to half the value of each resistor.
Emilio
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Partial Explanation
Hi Emilio,
Thanks for your continued remote troubleshooting. I will order the new tubes asap.
I was curious that the 2 resistors were almost exactly half of what they should be. If I accept your explanation that I am testing them incorrectly, then this makes sense that they aren't bad. What I am confused by is the R64 value and my measurement. What is the proper way, if any, to measure the value of a wirewound resistor or should I just assume that they are always working properly because of their design?
cheers,
craig.
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ww resistors
Hi Craig,
usually failure mechanism depends upon the type of component. Carbon resitors, either film or composition, can be more or less hygroscopic. No wonder than to find that old composition resistors could have lost 30 or even 40 % of their nominal value, due to a prolonged storage in a high humidity environment. When this happens, often resistors can be returned to their original value by a simple baking process. Sometimes they can readily recover some 10 % of their value, just heating their terminals with a soldering iron for a few seconds.
On the contrary, a wirewound resistor is not affected by moisture, unless the resistive wire is corroded. Anyway, in this case, you should observe an increase of its resistance value or even the opening of the resistor itself. Any resistance decrease could just derive from a short between adjacent turns. But, to have the resistance halved, we must assume that half of the turns in its winding are shorted and this is not likely. For this reason I assumed that you had measured their in-circuit value. You should have disconnect one end the resistor before the measure.
I bet anyway that, when you will put a new tube in the radio, all the voltages return more or less to their nominal values. Probably the same sensitivity of the audio amplifier will return to normal conditions. You had the gassy tube draining some 80 mA from the power supply, near to the saturation region. On the other side the good ECL86, was draining not more than 20 mA, due to its fairly high negative bias. By the way, the plate of the defective tube was dissipating some 13 W, well beyond its 9 W design limits.
My best wishes for your radio and of course for you,
Emilio
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Wirewound
I agree of course with everything Emilio Ciardiello says on Wirewound resistors. But additionally higher power ones (like multi-section droppers in TVs or AC/DC radio or Battery/Mains Valve set using a dropper and no Transformer) may read quite low when cold.
I measure them immediately after power removal after running for 5 minutes or so. Often they are a little high on load due to surface corrosion especially if the glaze is porous or cracked. Higher quality ones have an impervious porcelain type shiny finish and cheaper types have a porous matt cement coating.
As Emilio Ciardiello says as little as 1/2 usually means something is in parallel. Old high value composition parts seem to go high value, as much as x2 or x3 (20% or even nearly 100% ( x 2), often doesn't matter on grid "leak" resistors), but often low value cathode resistors go low in value (bad), about 10% to 20% can be acceptible. Medium value 2K to 100k resistors seem to change much less in value but often 20% to 30% is acceptible anyway.
It's really only on phase splitters, Bias networks etc, especially on Transistors, that much resistor accuracy is needed. On the capacitors the most critical values are Oscillator, IF and RF tuning which are often Ceramic or Mica and thus higher precision and less likely to drift. Next most critical values (and often waxed paper & foil on old sets) is the tone control & negative feedback parts.
All capacitors on grids isolating higher voltages (g1) or decoupling (g2) are leakage rather than value critical.
Emilo's good article link again Replacing old capacitors
I only check the likely critical (damage due to leakage) capacitors before power on and then if the Radio is performing OK I don't go "looking" for problems by checking other capacitors and resistors. I often as a final check measure the HT current on a battery set or HT voltage and HT drop (to estimate current) on a Mains set. Hum or motorboating tells me the electrolytics have "dried out" rather than being "leaky"
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Dotted Lines on Schematics
Hi Emilio and Michael,
Thanks for the great feedback and detailed explanation of wirewound resistors (and tube calculations).
I have another question... surprised? On the schematics, I've run across some confusing dotted lines that I'd like some clarity on.
The following image has easy lines to see:
between the R31 and R32 resistors as well as R41 and R42.
In the following image, there are two dotted lines that are hard to see:
Look closely at pins 2 to 5 and pins 3 to 5. Why are these lines dotted and if they are meant to be a connecting wire, what would be the effect if they are not connected?
Thanks in advance!
craig.
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Dotted lines
The dotted lines connecting the cursors of volume and tone controls are commonly used to indicate a mechanical connection. In this case two potentiometers are mechanically ganged on the same shaft.
The dotted connections between pins of socket for the stereo decoder indicate the jumpers to be made in order to close the audio paths, when the stereo decoder was not installed. Usually in countries where the stereophonic transmissions were not available, the radio was sold without the decoder and with a shorting plug. Dotted lines give the wiring diagram of this dummy plug.
Emilio
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Success!
Hi Emilio and Michael,
I wanted to thank you (and Harald) because tonight I plugged in a NOS tube that I bought for $6(!) and the voltages around the main electrolytic cap and ECL86 tubes all moved to the acceptable range! I put the chassis into the console and listened to it for an hour tonight and there are no noticeable problems related to the stereo playing through the speakers. Nice full sound, no speaker drop outs. Getting close to only having the refinishing to do.
There are a few minor things that I have some questions on.
1. The speaker balance knob doesn't seem to be able to turn the potentiometer shaft. Since I've never had this console working before (received it from deceased family members) I don't know if there's a problem with the potentiometer. I couldn't manually turn the balance part of the shaft when the inner dial was taken off. The volume control works well and I feel that one speaker is slightly louder than the other so I think that something is up. Any hints?
2. I saw another post about the turntable and being sure to use the right types of capacitors when replacing originals because of the AC vs DC issue. I went back and put the old capacitor on the turntable and things are running well again. However, the sound that comes out of the turntable is muffled to no better than 50% of what's expected and it has virtually no bass. I created a headphone to 5 pin DIN plug to run my digital music through the console and it works, but has exactly the same sound characteristics. This was done through the TR input plug.
Since it happens exactly the same with two different sources (PU and TR), I imagine that there's an issue outside the turntable in the chassis somewhere. I suspect that this is an easy fix since the FM channels prove that overall, the audio components are working well. Any thoughts on what the issue might be?
I'm excited to bring this up from the workspace tomorrow and start enjoying the radio in our living room as I wait for Spring to do the refinishing.
Thanks!
craig.
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locked shaft?
Hi Craig,
should you have a locked shaft on the balance control, refer to this article.
Unfortunately the manufacturer and the type of phono pick-up are not given in the model page. Anyway the poor sound from the phono pick-up could derive from worn or broken stylus tip. Another cause could be the wrong seating of the stylus, sometimes due to a defective, craked or melted, crystal transducer inside the pick-up. Try to gently slide the stylus on a fingertip. If you have an acceptable content of bass sounds, probably you just need a new stylus.
Emilio
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Locked Shaft Fixed
Thanks for the tip about the locked shaft and applying heat. It worked after about 15 minutes of heat application. Now I'm waiting for it to cool down so that I can work the shaft a little more, then start with the turpentine to clean out the tar.
cheers,
craig.
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