loewe-opta: 4054W; Rheingold, Restoring problems.

Respected Members,

The 40K, two watts resistor in the ECH81 pin 1 was burned.  I replaced it and the voltaje in pin 1 is of 69 volts instead of 23 volt that indicates the original circuit.  The resistor I replaced it for one of 5 watts and also is heated a lot.  The voltajes in the power main are normal in 240 volts.  The ECH81 tube was tested and is in good conditions. 
I thank some aid or suggestions. 

respectfully, Bruno.

 

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Dear Bruno,

any signals coming through at all? - what about voltage on pin 6 (anode) of ECH81 - 190v?    Measure actual volts drop across 40k.
 Good luck,
                  John.

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Dear John,

Thank you so much for your interest in my question. 
The radio is functioning well in all the bands.  The problem is that in the ECH81 pin 1 the voltaje is three times the normal and the 40K resistor works to very high temperature. 
On pin 6 (anode) the voltage is 203, and on the pin 1 40K 2W resistor drop voltage is 240 to 69 instead 240 to 23 according the original circuit.

This problem is a true puzzle for me. 
Thanks again and best regards, Bruno.

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Dear Bruno,

that sounds "spot on" to me, - the normal screen volts for an ECH81 with no cathode resistor and 40k. screen res. is usually around 70v. and I think I'm right in saying this remains fairly constant.
    I wonder if the 23v. is a result of measurement with a low resistance voltmeter? (it says something about 333 ohms/v. on the diagram! - unfortunately I don't read German too well).
    According to your figures, the 40k. is dissipating less than a Watt, so I wouldn't worry. (you might just check the 5nF. decoupler).

Good listening,
                         John.

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Very rarely have I seen this resistor fail in my German radios, but I do make an effort to change all the paper capacitors and electrolytics!

This resistor does get hot, it is a power resistor, and if you want to play it extra safe, go with a ten watt power resistor!

The 40K power resistors are a bit of a challenge to find these days.

I believe John is on the mark for 70volts, but when you change the paper capacitors check the voltage and report back.

If the ECH81 oscillates keep it,  the esoteric discusions of vacuum tube makes is not that important as making the radio safe and operational with new capacitors.

Omer


P.S. Did you adjust your Grundig to get your favorite station?

 

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Dear Jhon,

I am according to you in the normal screen volts for the ECH81.  I have revised other similar circuits and the voltage is under 100.  May be 23 volt value can correspond to an error in the diagram. 
With certain frequency I have found errors in the original circuits of other models. 

When I replaced the burned 40 K original resistor also I replaced the 5nF decoupler capacitor.  I am worried about the high work temperature for the 5 watt 40 K new resistor.  With few working hours already is being overshadowed.
 
Never I have seen a 5 watt screen power resistor in the ECH81! 

Dear Omer,

I expect to be able to expand the FM high end band in my Grundig 4010W the next weekend.  I will report the results. 

Thanks by the stimulating dialogue. 
Bruno.

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Bruno,

Here is some information that may reduce your concern about the resistor:
Power dissipation in a resistor can be calculated by squaring the voltage dropped across the resistor and dividing that number by the resistance.

With a screen grid voltage of 69 volts, the voltage drop across the resistor is 171 volts which results in a power dissipation of .73 watt.  (29,241 divided by 40,000 = .73)  With a screen grid voltage of 23 volts the voltage drop would be 217 volts which would result in a power dissipation of 1.18 watts.

The original resistor may have just failed with old age (common for a resistor dissipating a level of power close to its power rating) or the decoupling capacitor may have shorted.

The 5 watt resistor you are now using will probably have a life longer than the remainder of the radio.

Carbon resistors usually increase in resistance as they age, and this happens more quickly if the resistor operates at a higher temperature.  Metal film resistors are usually very stable as long as the power dissipation is no more than half the power rating of the resistor.

I hope this information helps.

Paul

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Dear Bruno,
                   I hope your "Rheingold" is working reliably, but could you please confirm whether you changed the original 40k resistor because it was o/c, gone "high", or simply because it looked so burnt?
Additionally, was the 5nF cap. shorted? (though I know you replaced it)
I've often found carbon film resistors blackened unrecognisably but still measuring o/k - I'd always replace with a larger one, though, of course.

Perhaps some expert can tell me why the very low voltages on the EF41 I.F. valves in particular - (23v. on the first 's anode, 38v. on the second's!)   I know  in an F/M only set the last I/F stage is sometimes used as a limiter, but could that be part of the reason here?

Kind regards,
                      John.

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Hello Paul,

Thank you so much for your refined answer. It contribute with supports the indication that the ECH81 screen resistor should be of 2 watts. 
Nevertheless a resistor of those characteristics burns quickly in few hours of use.  I have replaced one of of 5 watts and already is overshadows with few hours of use. 

I believe firmly that the original diagram has an error in the screen volatge of the ECH81 indicating 23 volts instead of + /- 70 volts. 

I would be very thanked to receive the opinion of an expert in this model and to confirm my opinion regarding the error in the diagram.  But the problem of warming-up of the 40K/5 watts resistor continues. 
Never I have seen another circuit that require 5 watts or more to feed the ECH81 screen. 

Respectfully,
Bruno.

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Hello Jhon,

When I did the first test in my model, called my attention the 40k/2watts burned resistor with a loose extreme and without contact.  I replaced it for one of equals characteristic and the 5 nF decoupler capacitor was leaking and was also replaced. 

The new resistor burned quickly and I replaced it, surpassing the original indication in the diagram, for one of 5 watts. 
At present the radio is working reliably but with the new 5 watts resistor very hot. 
A ghost does not permit that the resistor work more alleviated. 

The present reading in the EF41 anode is 22.1 volts according the diagram. 

thanks again,
Bruno.

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Hi Bruno,

How about if you recap that radio and check each of the resistors and their values, and see if the problem still exists!

After 50 some odd years, probably all the paper capacitors are bad and the electrolytic ones are probably questionable too.

Maybe, if you try to follow the circuit for bad components, you will find the components that are the cause of the problem.

How are the voltage readings for the other tubes?

If you cherish the old components so much, and like having the radio original, you can place them in a zip lock bag for later reference with the radio!

The main point here is that you wish to be able to use this radio.

Please get back to us, we will be waiting for your results.

Omer

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Hi Bruno,

comparing this schematics with, for instance http://www.radiomuseum.org/r/graetz_comedia.html which uses slightly the same principle, shows that the 23V may be wrong. The Graetz one has printed 70 Volts and 4.3 mA, which is no load at all. If you resistor becomes that hot there must be a problem ! What about voltages when removed the ECH81 ? Have you already checked another ECH81 in this radio set ?

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Gentlemen!

I am really curious to know what the rest of the voltages are for the other tubes!

How are the dropping resistors doing around the 50/50 mf can?

Any black power resistors in these radios should be replaced ASAP!

Why is the mA so high around the ECH81?

Omer

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Bruno,

Because you are still having a problem I have now examined the schematic for your radio and I believe there is a problem with the ECH81.

It is unfortunate that the designers chose to measure the voltages with a 333 ohms-per-volt meter because this causes the meter load to significantly lower all voltage readings that are not supplied from a low-impedance source.  In the case of this radio almost all voltage readings shown on the schematic are mostly useless except those for the power supply and some of the plate (anode) voltages.  It is this meter-loading issue that caused the incorrect 23 volt reading to be shown on the schematic.  Fortunately they have also provided current readings which allow the correct voltages to be calculated.

As a starting point please measure the power supply voltage.  Any difference from 240 volts will affect all of the other voltages in the radio.  An ideal situation would be to power the radio from a Variac and adjust the Variac to set the B+ voltage to the specification before making voltage measurements.  If a Variac is not available and the B+ voltage is not at the specified 240 volts, you can calculate the ratio between the specified voltage and the actual voltage.  Most of the voltages in the radio will be offset by this ratio.

I have calculated the proper voltages for the ECH81 using the currents shown on the schematic.  It appears that the voltages and currents not in parentheses are for FM and those in parentheses are for AM and SW.   The following voltages are calculated using the currents shown on the schematic.  The correct screen grid (pin 1) voltages are 68 volts for FM and 56 volts for AM.  The correct plate (anode) (pin 6) voltages are 202.5 volts for FM and 231 volts for AM.  Any digital meter, VTVM, or FETVM will have a high enough input impedance to measure these voltages correctly.

Please note that voltages should be measured with no station being received.  The signal strength of a received station significantly affects the voltages in the IF stages, particularly in AM, and can affect all voltages throughout the radio.  No FM reception can be obtained by disconnecting the antenna and tuning to a quiet spot.  This is more difficult in AM because there are so many static and interference sources in our modern world.  No AM reception can be accomplished by connecting a capacitor, perhaps 1nf, across the two connections to the ferrite antenna.

So now we must consider why the screen voltage on your ECH81 is low.  The first step should be to measure the other resistors connected to this tube.  These would be the 5K resistor supplying B+ to the plate (anode) (pin 6), the 1meg resistor connecting the control grid (pin 2) to the AVC bus, and the 50k grid 3 resistor (also connects to the triode control grid) (pins 7 & 9).  It would also be a good idea to measure the resistor that is unique to the triode (AM oscillator) section of this tube.  This is the 30k resistor supplying B+ to the plate (anode) (pin 8).  Replace any resistors that are out of tolerance.  I recommend metal film resistors.

Also, please verify (measure) the value of the 40k screen grid resistor that you have installed.  Even if this resistor would be connected directly from the 240 volt power supply to ground, the worst-case situation, a 40k resistor will only dissipate 1.44 watts and would draw 6.0 milliamps.

Have you tried a different ECH81 tube?  The tube in your radio may be gassy which would cause too much current flow.  This current flow would probably increase over the first 15 minutes of radio operation.  This would cause the screen and plate voltages to continue to drop during this period.  (This gassy-tube problem is rather common in output tubes, a much more serious situation because of the damage this can do to the output transformer and power supply.)  There could also be internal leakage from the screen grid to the suppressor grid (connected to ground through pin 3), but I believe that would affect radio operation.

Measure the screen grid, plate, and control grid voltages and report back what you measure.  The control grid voltage should be 0 volts in FM and should be slightly negative in AM.  This AM voltage will become more negative as an AM station is received, and could be as much as -20 volts.

Let me know what you find.

Paul

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Hello Paul,

Thanks by your interest in the restoration of this very nice radio that more than one headache has produced me.  Your  analysis is very interesting and motivating. 
The voltages were measured after 15 minutes with power supply and taking the precaution of tune not in a radio station.  You are in the correct thing: the values among parenthesis correspond to MW-SW.  The position drawn of the switches is in FM position. 

B + is 236 volts. 
The ECH81 voltages are: pin 1: 70 (85) pin 6: 200 (220) pin 8: 0 (100).

The resistors values they are all normal inside a margin of the 10%.  I have I check the pin6 5K, pin2 1M, pin 7-9 50K, and pin 8 30 K. 


The ECH81 tube is OK according to my Mercury 2000 tube tester.  It was measured for transconductance, shortcut and gas.  I replaced the tube by a TF NOS and the voltage values are the same. 

Assuming that the pin 1 23 volt value is wrong in the schema, the present values are similar to the ones that you calculated.  The problem persists in the pin1 40K resistor!  The replaced 40k, 5 watt is very hot and is beginning to blacken with few minutes of use! 

Thanks again, Bruno.

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Bruno,

Thanks for the voltage measurements. These inform me that there is not a serious problem now and your substitution of the tube eliminates a gassy tube problem.  I just want to be certain that there is not something happening "while you are not looking". There is nothing here that could explain an overheating resistor. I know this is frustrating for you to read because you say to yourself that you are looking at a resistor turning black.

For the discussion of the overheating resistor we can ignore every other part of the radio because the only factor that is important is the voltage across the resistor. With a B+ of 238 volts and a screen grid voltage of 70 volts in FM and 85 volts in AM the voltage across the resistor is 238 minus 70 which equals 168 volts for FM and 238 minus 85 volts which equals 153 volts in AM.  Since we are dealing with pure DC voltage here (there is a bupass capacitor on the screen grid which assures no AC), Ohm's law (voltage squared divided by the resistance) tells us the power dissipated in a resistor.  The dissipate power for a 40K resistor is 0.71 watt for FM and 0.59 watt for AM. It is not possible for these power levels to overheat a 5 watt resistor.  There are two possibilities with the resistor, either it is not 40K or it is not 5 watt. Have you verified that the resistor is overheating while the voltages are at these levels? Please measure the resistance of your resistor (you do not have to disconnect it since there are no other resistances in parallel) and verify that it measures 40k. What type of resistor do you have? Is it one of the tan rectangular "sand" resistors?

There is a possibility that the voltages are not staying at the levels that you measured. In your case the lower the screen grid (pin 1) voltage, the higher the power dissipation in the resistor. You could verify this by leaving your meter connected to pin 1 and monitoring the voltage over a few hours. However even if the tube would short out and the full supply voltage (238 volts) would be acress the resistor, the resistor would only be dissipating 1.42 watts which should not begin to overheat a 5 watt resistor. So the highest possible power that a 40k resistor could dissipate in this radio is 1.42 watts regardless of how or where it is connected since there is no voltage higher than 238 volts. Please measure your resistor! If, for instance it is really 4k instead of 40k than it would be dissipating 7.1 watts in FM and 5.9 watts in AM. I doubt that it is this far off because I don't think the ECH81 is capable of pulling enough screen-grid current to get the voltage readings this close to the correct readings - at least not for long before the screen grid would melt!

As a radio performance issue totally separate from the resistor issue, while the FM voltage readings on the ECH81 are essentially exact, the AM voltages are off a bit, particularly the screen grid. These AM readings indicate the tube is "turned on" a bit more (220 volts on pin 6 instead of the expected 231 volts) than the conditions of the schematic. Try taking measurements with the tuning set near the opposite end of the dial. Maybe the voltage on pins 7 and 9 is dependent on the oscillator frequency and will change the plate current (pin 6). Also, please measure the voltage on pin 2. It should be zero or slightly negative.

You can also provide pin 1, pin 2, and pin 5 voltage readings when a strong AM station is being received.  Under these conditions pin 2 voltage should be negative, pin 6 voltage should be higher than 220 volts, and pin 1 voltage may well be lower than the 85 volts you measured. These measurements might be helpful to remove any uncertainty.

Hang in there. We will get this fixed. There is no magic here!

Paul

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Hello Paul,

The 40k 5 watt "sand" resistor that I replaced first measured 39.6 k.  I replace it again for one of carbon tipe, 40k 3 watt  and now the resistor is not heated like the previous one. I believe that has its normal temperature of work. 
Never I thought that a new resistor could be heated excessively in spite of having a normal value inside the tolerance.  I think that the sand resistor was bad.
The voltages now are all inside the 15% of tolerance. 

After revising your refined comment regarding replacing the paper capacitors, I utilized for the first time a Heathkit capacitor checker IT-28, that measures capacitance and leakage, I could verify that the 100% of the paper capacitors presented leakage in voltages very lower to its performance. I replaced all the papers capacitors. 
The radio improved significantly its sound and sensibility.  I could not measure capacitance because surely the IT-28 is wrong calibrated.  May be I should acquire a modern capacitor tester.
 
Now I will restore the cabinet and when be finished, I will send some photos. 
Thanks again by your attentive aid and support. 
Bruno.

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Hi Bruno,

I'm glad to read that your radio is now working correctly!

An excellent site with a full explanation for replacing capacitors can be found at,http://www.antiqueradio.org/recap.htm.

Here, you can see which kinds need to be replaced and the ones that you can keep in the radio.

It is a given, to replace the paper capacitors in these radios!

I also would recommend checking ALL the resistors to see if they are with in spec and checking out all the solder joints to ground.

It is recommended to replace the black colored power resistors too.

P.S.  How is the Opus running?

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Hello Omer,

Thanks by your link. 
The opus 55 TS is working well.  The model have a great sound!. I have the problem of buzzing power transformer. 
The chassis rubber insulating are new but the noise transmits to the cabinet and is very unpleasant especially when tunes in low volume. 
I have a Telefunken Allegro S with the same problem. 
Seems that is a frequent problem in the TF radios. 
Bruno.

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Bruno,

I am glad your radio is now working properly.

On the buzzing transformer problem, have you tried Omer's fix using the GC red insulating varnish?  I have not needed to deal with this problem myself so I have not developed a fix, but Omer has had good success.

I believe your Heath IT-28 could be valuable for testing capacitors.  I have a similar Paco tester that I built from a kit and it works really well for testing paper capacitors.  A good point for these testers is that they actually test capacitor leakage using the rated voltage, something that the digital battery powered meters can't do.  This type capacitor testers also measure the capacitance using an ac test signal in a bridge circuit with a reference capacitor inside the tester.  With this bridge circuit type of test the capacitor value is measured under conditions more like those in a radio.  The capacitance test done by a digital multimeter is quite different and some capacitor defects can go undetected using a digital multimeter.   The accuracy of a tester like your Heath is dependent on the accuracy of the reference capacitors inside the tester, so the best way to get it as accurate as possible is to have high-accuracy capacitors for the bridge circuit inside the tester.  Heath may have already used precision capacitors, and if they were a type capacitor that is stable against changing value, unlike the capacitors I found in the German radios,  your tester may well be quite accurate.  I know that my Paco tester is still accurate enough without any repair, and it is older than your Heath.

If you want a really nice capacitor testor I recommend any of the Sencore LC series (I know they made an LC101, LC102, and LC103 and there may be other models).  I have an LC103 and it is the best way I know to test capacitors.  In addition to testing capacitance, inductance and leakage current with very high accuracy, it also measures ESR and dialectric dissipation on electrolytic capacitors.  An accurate ESR test is really helpful for finding bad filter capacitors.

Paul

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