grundig: Intermittent sudden volume changes.

I have been restoring my Grundig 2540U. After replacing all the electrolytic and paper capacitors it was working but the volume would start out loud with full closure of the magic eye then after about 5 seconds the eye would open and the volume would drop down. Occasionally, when listening to FM for a while, the volume would suddenly increase for several seconds then drop down again. 

After reviewing the schematics I realized that the PU/TR button when in the off position (not pressed) closes a switch that provides B+ to the magic eye, ECC85 and ECH81. I realized that I almost never pushed the PU/TR button since I have no aux. setup. After working the PU/TR button numerous times the intermittent volume problem has resolved. The FM, AM and SW volume is markedly improved. The magic eye closes and there is no intermittent volume change. 

My question. Is it possible that I cleaned the contacts on the PU/TR button by pushing it and exercising it? How are people cleaning the push button switches on these German radios since they are difficult to see inside of the switch box?

Thanks

Peter

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Hello Peter,

there are a lot of electrical contacts in your radios push-button unit. A lot of them were never actuated if you only use the FM-part of your radio. For example, the use of the PU-button breaks the complete power supply for the RF-tubes and the indicator tube, only the AF-part is powered when the PU-button is pressed. Another example is the triode system from the ECH81 tube - it is usually switched off if you only use FM, but is powered as the AM-oscillator if you hear AM.

So it is always a good idea to operate the "usually unused" parts of your radio occasionally, too. Therefore you repaired your Grundig while using the PU-button several times, you have cleaned the contacts doing this. Often this is enough to to, if the radio does a good job then, in other cases you can try to clean the contacts mechanically, but it is not always easy to reach their position. And please be very careful using any sprays - many of them are very aggressive and will destroy these contacts in short time, better avoid using sprays with unknown results !

Michael

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Some sprays have a delayed action where plastics or the paper-resin supports of switches disintegrate over a period of time.

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