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Old 8th Dec 2020, 1:07 am   #4
DMcMahon
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Location: Worthing, West Sussex, UK.
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Hello Terry,
As Ted suggests it could well be bad capacitors but not easy to prove without various tests, which would involve working on open chassis with various potential dangerous voltages present, so not advisable unless you know what you are doing, for safety reasons.

Some people would recommend wholesale capacitor replacement but a big job unless you really know what you are doing and there is potential for creating new problems and some maybe even many could be fine anyway.

There are 2 main areas/capacitor types to check out:-

1. The rectified DC + HT voltage, should be around +285V dc with a several volts max AC mains ripple. Should be checked/monitored for the period that the volume issue develops, for the R82 this is at electrolytic capacitor C37 (there are also lower associated voltages after this). For the TK 20 the HT voltage should be around +260 volts at C31. Low HT voltage/high ripple could indicate bad reservoir/smoothing electrolytic capacitors.

2. Grid coupling capacitors, normally paper (di-electric type) in these vintage of machines. There is a sticky Thread about these in the "Components and circuits" category. Basically they should not pass DC voltage, so little if any +ve voltage should be measured at the Control Grids of the valves, particularly important at the output valve.

David
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