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Old 28th Aug 2019, 3:02 pm   #1
Michael Maurice
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Default Beau Decca odd fault.

I am restoring a Beau Decca, including the radio section.

After changing all the capacitors the radio appeared dead, however checking voltages showed nothing and scoping around showed that there was audio at the anode of V3.

Tests also confirmed that although there was no audio at the top of the volume control, a buzz could be heard if I touch it.

I decided to do a quick test, I connected a 10nF capacitor between the anode of V3 and the volume control and got sound.

There is continuity across L17, 18 & 19, C32 was also replaced as part of the recap.

I could of course leave it as it is, but I'd rather fix it properly.

Has anyone got any thoughts as to what is going on?

I've attached a circuit diagram with the 'new' capacitor shown in red.
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Old 28th Aug 2019, 3:09 pm   #2
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Default Re: Beau Decca odd fault.

V4 defective (or not heating up due to open-circuit connections, etc)? It appears that's the return from the secondary (tap) of IFT3 to chassis.
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Old 28th Aug 2019, 3:12 pm   #3
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Default Re: Beau Decca odd fault.

Its certainly heating up but if you remove it, the sound improves.
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Old 28th Aug 2019, 3:28 pm   #4
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Default Re: Beau Decca odd fault.

I've just tried another EB34 and it makes no difference, however for a split second as it warms up, you get sound.
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Old 28th Aug 2019, 3:49 pm   #5
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Default Re: Beau Decca odd fault.

Instability?...disappearing when extra C is introduced to V3's anode circuit?

C21 OC?

Just throwing ideas into the mix.

Lawrence.

Last edited by ms660; 28th Aug 2019 at 4:03 pm. Reason: another suggestion
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Old 28th Aug 2019, 6:58 pm   #6
Michael Maurice
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Default Re: Beau Decca odd fault.

I think I’ve solved it, but not sure if its reliable.

There appeared to have been a bad connection, the ground connection to the EB34’s cathodes was loose, so tightened it and other ground connections and it worked.

The difference of the scope traces was interesting, when the fault was present I got pure but distorted audio on the anode of V3, whereas when it worked, there was HF and the audio formed an envelope.
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