[ARC5] A hardly relaxing oscillator

[bcarling at cfl.rr.com]

Hi Mike,

I wonder why there is a neon bulb on the transformer? Is it a pilot light? Or is it intended to 
oscillate?

Self-oscillation and/or motorboating are not uncommon in audio amplifiers.
Both very undesirable. I would change out the coupling caps and also
check your resistor values. Sometimes they can change a lot, going higher than the original 
value.

There are ways to isolate this to a particular stage. Temporarily connecting a .01 uF at 600V 
cap to ground at various different places may help isolate it. Start at the volume control. Then 
work forward.

Bry Carling, AF4K

On 2 Feb 2015 at 7:24, mike Kana wrote:

> Hi all - Im doing some referb on a modified reciever (12V filaments) - when spinning up
> the 12V (!) dynamotor that came with the radio, the neon bulb on the audio transformer
> lights up and the audio has a loud oscillation - reminds me of the old 'relaxation oscillators'.
> Im going to do a full recap and I have isolated the issue to the audio section.  I figure the
> new caps might fix the issue but has anyone run into this before?
> 73
> Mike
> AA9IL
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