[Amps] 425 Amp

[Jim Brown]

On 4/4/2013 3:06 AM, Andy Cook, G4PIQ wrote:
> Do check the HV filter capacitors which will are very suspect by this age of
> amp. I've found that when they fail, the overdrive light comes on with just
> a few watts of drive but not with the normal intensity - I suspect this may
> be related to the HV supply being deeply modulated with 100/120 Hz.

My experience has  been one cap in the HV string will fail open, and 
I'll get reports of a raspy CW note. That has happened to me 2-3 times, 
and the overdrive light did NOT come on. All three of my 425s came to me 
used. With one of them, I replaced one cap in the string and several 
years later it's still running fine. With one, after I replaced the 
first cap, another failed soon after, and I replaced it. With the third, 
for one reason or another I replaced the whole string. I don't remember 
why.

The overdrive light is monitoring grid current, and is designed to come 
on when it's greater than 120mA. The schematic on my website may help 
your troubleshooting efforts.

http://audiosystemsgroup.com/publish.htm

73, Jim K9YC