More Fisher 500 power supply chicanery...

[GordonW]

Word to the wise- if you get a Fisher 500 (this one was a 'C', BTW) that sounds "wimpy", and burns up the 270 to 470 ohm (depending on revision) 2 watt resistor from the center of the voltage doubler caps (connected to the junction between the two caps in the doubler) to the power supply cap running the front-end circuitry, it's the bottom cap in the voltage doubler, gone open-circuit.

It's a real puzzle... until you start measuring *AC* voltages. DC looked right (around 4 volts DC across a 270 ohm resistor, which ostensibly is only like 60 milliwatts or so)... but NOT SO, if you measure AC voltage across the resistor. IIRC, I was getting like 25 VOLTS AC... probably like 2.5+ watts CONTINUOUS across a 2 watt resistor, at idle- and even more when you cranked up the volume!

Tacked in a 220uf 250v cap across the bottom voltage doubler cap... the ripple went down to less than 2 volts AC... DC voltage went up to about 6V. NOW everything worked. I bit the bullet, and replaced BOTH doubler caps (220uf 250v caps on a little tag board under where the "top" voltage doubler cap used to go)... Ahhhh... NOW THAT'S the way it's supposed to sound! :thmbsp:

So, when people recommend replacing those voltage doubler caps... it's REALLY good advice!!

Regards,
Gordon.

 

[sloober]

Best approach with these 500c-800c amps is to just replace all the caps including the power supply caps. My first 800c I used the SDS board. Second one I did with individual caps under the chassis. I STRONGLY recommend the SDS option. Much easier to figure it all out. John