Counterpoint SA100 one channel doesn´t work

[Orgonom]

Want to change Fet Outputstage to Mosfet (IRF 240/9240).

I found a schematic from an italien guy, who mention that it could
be changed to IRF Devices. But after reading the schematic, i saw
that the tube driving stage worked only with 1 or 2 mA.
Way to low for driving these Mosfets.

Has anybody have expirience with these Amp´s?

Maybe adding a fet follower to drive the mosfet?

It does´t seem to be a good design, or am i wrong?

Sorry for the bad english but it isn´t my native language.

Thanks, Andreas

[stokessd]

Yeah, I have experience with that amp. I used to use one to almost drive a pair of Eminent Technology LFT-VI's. The output stage melted down on me after a couple years. I sent it to counterpoint, $200 later, I got it back and within 3 minutes it melted down again (different speakers cables etc). I sent it back and they fixed it again (free) and it worked when I got it back. As soon as I realized it worked again I sold it. It was a good sounding amp, but didn't have a particularly good damping factor and couldn't control tough loads like a solid state amp can. I've got the service manual if you want it (I'd have to xerox it and mail it).

The output transistor power rails have fuses, but the FET's are a faster fuse than the fuses, and they melt, then the fuses pop. Handy huh?


If memory serves me right, the output stage used insanely tightly matched transistors due to not having much thermal compensation and no feedback to speak of.

Alta Vista Audio (http://www.altavistaaudio.com/) still repairs and rebuilds these units, although not to original form.

I really like the build quality and the sound quality of these units. I still use a modified SA-1000 pre-amp in my system.


Sheldon

[pothound]

i also have a counterpoint sa100 with a channel out. if i can find an easy way to remove the circuit board, i will change the electrolytic caps on the board. there is a heat problem with d60, in5365 36v 5w zenerdiode causing 10 mfd cap to get hot. my unit is 15 years old.
any help appreciated
thank you
pothound

[anatech]

Hi Andreas, pothound,
Now there is an amp I know extremely well.

The outputs are made by IR. You must match them extremely tightly as there is zero sharing between devices. I normally takes me 20 ~ 30 devices of each to get a matched set. Set the bias a little low and the transformer will last longer. Check the darn relay. The heater supply will have a bad first filter cap. Gate resistors and zeners are shot as well. It takes the better part of a day to get the bias set correctly.

The 10 uF caps are dead, the board is crispy. New zeners and resistors of the top as well. Bad design. If you buy a matched set from M.E. (not me), make sure they are in fact good. You are getting "pulls" from amps he has worked on. He will tell you this.

His upgrades sound worse, just know this if you send it in.

-Chris