Counterpoint Amp Repair

[Johnloudb]

My Counterpoint SA-100 broke. It started having problems shortly after I replaced the tubes in it, with Electro-Harmonix 6922 tubes. The left channel would make distortion and later the right channel too, especially at low frequencies. I just let it run like that for a year since this was just used for TV speakers. But, then things got worse and the left channel quit working altogether.

I see the gate stopper resistors are blown on the N-Channel devices. They smolder and smoke when turned on. So, the output transistors are toast as well.





I don't know what blew the transistors though, unless it happened when a tube died? Maybe a large transient blew the gates? But the gates are protected with diodes. It looks like the tubes are capacitively coupled to the gates with Wima 1uF caps. Those might need to be replaced as well.

Sooo ... I'd like to find a schematic for this, if it's out there. It would help, though I think I can do the repair without one. The left channel seems to work okay now with the new JJ 6922 tubes.

It seems to be just a matter of blown output transistors. They are the 75 watt IRF130 and I can get replacements at Mouser.

One other thing. Both channels have been running really cool (ice cold) for years. They used to run fairly hot. I didn't think MOSFETs need bias adjustment, but I guess they might after all this time. Or maybe Counterpoint adjusted the bias to run cooler after I stuffed it under a cabinet once and it over heated, and got damaged? Well, probably not.

The potentiometers for bias and DC offset are labeled, so that should be easy enough to fix. Got to these babies cooking again!

If I get the problem fixed I may also do some mods, but one thing at a time.

[NickS]

Would this help?

[Johnloudb]

Yes, Thank You, Thank You!!!

Okay, well the negative rail fuse is also blown. So, I'm still a little stumped as to where all current passing through the gate resistors of the N-channel FETs is going.

I guess I'll pull the output transistors out, replace the fuse, and then check the bias voltages and see if the rest of the circuit okay.

[NickS]

The only thing I see that can burn the gate resistors is maybe one or both of the coupling caps (C3 & C4 on the schematic) are shorted and put about 225v to the gates. This would have blown the snubber zeners also. I would pull out those caps and check, or just replace them. Also check voltages, without the caps in the circuit, on the tube and solid state side separately to try and isolate a problem before reinstalling them. I would also replace the bias adjust transistor and check all the power supply voltages before installing the new output transistors.

[Johnloudb]

Okay, I took your advice and looked into it. That seemed like most logical explanation, but then I think I found the problem.

Okay, well this amp is actually an SA12 that was supposedly modified to be a SA100. We never were sure because of a mix-up we had with the dealer, who said they didn't modify it cause they didn't bill us for it. But, months later he said they did, but he didn't bill us for it cause of the mix-up.

But, it never did look modified to me and this circuit board is the same board as the SA12 (an 80 watt amp).

Anyway, I turned it on and measured 157 volts across the coupling cap. And the the heater supply only puts out 257 volts.

And there is a relay which connects the gates to ground on turn on to prevent thumps and stuff. So, while it's warming up at least, for a couple minutes, there is 44 volts across the gate resistors.

When the relay turns off there should be 44 volts at the gate, and not much current should pass through the protection zeners. But I didn't wait that long to measure it cause if the resistors blow then there goes the relay and maybe power supply.

So, that's why they are smoking! They are dissipating 4.6 watts.

So, I think I'll replace some of things you mentioned anyways just to be careful, and maybe do some upgrades.

Glad the tubes and other components are likely okay. Anyway, it's good to get your help on this.

[Johnloudb]

Okay, need new output transistors!

This amp has RFM12P1C, P - channel MOSFETs which seem to be unobtainable.

So, any votes out there? This amp has no source resistors, so I may add those too.

I hope to find some with at TO-3 case so they will fit right in the socket. But that may be out. They need to be at least 75 Watt. I'll see what I can turn up.

[NickS]

Read here: http://www.altavistaaudio.com/SA_Biasing.htm

An IRF9130 or IRF9230 might work. They are available from http://www.newark.com/

[Johnloudb]

Thanks a bunch Nick! I checked out Mike Elliot's instructions, and I don't think I'll be buying 1000 IRF130's and IRF9130's, burning them in, and matching them. :B

So, I guess that means using source resistors at the output. I think I'll fix one channel also replacing the diodes and gate resistors, of course. I'll check out the relay and Vbe multiplier (2N2222 transistor) make sure those are working.

I'm glad Newark has those transistors.

That's a start ... then I'll compare both channels and if the new channel sounds good I'll do the left channel too and some other mods.

[Johnloudb]

Well, I see that the only place I could find the IRF130 wants $38.00 each! Not going to happen.

Newark has some parts left of the IRF9230 and IRF230 parts, and would work but not as well as the IRF130/IRF9130 cause they have about 3 times the on resistance, 0.6 ohms & 0.8 ohms and this amp uses no feedback on the output stage. So adding that to the 0.22 ohms emitter resistors the damping factor would be pretty bad. Mike Elliot said adding the emitter resistors also hurt sound quality.

So, I may find some other TO-220 mosfets with low on resistance and low capacitance to fix the amp.

Not sure what I will to do at this point though.

[Amphiprion]

Hi John,

Do you have a data sheet for the original transistors? I've been looking but haven't found one yet. I might be able to help find a suitable replacement.

[Johnloudb]

The Newark site has a .pdf data sheet at the bottom of the page.

For the IRF130:

http://www.newark.com/international-rectifier/irf130/n-ch-mosfet-100v-14a-to-204aa/dp/06F8119

For the IRF9130:

http://www.newark.com/international-rectifier/irf9130/p-ch-mosfet-100v-11a-to-204aa/dp/06F8453?Ntt=IRF9130

Though, I'm not sure which way I'm going on this project right now. If I think I can repair it so it sounds as good or better, without spending too much, I will probably do that. But I enjoy just thinking about different things too, and how I could maybe make an improvement to the amp.

Right now I'm thinking maybe paralleling four IRF520 and its compliment, four IRF9520, for each channel. Even with the .22 ohm source resistors on each source the output resistance would be only .12.

Plus, it would have an overall higher transconductance and just slightly more capacitance.

Yeah, I welcome any ideas really, on parts or circuit mods. I'm just kind of brain storming right now.

[LarsH]

John, did you complete your Counterpoint repair? If so, what parts did you use? I see that the Newark IRF130/IRF9130 have gone down in price. My SA 220 keeps smoking me and i would very much like to see some success on your part.

[phi70]

IRFP240 and IRFP9240 are about $3 a piece, they are TO247 but can easily be mounted on the heatsinks.
1 full order or magnitude cheaper than the mythical Exicons
Modern production, I dont see a need for matching even.
just turn the bias pot to minimum (full clockwise),
Just get a bunch of ~400ohm gate resistors, 1N4740 zeners, and 0.1-0.2ohm wirewounds for source (output) resistors and any SA12, SA100, SA20 can be revived in no time.

[Johnloudb]

Thanks Phi70, Sounds like a plan. Funny you should post now … I haven’t posted here in so long. Just started checking in recently. I really like this forum and the people here, just have had so much going on in my life.

That is a fine transistor and I have a bunch of matched ones for another project.