Counterpoint SA-220 problem...

[Pars]

My brother's SA-220 started smoking the other day and one channel quit working. I am taking a look at it for him, but don't have schematics for this.

U3 (AD42020-1) and U4 (AD736JN) both split in half. The portion of the board above these has smoke damage from this. VR1 and VR2 trimpots are both charred and the board below them is also damaged, though I haven't taken it apart to inspect how much damage there is.

I figured before I did much on this, that I would try to determine whether the output FETs are still good or not. I don't measure any shorts, but they also do not ohm out the same as the good channel.

Any suggestions? My brother would like this fixed if possible, but not looking real good right now.

If anyone has schematics, I would appreciate a copy also. I do have the SA100 schematic that was floating around, but this problem at least so far seems to have started in the thermal tracking portion of the board which isn't on there. Thanks for any help, and I can post pics if they would help.

[Pars]

Bump.

Chris (anatek)? You still around?

Any hints on determining if the FETs are still good? I'll have to find the datasheets for these to determine the pinout. If the FETs are good, I would also guess I will need a new PCB, so if anyone has a line on one shoot me a PM or email. I can't find a datasheet (or anything) on Analog Devices site for the AD42020-1 also. I saw someone had a couple for sale here about a year ago or so. Not sure if you can just disable the thermal tracking portion of the board or not. Schematics would again help. Thanks for any help.

[jackand08]

Hi there!
The IC's that you said are blown are true RMS-DC converters so are probably used in somekind of protection circuit along with a relay or maybe a VU meter (if there is one on the front panel).

Checking the output FET's would be a good idea (a quick google provides plenty of guides) because if the protection circuit or VU meter IC's have blown up as you described, then it must have been a pretty catastrophic failure.

It may also help if your brother could descibe the kind of failure that he experienced. When it started smoking, did the speaker cone move in any strange ways, did anything actually explode or did it just smoke? This might help us determine the problem further.

Others seem to point towards the AltaVista audio site to find schematics but at $75 a pop I wouldn't bother at all.

Hopefully some more people will see this thread and provide some more answers.

[Pars]

His wife was home at the time, and was going to listen to something so turned the amp on. He usually leaves it on all the time (I've warned him about these before) but noticed a funny smell the night before so had turned it off. Apparently it just started smoking; she thought it was the preamp, so I'm not sure if she turned the amp off or not.

It looks like I will probably have to pull the output devices to check them; it they are toast, most likely game over on this amp. Ohming them out in circuit isn't telling me much other than they aren't shorted. I'm not sure I'm up to designing a replacement output section using the Exutech (???) lateral FETs or something else for this. AltaVista audio is up to $400 or so for replacement pulls, and it would likely need an entire output PCB as well. The $2k or whatever he is charging for the "Basic" upgrade isn't an option either.

[jackand08]

Strange, the protection must have kicked in properly (because there were no reports of abnormal speaker movement) but then because of the extent of the failure, the actual protection IC must have blown too. This will probably have blown the fuse and turned the affected section off.

For the protection IC to have blown like this, the FET's were most likely shorted and supplying the power rails to the protection circuit. Replacing the FET's would be a pig of a job too, firstly because the originals are so hard to find and also the fact that they have to be closely matched.

Short from sending it off to AltaVista and paying some serious money for them to repair it, or to buy one of their diagrams (still mega $$$), I don't think there is much else that can be done apart from maybe trying to contact someone on the forum who knows more about this gear.

[Pars]

Thanks. From reading previous threads, the protection circuitry has been described as an analog computer which attempts to sense the heatsink temperature. There are protection relays (input signal perhaps?) as well as rail fuses on each of the two channel output boards. I'm still not convinced the output FETs are blown, as I would expect to see some shorts. If they are, then FET replacement with the same type would be out of the question. The speakers were Apogees, so hopefully that side was not damaged. I think my brother has spare ribbons however.

[jackand08]

If the amp had the AD736 IC inside though, surely it must have some kind of output protection from DC as well. A true RMS-DC converter probably wouldn't have a place in just sensing heatsink temperature and would instead be used to monitor the output instead (oscillation, DC maybe?). The analog computer that you talk about may be a simple microcontroller that reads the DC output of the converter and acts on this as an input. Depending on their size, the protection relays could be for the inputs (startup/mute??) if they were small but if they are bigger, higher amperage and voltage units, then they are most likely for the outputs (which would make more sense). The protection circuit may also incorporate some kind of heatsink temperature sensing to turn the amp off if it overheats too.

As there is no schematic freely available for the amp, I am making quite a few assumptions here. It may help to get the problem solved though.

[Pars]

I'm hoping that Chris (anatek) weighs in here as he used to own/run a Counterpoint service center. I pm'd him, but know he has had health issues the last couple of years or so.

The relays are small signal type, so input would be my guess. The SA100 schematic (not official) floating around also shows input relays (from the tube section to the SS output stage).

[quinnling]

Pars,

I have a blown SA20 with one bad channel. I planning to try this mod/repair from Denis Vilfort: Hafler DH-200/220 Mods

I'll use his output stage with an Aikido configure with a White Cathode Follower.

You can have my SA20 Board and what I believe is the good channel's mosfet.

I'm not sure what is the difference in the circuit board between the SA220 and the SA20.

Let me know if you are interested
QuinnLing

[Pars]

^ Thanks for the offer. I'll have to check to see if an SA20 board would work or not.