I realize this was long ago, But I'm also considering using a white follower as the buffer stage in my SA-220. On paper it appears to be a good compromise. I do have concerns about the fragile gates of those irreplaceable Mosfets (uncontrolled oscillation, undamped transients, etc) Did you end up doing this, or know of anyone how has? Or for that matter any other mods?
hello
I am reviving this thread, not common to have a SA-220 on the workbench...
this amp has one channel blown (at least, visually) all output transistors are shorted, and so are the gate resistors (on the + side). The transistors are
RFM10N15 and IRF9241, and wonder if this is correct.
There is also a resistor blown in the main board near the led wires, R140
I checked the regulators and the MJ340's and they are OK
no user manual, and of course no schematic...
any hint will be appreciated
regards
Alexander
I am reviving this thread, not common to have a SA-220 on the workbench...
this amp has one channel blown (at least, visually) all output transistors are shorted, and so are the gate resistors (on the + side). The transistors are
RFM10N15 and IRF9241, and wonder if this is correct.
There is also a resistor blown in the main board near the led wires, R140
I checked the regulators and the MJ340's and they are OK
no user manual, and of course no schematic...
any hint will be appreciated
regards
Alexander
Not sure if anyone will respond or not. We wound up abandoning fixing the SA220 because I fried the transformer inadvertently. My brother moved on to a Class D amp that he put together, and honestly likes it better than the Counterpoint on his Apogees.
Your transistor numbers appear to be correct. The problem with these amps is that with no source (or is it drain?) resistors, the MOSFETs had to matched incredibly tightly. Couple that with the fact that it is difficult to get them, let alone suitable quantities to match that closely, and you are kind of between a rock and a hard place, so to speak.
As far as I know, there are no replacement output stages since Mike Elliot is no longer doing business as Alta Vista.
Your transistor numbers appear to be correct. The problem with these amps is that with no source (or is it drain?) resistors, the MOSFETs had to matched incredibly tightly. Couple that with the fact that it is difficult to get them, let alone suitable quantities to match that closely, and you are kind of between a rock and a hard place, so to speak.
As far as I know, there are no replacement output stages since Mike Elliot is no longer doing business as Alta Vista.
A few thoughts...
1. While Alta-Vista Audio is closed, he does list a way to obtain a service manual for $50 which includes a schematic.
2. While the FETs are not available from AVA or on the open market, GreenStreet Audio is offering a FET-replacement/upgrade for the SA-12/100 series. While his upgrade design would be a LOT easier to apply to a SA-20, it may be possible to do apply it to an SA-220, especially after getting the service manual. I expect he could supply the FETs he uses and technical expertise, for a price.
Greg in Mississippi
P.S. Pars, bummer you didn't post anything about killing the transformer, I have a spare SA-20 transformer here (after doing the AVA Plitron upgrade). If you still have the carcass & any interest to re-kindle that project, contact me via PM.
1. While Alta-Vista Audio is closed, he does list a way to obtain a service manual for $50 which includes a schematic.
2. While the FETs are not available from AVA or on the open market, GreenStreet Audio is offering a FET-replacement/upgrade for the SA-12/100 series. While his upgrade design would be a LOT easier to apply to a SA-20, it may be possible to do apply it to an SA-220, especially after getting the service manual. I expect he could supply the FETs he uses and technical expertise, for a price.
Greg in Mississippi
P.S. Pars, bummer you didn't post anything about killing the transformer, I have a spare SA-20 transformer here (after doing the AVA Plitron upgrade). If you still have the carcass & any interest to re-kindle that project, contact me via PM.
Greg,
Thanks for the offer, but my brother sold the carcass (before Alta Vista closed). This was probably the least enjoyable audio repair/DIY project I have done, as this amp is a pain in the *** to work on.
Greenstreet is using Exicon devices in a TO-247 package. The Exicon devices are not pin compatible with the SA220 MOSFETs, and you still have the extremely tight matching to consider, (unless you were to use source resistors, which was the method I had nearly working until the tranformer failed.)
I think I had emailed them back when I was helping my brother, asking if they were considering offering anything for SA20/220, and they said no.
Thanks for the offer, but my brother sold the carcass (before Alta Vista closed). This was probably the least enjoyable audio repair/DIY project I have done, as this amp is a pain in the *** to work on.
Greenstreet is using Exicon devices in a TO-247 package. The Exicon devices are not pin compatible with the SA220 MOSFETs, and you still have the extremely tight matching to consider, (unless you were to use source resistors, which was the method I had nearly working until the tranformer failed.)
I think I had emailed them back when I was helping my brother, asking if they were considering offering anything for SA20/220, and they said no.
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Finding and selecting matched output MOSFETs to the required match is not practical.
It is possible to use unmatched MOSFETs and add source resistors, which is what everyone else except Counterpoint did/does on their amps. The penalty is an amplifier output impedance which is higher (because the global feedback network does not include the output stage) but still lower than many amps on sale.
And, even then the available TO204 package MOSFETs are still very limited and expensive.
Or replace the output stage with something else completely -do both channels at the same time and sell off the one functioning set of MOSFETS to someone.
Or gut the amp, and replace the circuitry with something else that is happy to run off +/-70V rails.
5W. You want to be able to handle 1/4 of the power rail fuse current.
And low inductive. Think parallel metal films
I'm irritated as when Mike closed shop, he had a good 50+ of the output boards for the SA-220, just lying about. From all those conversions he did, to the newer design.
I accidentally damaged one of my 220 output boards, and needed a new one. That's how I found out he had them. dirt cheap too, $25 each. Fully functional.
I waited too long. He closed up and those boards went away. I wish I knew where they went...
I did indeed buy the schematic, for the 20/220 and the 200/400.
I accidentally damaged one of my 220 output boards, and needed a new one. That's how I found out he had them. dirt cheap too, $25 each. Fully functional.
I waited too long. He closed up and those boards went away. I wish I knew where they went...
I did indeed buy the schematic, for the 20/220 and the 200/400.
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It's just a normally closed thermal switch, I can't remember the temperature, but 80°C isn't uncommon. They may have used a higher temperature. I have never had to replace one except in some other pieces of equipment when people destroy them. The temperature should be marked on the case and may be Fahrenheit or Celsius. It doesn't carry a high current.
The thermal cutoff was mounted on the heatsinks - one per side, at the bottom, below the output transistors. I am pretty sure the temperature was lower than Chris's 80°C. If you aren't replacing an original one with the markings on it, I can probably dig deep enough into the archives to find a part number