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Old 19th June 2002, 08:38 PM   #1
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Question Repairing SAE 2200

Hi all,

I have read many interesting threads while lurking here, but this is my first post. I am thinking of repairing an SAE 2200 amp that I bought recently, and I'd like to know if anyone is willing to check my diagnostics and provide pointers. My eventual goal is to use the case and power supply to build a "Leach" amp, but I thought I might try repairing the output stage first for the experience. My poking and prodding has yielded the following:

On one channel, the 2 output transistors and previous 2 (VA stage?) transistors are dead shorts, and the 62ohm resistor is open (yellow).

On the other channel (working, I was told, but I have not checked), one of the 10ohm resistors is open (blue).

The other 10 and 62ohm resistors (in purple if not otherwise colored) are looking rather scorched, but test fine. The 200pF capacitors nearby are scorched as well, apparently from being close to the resistors.

My questions are: Does this appear to be a typical "blown" output stage from some user abuse? If so, I might expect it to not blow again after repair. Are there any other parts I should look closely at?

I plan to replace to 4 transistors, and all the suspicious resistors and capacitors.

Finally, At wardsweb.com I found a part conversion table. For SAE 8 and 9, it lists easily obtainable motorola parts, but lists one NPN and one PNP for each type. I assume I should follow the schematic and get a PNP for SAE8 and NPN for SAE9.

I have not found the motorola replacement for the other transistors (listed as MJW21192/91), but I did find the set MJF122/27, which seem to have similar specs (i.e. 100v, 5A TO220). Would there be any pitfalls to using these parts?

I did not pay very much for the amp, so I'm willing to experiment. I'd rather not blow too many output transistors, though, if I can help it!

Thanks very much for any assistance.

Cheers, Dan
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Old 19th June 2002, 09:04 PM   #2
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I should replace all the coloured parts, and check the other transistors of the prestage (isn't shown on the picture).

If you have a multimeter with a diode tester you have to measure a diode between the base and the emitter, and between the collector and the base, if your multimeter detects a short circuit (beep) or your multimeter cannot detect a diode the transistor must be replaced.

Try if the other channel still works, it's always easier to repair something if you can compare with a working example ;-)

best regards,

HB.
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Old 19th June 2002, 09:24 PM   #3
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HB,

Thanks for that diagram. I think I knew about the "diode" test for transistoers before, but I could not think of it and was at a loss last night except to check for shorts. I'll look at the input transistors tonight.

Cheers, Dan
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Old 19th June 2002, 09:42 PM   #4
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Your Q17 should be an NPN and Q19 should be
a PNP. I believe your schematic has an error in
this regard.
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Old 19th June 2002, 09:44 PM   #5
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thanks Nelson I didn't noticed that
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Old 19th June 2002, 10:11 PM   #6
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Default Doh!

Thanks Mr. Pass,

I was wondering about that last night as well - they don't work that way, do they!

I double checked the parts list, and the parts are identified as you said, opposite of what is drawn and written on the schematic.

It's interesting, because apparently this schematic is official SAE work. I downloaded it from wardsweb.com as well.

Cheers, Dan
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Old 19th June 2002, 10:18 PM   #7
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Just in case anyone is interested,

I was looking at my colored schematic, and I just noticed that all of the bad parts form a circuit from +V to -V, so when one transistor went, it must have sent an awful surge through that circuit, burning all the parts until one of them failed open. Probably all the experienced amp folks knew this all along.

This makes me think that the 0.2ohm resistors in the output stage are also fried. I was hoping not to get into that board because it is mounted upside down to the heat sink using the soldered output transistors. I'll have to remove them all...

I'm having a lot of fun with this and I haven't even replaced one resistor yet!

CHeers, Dan
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Old 20th June 2002, 07:25 AM   #8
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yep, you're right!

when one output transistor is short-circuited when the other starts to conduct the fireworks begin

just another tip: look at the solderside of the pcb and search for brown coloured zones; this way you can find out which components went very hot and are probably toasted anyway.

HB.
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Old 20th June 2002, 06:18 PM   #9
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I would replace all the outputs, drivers, and the bias
devices.
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Old 20th June 2002, 07:21 PM   #10
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Default Cool

So I'll be replacing all the components colored in green in the blown channel, and looking closely at the transistors in yellow. If there's a problem there, I'll also look at their associated passive components. It appears that in that stage of the amp, the transistors are pretty well protected from excess current, but on the other hand, they are pretty small devices too.

Thanks for all your help,

Dan
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