AB international 9620 amp with blown channel

[xplod1236]

I have acquired a schematic for this amplifier, but I don't want to post it here due to copyright. I was wondering if anyone would be willing to shoot me an email, take a look at the schematic, and suggest which other components (besides the obviously fried ones) to check/replace. I don't want to put in ~$80 worth of transistors (what I found so far) only to have them fry again. My email is [my username] @gmail.com. Thanks in advance.

[nigelwright7557]

Originally posted by xplod1236
I have acquired a schematic for this amplifier, but I don't want to post it here due to copyright. I was wondering if anyone would be willing to shoot me an email, take a look at the schematic, and suggest which other components (besides the obviously fried ones) to check/replace. I don't want to put in ~$80 worth of transistors (what I found so far) only to have them fry again. My email is [my username] @gmail.com. Thanks in advance.

When I build my amps I always fire them up without the output transistors connected.
I connect the output back into the LTP and then fire it up and check voltages. I always check the bias voltage is set to a minimum at this point.
Only if this is OK do I consider reconnecting the output transistors.

If it fires up all and well, I then set the bias using a sig gen and a scope.

[megajocke]

That is a good tip. If the driver transistors have emitter resistors going to output (assuming a darlington type output here) then just leaving out output transistors is a good test. A series lightbulb in line with the mains is also a good idea.

Are the outputs blown on this amp? How many and which type? $80 in transistors sounds like a lot even for a big PA amp.

[nigelwright7557]

Originally posted by megajocke
That is a good tip. If the driver transistors have emitter resistors going to output (assuming a darlington type output here) then just leaving out output transistors is a good test. A series lightbulb in line with the mains is also a good idea.

Are the outputs blown on this amp? How many and which type? $80 in transistors sounds like a lot even for a big PA amp.

I usually find if the output transistors are blown then they go short across collector-emitter or drain-source.

A lamp across the input is a good idea.

I sometimes put in lower value fuses in the hope they will go before the output transistors. This is a juggling act, as too low a fuse and the switch on surge will blow them and too high and the output transistor die before the fuse blows.

[xplod1236]

One out of 6 (in parallel) MJ21194 output transistors is shorted. I will be replacing all 6. There are also 5 MJL21193s and 5 MJL21194s, which look to be in series with the outputs, which are completely blown up (melted legs, holes in them). Each one of those is $4-5 at digikey, so the total comes out to ~80, with a few smaller parts which I didn't mention.

[nigelwright7557]

Originally posted by xplod1236
One out of 6 (in parallel) MJ21194 output transistors is shorted. I will be replacing all 6. There are also 5 MJL21193s and 5 MJL21194s, which look to be in series with the outputs, which are completely blown up (melted legs, holes in them). Each one of those is $4-5 at digikey, so the total comes out to ~80, with a few smaller parts which I didn't mention.

Thats quite a lot of money, that is why I use cheap MOSFETS in my designs. £2 each.

I would also have a dig around to see what might have caused all this damage.

Has your speaker fried ?
Is there a short in the speaker cable ?

I would seriously take my earlier advice and dont switch on first with all the output transistors connected.

Jus ttake short careful steps and things should be OK.

[nigelwright7557]

Originally posted by nigelwright7557


Thats quite a lot of money, that is why I use cheap MOSFETS in my designs. £2 each.

I would also have a dig around to see what might have caused all this damage.

Has your speaker fried ?
Is there a short in the speaker cable ?

I would seriously take my earlier advice and dont switch on first with all the output transistors connected.

Jus ttake short careful steps and things should be OK.

Another trick I sometimes try is after checking the driver board voltages is to not connect all the output transistors but just one set on each phase. Then if you do blow them up its not a full set !

[megajocke]

Have you seen that the MJ(L)21195/MJ(L)21196 are a little bit cheaper? They are a slightly newer, slightly better performance part. Almost identical, I mailed Onsemi once on the difference. I think it was that the 95/96 has a bit better gain linearity due to a new process.

Does it use both MJs and MJLs? Melted holes and stuff? I suggest you check the board for vaporized traces then too :P Driver transistors, emitter resistors, base resistors and so on. VI limiter circuit resistors, transistors etc. if used.

These use some kind of Class G? Output transistors in series with (a) diode(s) supplying middle rail to the junction? Commutation diodes could be bad.

edit: found it. Onsemi technical support said:
"The MJL21195 is considered a new generation of Perforated Emitter designs, better gain linearity and VBE(on) distribution but other than that parts are very similar. " [to MJL21193]

[Eva]

Judging by the amount of power transistors, this is either a class G or class H amplifier and it's a bit harder to repair than common ones, particularly considering that all the rail switches (MJLs) seem to be destroyed.

I'm curious about the schematic of this newer model as I only have experience with the older AB 1100A (which uses MJ1502x stuff and +100/+50V/-50V/-100V rails and allows the rails to drop to 0V). Could you send it to me? evapowah at hotmail dot com

[djk]

"cheap MOSFETS in my designs. £2 each."

£2 = $4, looks like the same price to me, except they won't handle the power that the TO-3 cased MJs will.

In any event, this is a repair, not a new build. Kind of hard to make those £2 MOSFETS fit in (pin-outs are different if nothing else).

Knowing the general rail switching design AB uses (including a 0V rail switch), I wouldn't try to fire it up without the outputs. Use a lightbulb in series with one side of the line cord, 75W~100W should be fine for 120V.

This amp will do 4KW bridged, work safe!