swapped TIP35/36C overheating/blowing fuse immediately

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Hi all,

I'm trying to revive an old AudioChoice QM4 PA amp. I've replaced the final outs with a TIP35/36C pair, but it keeps blowing fuses. The 35/36 are both ridiculously hot, just from turning it on. I'll try to attach the schematic. It shows the 35/36 and TIP29/30 pair already matched, but that's not the way this one was. It had C5198/A1941s in it before. The c4793/A1837 pair that drives them are still in there. Do I need to update those to 29/30s, or...what am I looking for?

(Trying to flashback to my high school/military electronics...:dice:)

-runnamukk
Hollywood, MD
 

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No shut it down. Yer looking for more blown devices, usually it's prudent to replace the stage/s preceeding where the last damaged transistor was found even if they test good. Use an ohm meter to check 2 junctions for each transistor again. You probably didn't find all the bad parts before you started testing. Q5 and Q6 are likely suspects here.
 
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First step... fit a light bulb tester in the AC supply to stop damage.

Apply a shorting link across Q4 from C-E to remove bias from output stage. Check and confirm R25 and R26 are OK.
If the outputs have failed there is a fair chance the drivers have too.

Now switch on and the bulb should not be lit. If it is there is a fault.

The outcome of these tests determine what steps to take next.
 
If you have changed the output transistors then you need to rebias the amp.

Problem - the amp has no bias control (see schematic post 1). Maye changing R12 to resistor plus potnetiometer (thinking 220 plus 200 ohm pot) YMMV
Need to measure the bias if this is the problem.
Should also put a scope on the output to see if HF oscillation.
Just looking at the resistor values, would guess slightly under biased unless something else is wrong. Just my 2 cents.
 
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