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Old 13th March 2011, 05:12 PM   #1
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Default swapped TIP35/36C overheating/blowing fuse immediately

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...)

-runnamukk
Hollywood, MD
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File Type: pdf QM4 - Power Amp Schematic.pdf (58.4 KB, 109 views)
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Old 13th March 2011, 06:03 PM   #2
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Have you got a shorted jack socket at the output? TIPS are only current limited, could get hot if long-term short present.

Perhaps bias transistor Q4 is dead or open circuit in the base.

John
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Old 13th March 2011, 06:17 PM   #3
lineup is offline lineup  Sweden
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maybe you need base resistors for TIP35/36
10 ohm close to base pin
or
it maybe you should reduce R11 to get lower idle
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Old 13th March 2011, 06:27 PM   #4
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Hi,
Check the voltage at the bases of Q7and Q8. It will tell you if they should be on or off. When do the check short the input to the amplifier.
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Old 13th March 2011, 07:08 PM   #5
infinia is offline infinia  United States
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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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Old 13th March 2011, 08:00 PM   #6
Mooly is offline Mooly  United Kingdom
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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.
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Old 14th March 2011, 12:14 AM   #7
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If you have changed the output transistors then you need to rebias the amp.
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Old 14th March 2011, 12:23 AM   #8
sregor is offline sregor  United States
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Quote:
Originally Posted by nigelwright7557 View Post
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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Old 14th March 2011, 08:47 AM   #9
lineup is offline lineup  Sweden
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Quote:
Originally Posted by nigelwright7557 View Post
If you have changed the output transistors then you need to rebias the amp.
I think this is the most probable.
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Old 14th March 2011, 08:05 PM   #10
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All good adivce. Particularly check all the previous transistors. Probably a driver has gone short too.

You should be able to check them in situ with an analogue meter or a digital one with a diode test position.

John
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