Help repairing RadioShack 250watt amp

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longblock454

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Joined 2004
#1
Hello all!

I was given this RadioShack 250watt stereo amp. The amp blows the missing fuse in the picture below the second the amp is powered on. I had originally thought the capacitor (one missing) was shorted, so I removed it, but that was not the cause. The power supply appears OK with the positive and negative showing right at 40v. From left to right the power transistors are as listed:

2sc5200
2sc5200
2sa1943
2sa1943

2sc5200
2sc5200
2sa1943
2sa1943



Any suggestions on what to test next? Could one of the power transistors be blown, is there any method for checking while soldered to the board?

Many thanks
Andy

amp.jpg
 
#2
It is certainly plausible that the output transistors are shorted. Assuming you have a multi-meter, check the continuity (or use the diode check) between each leg of the transistors. You won't get the correct diode voltage with the part on the board, but you can at least see if they are shorted.

Once you check the outputs, don't stop there; check the drivers and any other transistors as well. Good luck with the repair.

Regards,
David
 
#4
first check that nothing across the output
is making a shortcut

also if you can, inspect the backside of pcb, looking for something causing shortcut or looking like burnt

check the wiring for something bad

try to see if there is something looking like burnt
(black or miscoloured) by heat in the pcb
around those 8 big white power resistors
and close to output rail + the power transistors would be places to inspect

one thing is clear
there IS a shortcut somewhere
a shortcut that is in the power output stage
or something else that cause very high current to flow in output


It may be one of those 8 power transistors
that has got an internal shortcut.

There is a lot of work to see, if one or several of those 8 transistors are damaged
I guess you have to remove them and do some measuring on them
with your multimeter.
 
#5
It would be very unusual for it not to be the ones in the pair next to it. There are two transistors per rail per channel on that amp. I would personally replace them all with the MJL transistors from motorola (free if you can get samples).
 
#7
longblock454 said:
I found it! The first 2sc5200 on the very left is shorted! I removed each one till it stopped blowing fuses. Searching reviled that the 2sc5200 has a matching 2sa1943, how do I know which one of the 2sa1943s is it's pair?

Thanks again.
Click to expand...

they really are not pairs by 2 and 2 like that (2SC5200/2SA1943)

----------------------------------
for each channel, left and right, like this:
----------------------------------
there are 2 x 2SC5200 in parallel with each its own WHITE power resistor
and 2 x 2SA1943 in parallel the same way

2 transistors on the positive voltage side (NPN = 2SC5200)
and 2 on the negative voltage side (PNP = 2SA1943)

Remove the bad one.
And buy & replace with one new 2SC5200
 
#9
stay with 2SC5200, as much as you can
change to something else will probably only cause problems.

By the way,
2SC5200 and 2SA1943 are among the best Power Transistors you can find.
So, you have a very good power amplifier there.
Hope you get it to work with only replacing that one device.


What caused the 2SC5200 to become damaged is one important question :att'n:
If that whatever problem still is around, then some transistor may blow again.

But hopefully was a temporary SHORCUT across the Speaker Output terminal.
By some reason caused by the former user 😉
 
#12
you probably have one of the 2SA1943 transistors shorted also. on all the output transistors, use an ohmmeter to check between the center (collector) and the lead on the far right (emitter). they should read open. if one shows just a few ohms or less, it's shorted.
 
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