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Sherwood 7650 blows left ch speaker fuse

kappclark

kappclark

AK Subscriber
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So I found a freebie at the dump - Sherwood S-7650 CP in good shape -- so why at the dump ? Of course, I took it home to play with it.

Did not bother with DBT, hooked up antenna, but found no left channel using headphones ... then a bit of smoke .. unplugged .. checked 4 Amp speaker fuses .. l ch blown... replaced fuse, turned on, and immediately blown fuse in L Cha

Generally, what could this indicate ?? Blown output transistors ?? something shorted is killing the fuse in the left channel ...

Right channel is great ... when I switch from stereo FM to mono with front switch, it adds a distinct hum (not when in stereo mode)

Any ideas appreciated ... maybe a summer project for me ..
 
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I do just so I don't get eronious readings from other parts in the circuit.

Test them like you would a diode between the base and emitter.

If you happen to find one or more that are actually shorted, you will need to check its associated driver and pre driver transistors as sometimes the short backs up depending on the severity. Emitter resistors will also need to be checked.

Since your right channel works, you have good working transistors and resistors to compare them to.
 
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Thanks - looks like the 4 are not soldered, but wire-wrapped ... what is best way to 'undo' the wirewrapping ? needle nosed pliers ??
 
Haven't seen a pic of your unit, but if it has four total output transistors, you will only need to check two as it probably uses two for the left channel and two for the right channel.

Figure out which two are the good working channel and leave those alone.

I use needle nose plyers but be very careful not to break the wires right up next to the substrate of the transistor or you will have to replace them for sure.
 
Those are TO-3 type of transistors and are in sockets so there is no need to undo the wire wrap. Just remove the two philips head screws in both and they just unplug straight out.

It looks like both transistors have been replaced before so I will bet that that is the bad left channel and that one or both are bad.

Even if only one is bad, you need to replace both with identical types.

It looks like the good right channel has its original Toshiba TO-3 transistors so try and find some of those maybe in a good Motorola replacement.

Be sure to check the drivers, pre drivers, and emitter resistors in the bad left channel also.
 
Wow - you just saved me some serious time ... cannot thank you enough for pointing out I can unplug straight out ... a picture tells 1000 words..

I did find some compatible replacements available at Newark. Will remove, test the transistors .. will require new paste when reinstalling ..assuming I can use thermal paste I use for CPUs + heatsink ..

The Sherwood service manual says orig parts are 2SD427 and 2SB557. can be replaced with mj21193/mj21194 ..

HERE is AK thread on suitable replacements for the transistors... (once more, AK clears things up ! )
 
Making progress ... was able to get the first output transistor 2sb557 (PNP) removed, and tested it. This is the guide I used:
upload_2022-6-13_18-57-25.png
In diode test mode, the only tests which were different were the last 2 (Emitter to Collector and collector to emitter... I get a steady beep showing continuity -- not a good thing ??
 
I would check between the base and the emitter. The diode symbol is always between these two.

I would check the 2 stock transistors first. They should test the same.
 
THX - further testing sometime today ---

(red) base - (black) emitter = OL (no reading) ...

emitter - base = 511
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going to check the stock transistors
 
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The new ones for the left channel...surgery tomorrow
 

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Be sure to check the emitter resistors in that channel. If one or both are open, it might not work properly or damage the new outputs.
 
Tested new transistors -- all readings correct .. new outputs installed with new mica and silicon heat paste. checked emitter resistors for L.CH = both measure 0.5..

Unfortunately, after re-connecting, the left ch is still out ... 4 A fuse blew imediately when power switch was thrown ...right channel strong -- tone controls work etc .. I guess I need to look further upstream in the audio output circuit..
 
Better pull those new outputs and check them again because since the fuse blew, one may have shorted due to something else not being right in the circuit.

You really should have a dim bulb tester when replacing outputs just in case something happens and it did.
 
I do have dbt, and will use on next power up
==============
** I hooked up dbt with 100 W bulb --- initial brightness, then steady dim ...

Then , I replaced the 4A blown fuse ... hooked up via dbt, and steady bright bulb ...
 
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