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CR-2020 Left channel is gone, fuses popped, some troubleshooting, where to next?

hertzdonut

hertzdonut

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My 2020 has been working fine, no issues as a bedroom receiver. (Light use). Background info: All service bulletins were completed, as well as other repair work to bring this unit back to life, about a year ago. (With help from multiple contributors on AK!)

During recent use, I heard a couple of pops, and the left channel went silent. Swapped speaker wire to confirm it was the receiver, and it was. Right channel is fine, relay still clicked upon power up. Just no left channel.

I opened it up, and the left board of the pair of the Main Circuit board 1 had the two blown fuses. I replaced those (5A 250v) and applied power. The new ones gave a little spark, and that's all she wrote for them.

Having a hunch it might be the left channel outputs, I pulled them both (they were originals, 424 and 554). The 424 was crap, but the 554 tested fine. (Six way diode test).

I ordered replacements, two sets of the 21193 and 21194 (just in case) (Mouser #'s 863-MJ21193G and 863-MJ21194G).

I installed the 21193, reinstalled the 554, put in another set of fuses, and gave it try. Sparks, and another set of fuses gone.

I pulled the outputs again, my new output (21193) tested fine, but it appears the 554 is gone. Well, glad I ordered what I did.

Then, from the same board, I pulled and tested TR611 and TR612. They both tested good.

And that's where I'm at right now. Not sure what's the next component I should be looking at. Or measure. I intend to put this on my DBT when appropriate, (no sense blowing more fuses!) but measurements can't be taken while the unit is on a DBT, can they? And I still have the service manual, of course.

Thanks in advance for any help!

- Jeff
 
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Outputs should always be replaced as complementary pairs. If one output transistor craps out. Its compliment probably took a hit as well. Hopefully, the new 21193 was beefy enough to withstand the shock of the 554 crapping out.
 
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Yes, it tested good when I pulled it out after the second round of fuse popping.

Unless you have a different suggestion, I could put the unit on my DBT, after putting in the new outputs and new fuses and see if the bulb dims or not.
 
I found the real culprit...somewhere I read that the replacement for the 424 was 21193. Well, that's what my brain absorbed. :dunno:

Yes, I know now, that's backwards. The 21194 is for the 424 and the 21193 is for the 554. No wonder the fuses popped.

Anyway, I dropped the 21194 in the TR613 slot, the 21193 in TR614 (correctly orienting the base/emitters) and put the unit on the DBT. Went from bright to dim. Nice.

Removed the DBT, plugged in my speakers, fired it back up, and beautiful sound is now pouring out of both channels.

Thanks for you help, avionic!

- Jeff
 
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