Help with CR-640

This one came to me in rough shape. Very dirty, missing F803, rectifier diodes popped. I replaced all the caps in the power supply and main amp, as well as the bad diodes. I now have voltage where before I did not (e.g. dial lamps) and the tuner seems to be working (meter jumps when turning dial). However, I'm still in protect mode, so no sound. I'm hoping you guys can provide a clue. I've taken voltages on each of the pre- and power-drive IC's in hopes that someone can tell me what to do next.

I have replacements for all 4 ICs on hand, though I can't be sure they are all good (sourced from ebay & utsource). I smell a wumpus.

I don't see any "-" signs on any voltages?

Making reasonable guesses (and guesses are all that one car really provide remotely, anyway), I'd say the right channel power pack has failed with one of its BE junctions (at least) gone open. There's something funky with the left channel to have such a high DC offset (volt and a half or so on the amplifier output, pins 3 & 8 of the power pack), but possibly it's within offset adjustment range.

My suggestions:

Remove the right power pack.

Power up with a "dim bulb" rig.

Verify that the left channel output can be brought to very near zero with the offset adjustment, and that afterwards the protection relay engages.

Power down and check the emitter resistors on both channels (verify 1 ohm resistance between pins 3 & 8 of the power packs - my guess is there's no problem here).

Optionally, do the check for predriver health discussed in the "Ongoing saga" thread - my gess is there's no problem here.

Replace the right power pack, power up via dim bulb again and verify that both outputs can be adjusted to zero and that the protection relay engages.

Power up normally and test amplifiers at full power with 8 ohm loads - or perhaps with 4 ohm loads, if you're feeling lucky and/or you intend to run the receiver with two sets of speakers.

Good luck,

chazix

i don't see any negative voltages marked ...
it does however look like the right channel is blown

Argh, my tube origins are showing....not used to dealing with positive and negative voltages. Here are fresh readings from the power ICs:



chazix, thanks for the detailed troubleshooting plan, I'll do that later today.

Well, I removed the right output, powered up on a dim bulb and was able to get the left output down to about 0.15v at full rotation. So I put in a replacement IC on the right output (STK0050II). Powered up again. Still in protection, and right side voltages are still out of whack. I did verify that there's 1 ohm between pins 3 and 8 on both outputs.

Any ideas what to check next? Right side pre-driver?

Well, one approach would be to posit that there's nothing EXCEPT the predriver that's likely to be the source of the continuing right channel trouble, and therefore to go ahead and replace it. (This reasoning assumes that the replacement parts are good, and of course there's a bit of room for doubt about that.)

Did the receiver come out of protection with the right channel power pack removed? It should have, once you got the left channel offset trimmed down.

I'm concerned that you were only able to get the left channel to 0.15V. Something might still be amiss there.

BTW, there were still no minus signs visible (to me, at least) in your latest readings. For anyone to really analyze them, it's kinda important to know the polarities

chazix

Dang, I must have accidentally posted the same numbers. I've edited my post above to contain the correct image.

It didn't come out of protection with the right side driver removed.

It did seem wrong that I would be maxed out on the trimmer and still not at zero, I'll have to take another look at the left side.

I'm afraid I can't think of anything solid to suggest at this point. If the protection circuit didn't engage the relay when the right power pack was removed (*) and the left channel was down to 0.1V, then I'd start looking at the protection circuit itself.

If it was me, I'd also be double-checking my re-cap work,

(*) BTW, you said "didn't come out of protection with the right side driver removed". Did you remove the power pack (the part that's mounted to a heat sink) or the pre-driver (the free-standing 15-pin part)? What I had in mind was the power pack.

chazix

The power pack is what I removed. I'll take a look at the protection circuit. Since the voltages on the right side are still funky, I may also try one of my replacement pre-drivers on that side.

Hi Folks,

Been away from the Yam for a bit. Back at it now. After replacing both the power pack and pre-driver on the right side, I've successfully set the DC offset to 0 volts! Still won't come out of protect....checked the voltage across the coil of the relay: nada. I'm guessing one of the transistors in the protection circuit is popped.

Oy, looks like everything in the protect circuit may be fried. So far:
D901? Dead.
D902? Already replaced (was dead, bloated)
TR901? Dead.

Whew, quite a project! Perhaps the protection circuit damage is aftermath from the rectifier failure, though at a glance it's not obvious to me why that would be. Or maybe the back-EMF diode across the relay failed with a short spontaneously and took some friends down with it.

At least you're in territory where it's less of a problem to get good replacement parts now.

Nice going,

chazix

Victory!

I replaced all the transistors in the protection circuit. Bam! Natural sound in the house!

Subs were:
TR901: 2SC2320 (ecb) -> KSC2383 (ecb)
TR902: 2SA999 (bce) -> KSA992 (ecb)
TR903: 2SC2320 (ecb) -> KSC2383 (ecb)
TR904: 2SD736 (ecb) -> KSD1616(bce)