CR-840 — don't always assume the worst

[cnolanh]

I've got a CR-840 as the anchor of my office system. It plays music softly most days, once in a while gets to play at medium volume, and it never gets cranked. This Yamaha is a beautiful machine that has worked at nearly 100% for many years (only flaw is that the Low Filter switch won't stay in, which doesn't matter because I never use that feature).

Yesterday I was listening to music at low volume when I heard a soft squeal and then the R channel cut out altogether. Crap, I thought, one of those infamous power amp modules has blown. I started thinking about what to replace the receiver with, and about parting it out or selling it to someone else to mess with.

Then today I had more time to troubleshoot. First discovery: It put out normal sound to both channels of headphones, though the R speaker still didn't work at all. Hmmm. I wondered if this model could have a separate small amp just for the headphones.

Second discovery: Switching the speaker wires at the amp revealed that the receiver's R channel does work, and that my R speaker did not work when powered by either channel of the receiver. I couldn't see any sign of a break or short in the decent speaker wire, and further testing revealed that the speaker itself had stopped playing even with a known good signal.

Third discovery: There's a fuse on each of these Phase Technology PC65 speakers (I had forgotten that) and indeed the R speaker had its fuse blown. Putting a new fuse in returned the sound immediately. Whew.

The speakers call for 2A fuses, and I discovered 2.5A fuses in both of them. I'll replace them with the correct value fuses. I still don't know what caused that speaker's fuse to blow, and I suppose it still could point to something amiss with with the CR-840. For now, I'm glad that she still sings.

 

[drumbum]

I would endeavor to check dc offset and bias for starters, tout suite.

 

[avionic]

I would endeavor to check dc offset and bias for starters, tout suite.

For starters...These have " chip amplifiers" no bias adjustment.However there is a DC offset/balance adjustment.If the protect relay "clicks" then the DC offset must be pretty damn close.Wouldn't hurt to check it and fine tune it.

 

[Hajidub]

Would abide by Avionic's advice before running again with that larger fuse (it's there for a reason). GL.

 

[cnolanh]

Thanks, all. The relay does click as normal, and the receiver does not get abnormally hot, as far as I can tell. I will try soon to check the DC offset.

 

[avionic]

and the receiver does not get abnormally hot,

DC offset will not help that issue.

 

[NH-MAN]

Could be just fatigue on the wire of the fuse from vibrations?

 

[Skywatcher]

Is it still working? If the one speaker randomly blew it's fuse, then I would suspect there was indeed a DC "surge" from the amp. The pre-driver modules do not have a heat sink on them and are known to be the initial failure sometimes. If it is still working, put a heat sink on each pre-driver module and hope the culprit hasn't been permanently damaged. There are two Metal Oxide 390 Ohm (I believe 2 watt) near the pre-drivers. Look to see if one (or both) of those has faded color bands. If so, that would indicate which module is suffereing the worst. Pull the resistors and check to see if they have drifted high or not. There are also 2 more 1/2 watt 390 Ohm resistors out near the output modules. Check them too.

 

[cnolanh]

Thanks for the detail about what to look for, Skywatcher. Yes, the receiver still seems to be working normally. I'll open the case when I get a chance and see about adding heat sinks to those modules — I know that's good practice for these.