[raymondbeautrix]

Hello Everyone,

My amp started acting a little funny since about a month ago when I got some popping noises from my left speaker and noticed one of the 4 left KT88s was glowing abnormally bright.  After turning off for a few moments then back on, it started acting normally again.  This problem came up just a couple more times, then for a couple weeks was fine.

Just today, however, I got the same popping noises, but this time the left channel won't come back on.  I decided looking at the built-in bias meter, and noticed that all of the left KT88's are showing 0 voltage which won't respond to turning the knobs.  The right KT88s showed a voltage and could be adjusted properly.  I tried swapping my right and left KT88s to factor out the tubes.

Here's a small list of pertinent questions:
1) Can anyone deduce any sort of diagnosis based on these conditions, or offer a shortlist for possible issues?
2) Explaining methods for applying a fix or for properly diagnosing the problem.
3) Non-local options for repair and ballpark cost.
4) Cost estimate for a local 'electronics fix-it guy'.

Here's a link to the manual in case this would help: http://www.elusivedisc.com/CAYIAA100T.pdf

Any help would be greatly appreciated.

Thanks,
Isaac

[raymondbeautrix]

This is a longshot, but if anyone happens to know how to obtain a schematic for this amp, I do have an electrical engineer friend.

[PhilNYC]

Have you tried contacting the distributor?  http://cayinusa.com/

[Niteshade]

It sounds like  a coupling cap went bad and/or there is an issue with the bias supply's feed to the left side. There could be a fuse on the left side that blew after the glowing plates too.

A schematic would be nice, but since he has the right side to work against, he should be able to locate the problem. I doubt it will be an expensive fix.

[raymondbeautrix]

Phil: Yes, I emailed the US distributor and am waiting for a reply.  I would imagine they're likely to have the schematic, but I'm unsure whether they'd charge for it or consider it a 'trade secret'.  If I remember correctly, you work for the US distributor?

Niteshade: Thanks for the helpful tips!  If I went and bought a multimeter, are there any simple tests I could run?  I did take a circuits lab for my undergrad physics degree, so I have some limited background.  But my practical knowledge is very limited because it was mostly theory-based.

[PhilNYC]

Yes, I'm a dealer for Cayin....

FWiW, Steve Leung (the distributor) is horrible at replying to email, but is very accessible via phone.  I'd recommend you give him a call...!

[JoshK]

Blair (Niteshade) has much more experience with these things than I, but I would have guessed the cathode resistor fried.  It seems to me that would explain the 0 volts bias.  However, there is many more ways to bias than cathode bias.

Curious, is this a PCB or point to point design?  I'd guess PCB.  Point to point is easier to work on but either can be repaired.  Should be an easy fix since you mention no smoke or burning smell.

[JohnR]

According to the manual each tube has independent bias adjustment so I would guess there won't be a cathode resistor to speak of. This is not a good time to start learning how tube power amp circuits work and I don't recommend it. Nonetheless since this is the Lab, if it were mine I guess would start by measuring the voltage at the grids on the power tube sockets, on a variac with reduced voltage, and work backwards from there. Since it was intermittent for a while I would be looking at the pots and switches in the bias circuit with suspicion.

[raymondbeautrix]

Thanks a lot for a the added feedback guys.  As for the bias meter, it is important to note that I'm also getting 0 voltage through one of my right channel KT88s.  But since the right channel works, I'm wondering if there also isn't some general issue with my bias meter.  I'm not sure if this info would help diagnose the left channel issue or not.

I should point out that I have no real experience getting inside an amp.  I want to get a multimeter to try some of this out myself, but my friend (electrical engineer) will probably have a part in it.

[raymondbeautrix]

Ok, same thing JUST happened to my right channel.  I here some popping sounds (with the vol muted), look over at one of my right tubes and see it light up, a bright incandescent spark, then silence.  Both channels now do not function.

Does this new information help us make any new deductions/diagnoses?

[JohnR]

Sounds like you've blown a tube by running it without any bias. Or possibly worse.

[Niteshade]

I must say that this is a bad month for amplifiers. 

The good thing about stereo amplifiers is it's easy to compare one channel to the other since they rarely both go bad. Time to compare voltages!

Some good advice: Have a 100 ohm 5W resistor around so the capacitors can be safely drained. If you test with no tubes, this is an absolute must. Even with tubes, there could be residual voltage since they quit conducting after their cool.