Many moons ago Gerald rang (his name's not really Gerald, but the names have been changed to protect the innocent)
"I've got this Copland Hybrid amp, it sounds thin and distorted on one channel, mainly in the mid-range, can you take a look?"
Yeah, why not? (in hindsight, there are now a good dozen reasons I can think of why not...)
It's superbly made in Denmark, and weighs a lot!
... the weight mainly due to the huge toroidial transformer, and gargantuan heatsinks!
Look at all that smoothing.
The problem quickly showed itself.... something in the back of my mind said "this sounds valvey to me" .... but swapping the two 12AU7's (that's ECC82 to you and me) made no difference. I eventually traced it down to a thermally poor bias transistor on the right channel... or so I thought.
The amplifier behaved faultlessly on test with my Rotel pre-amp driving it, so I sent it back....
About a week later the phone rang again ... it's Gerald
"It's doing it again" ...
Send it back....
It arrived. I put it on the bench, and listened to it all night, as I was working on other things... It performed faultlessly.
I removed my power amp from my hifi to give it a damn good test .... months it sat there, performing beautifully.
.... then ... it did it...
"I've got you now!" ....
Put it back on the bench, and it was fine! No amount of tapping or banging would get it to fault....
I removed the PCB (not an easy task in itself) and checked every soldered joint. Not one was poor.I was about to reassemble it, and thought I'd just check the bias again... and sure enough the right channel had drifted a bit. I adjusted it, and put it back on test.... then about 10 mins later, it faulted again!!! I changed every cap in the right hand bias circuit.
Working again...
After a week of perfect operation, it was returned to Gerald...
... where it ran for a couple of days ... and came bouncing back once more....
On the bench, and the fault was immediately obvious. Bias on the right hand channel was miles out.
As I probed my way through the circuit, the fault was progressively putting itself right.... the DC bias to the bases of the output pairs was decreasing. I traced it all the way back to the drivers, when the fault just wasn't there anymore.... I switched the amp off and went for a nice cup of tea....
I came back, and switched the amp on. Excellent fault present for a few minutes, then the bias was decreasing to a sensible level once again. Out with some "freezer spray on the cheap". Output was connected to a dummy load, and monitored on the scope. Finally some progress. Freezing and cooling a transistor in the voltage amp would cause the fault to come and go...
Gotcha....
... I hope
... and so does Gerald...
Superb diagnostic skills you have Mr Doswell. I admire your perseverance, I'd have given up probably. Reminds me of an American amplifier which had one of these on/off faults. The fault occurring mainly at the owners home but only for a few seconds here. It was one of those with a myriad of output transistors, Accuphase? Turned out one transistor was going temporarily S/C, I'm glad I never do this stuff now and leave it to the experts like your good self!
ReplyDeleteI've had a similar fault on a Linn Axis supply, which would intermittently blow the input fuse. The was one of the drive transistors going short every third blue-moon. I will NOT be beaten by mere machinery!!!
DeleteHi Doz. I am working on a Copland CSA-515 and would like to ask how to bias the amp. I saw two trimmer on each channel, and I guess one trimmer is for bias and the other trimmer is probably for DC offset. I am desperate to find information about how to do the bias. Could you please let me know? My email is liemth@yahoo.com. Thank you so much!
ReplyDeleteNot sure how to use this site so I post it again. I am working on a Copland CSA-515 and need instruction to bias the amp. Could you please let me know. Greatly appreciate your help!
ReplyDeletehttps://audiokarma.org/forums/index.php?threads/copland-csa-515-bias-adjustment-instruction.1024310/
DeleteThat is my post. Could you let me know the location of the leads?
DeleteI'm sorry, I don't remember.
DeleteIntermittent defects always require perseverance - usually on the part of the customer. :)
ReplyDelete