Hi, I recently resurrected my Edison 12 power amp which had been in storage for nigh on 15 years and had new matched valves before being put away. But it had always had a problem on switch off..a chirrup through one speaker.
I have found a friendly and valve experienced qualified engineer who has established that one channel, on switch off, goes unstable in the RF range...immediate switch on causes the amp to oscillate.
Not taking anything from him but to speed up diagnosis..any one got any ideas? I have supplied him the schematic copied from another Edison 12 thread so he has that but it is quite apparent that many of the values are different....
Anyway..any advice!?
I have found a friendly and valve experienced qualified engineer who has established that one channel, on switch off, goes unstable in the RF range...immediate switch on causes the amp to oscillate.
Not taking anything from him but to speed up diagnosis..any one got any ideas? I have supplied him the schematic copied from another Edison 12 thread so he has that but it is quite apparent that many of the values are different....
Anyway..any advice!?
If the diagram in post 3# is correct,
And the fault is in one channel...
Check the bias voltage (output tubes) across the cathode resistors on the good and bad channel
Post the values that you get..
Its important that when you take these measurements (make sure a load or speaker is connected or you will damge the OP Tx)..(Also dont start swapping tubes about etc or the results are of no use)
Regards
M. Gregg
And the fault is in one channel...
Check the bias voltage (output tubes) across the cathode resistors on the good and bad channel
Post the values that you get..
Its important that when you take these measurements (make sure a load or speaker is connected or you will damge the OP Tx)..(Also dont start swapping tubes about etc or the results are of no use)
Regards
M. Gregg
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Hi,
Thanks Flatheadmurre (and M Gregg)...I was just logging on to post the circuit!
Any useful tips I will pass on to the engineer who has it..this is an area in which I have no qualification...
I posted on the off chance that someone may have had the same problem and solved it...so thank you all for any input!
Thanks Flatheadmurre (and M Gregg)...I was just logging on to post the circuit!
Any useful tips I will pass on to the engineer who has it..this is an area in which I have no qualification...
I posted on the off chance that someone may have had the same problem and solved it...so thank you all for any input!
It's a stereo amp...one mother board, but as I say it is with the engineer..I don't have it "in front of me"..and nor am I qualified to tinker with it!!
I was just going to pass on any snippets of advice I may get on the forum from those who may be have had this amp and the exact same problem.. (and without trying to insult his intelligence...or any of yours!!) He is Bsc qualified electronics engineer and of the age that he grew up with valve amps so he probably will track it down, but "time is money" as they say and directing him to the most likely cause might save some grief..and money!
I was just going to pass on any snippets of advice I may get on the forum from those who may be have had this amp and the exact same problem.. (and without trying to insult his intelligence...or any of yours!!) He is Bsc qualified electronics engineer and of the age that he grew up with valve amps so he probably will track it down, but "time is money" as they say and directing him to the most likely cause might save some grief..and money!
To be honest,
I can tell you what to measure...and direct from there..
The "it could be this and that, is real hit and miss!"
Regards
M. Gregg
This is what I would do..
I guess he will do something similar...
If the fault is in one channel then with a bias check we can tell if its a coupling cap problem. (OP tubes)
If that is ok check the voltage across the 470K on the grid each tube..
if thats ok pull the driver tube watch for any red plating..
See if the fault is still there power on/off...
Untill we have the results its hard to go further...
Regards
M. Gregg
I guess he will do something similar...
If the fault is in one channel then with a bias check we can tell if its a coupling cap problem. (OP tubes)
If that is ok check the voltage across the 470K on the grid each tube..
if thats ok pull the driver tube watch for any red plating..
See if the fault is still there power on/off...
Untill we have the results its hard to go further...
Regards
M. Gregg
I understand he has already changed some resistors and caps..and re-soldered some of the valve base connections...
M Gregg is right of course!! If we are saying it could be one of many things it is nigh on impossible to achieve a result at "arms length"
Thanks for all the helpful comments though...I will pass them on...
M Gregg is right of course!! If we are saying it could be one of many things it is nigh on impossible to achieve a result at "arms length"
Thanks for all the helpful comments though...I will pass them on...
The bottom line is this,,
He may not want any input from us..but give him the link to this page..
If he wants we can work with him..
Regards
M. Gregg
If an amp oscillates as it powers down then this could be a sign of conditional stability: the locus of loop gain goes beyond the (1,0) point in the complex plane but manages to loop the right side of it. Dropping the gain (by powering down) causes oscillation, which is then self-limiting so keeps the gain low. Yes, you can cause oscillation by reducing loop gain!
Most likely cause is a fault in one of the compensation components, but it could be the output stage. You need to find out whether it is the whole loop or just one stage. Blindly changing components is expensive and might even make things worse if component placement is the problem.
Most likely cause is a fault in one of the compensation components, but it could be the output stage. You need to find out whether it is the whole loop or just one stage. Blindly changing components is expensive and might even make things worse if component placement is the problem.
Sounding like a broken record.
The amp is working fine when all components are in good condition.
So Checking the caps, resistors and looking over the solder joints is my bet.
If you´re not able to verify if a component is good or not without changing to a new one then let someone else.
Keep in mind that the engineer of the amp is same man as made Audion and Audionote so the design is simple but ok.
The amp is working fine when all components are in good condition.
So Checking the caps, resistors and looking over the solder joints is my bet.
If you´re not able to verify if a component is good or not without changing to a new one then let someone else.
Keep in mind that the engineer of the amp is same man as made Audion and Audionote so the design is simple but ok.
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