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I have posted about some months ago, but I want to start fresh. A AB763 Deluxe Reverb of mixed age came in for some general maintenance. I did a few things, including the cathode caps, a cleaning, biasing and some other minor stuff. About a week later the customer says it is sounding thin, ratty and choked, so he brings it back. It was indeed sounding ratty, with a loss of power, and less volume with a choked effect. It had not done this while I had it, or for the first week he had it back. I probed, wiggled and tried to find any kind of mechanical relation, but found none. I attempted to probe the preamp tube V2, and it made the usual expected pop , and came out of fault mode. It sounded just fine after that, and it wouldn't go back into failure mode after hours of trying, so I sent it back.
About a week later he brings it back, saying it did it again yesterday. I suggest even though the filter caps look and measure good, that we replace them. He agrees. This time I keep it for about a week. I do everything I can think of to try to make it go into failure mode, but it only does it twice after dozens of power cycles, and always clears up the second I probe something. And then it won't do it again. I can't afford to clog up my bench with trying to make this amp go into failure mode with such a seldom occurrence, so I send it back for him to try.
Again it fails at his house, and he brings it back. Same as other times, I cannot measure anything because as soon as I try it comes out of failure mode. Thinking it may be an intermittant arcing OT, I suggest we try that, and he agrees. I put in a ClassicTone replacement, and send it back to him. He texts me the next day that the amp sounds GREAT, better than ever, and that I am the best tech, etc. I tell him let's just wait a while before we take out bows.
Sure enough three weeks later after performing fine at his house, he turns it on one day and it is in failure mode. He brings it to me,and sure enough it is making that ratty, low output ringing crap sound, almost like a ring modulator in the mix. And when playing an open string, as it dies down, rather than going steadily into silence, at maybe the 10% point it abruptly cuts out. I pull the chassis and get it on the bench and it is still in fault mode. I put my scope on the speaker jack with a load and when I play an open string, I can hear the fault, and the scope shows a noisy and much busier trace than what I would expect to see. Putting a sine wave from my generator of the same note, I cannot hear the artifact, but I can see the wave is distorted. On the down slope side about halfway between the upper and lower peaks, the trace goes horizontal for a bit before resuming. It does not do this on the up slope.Crossover distortion?
Once again as I touch a probe to a plate of V2, it makes the usual pop, and snaps out of it. That was 1.5 days ago, and it hasn't failed since after about a dozen attempts of powering off and on for differing amounts of time. One thing I have failed to determine is if the failure is in both Normal and Rev channels. I would really like to know this. I can't afford to keep this on my bench endlessly, so I am going to send it back with instructions to find out if it is in both channels, I am also going to swap the reverb driver with the PI 12AT7, see what becomes of that.
So here is what I have done thusfar:
Probed to look for mechanical failure, none found
swapped out all tubes one at a time
replaced all electrolytics
freeze sprayed the entire board in sections
replaced OT
also:
replaced screen resistors (for another issue)
replaced a couple of plate resistors (for another issue)
verified grounds
Good gravy, this has been a challenge. It's hard to fix a problem when it only rears it's ugly head once in a while, and then runs away when you get your tools out.
http://ampwares.com/schematics/deluxe_reverb_ab763.pdf
About a week later he brings it back, saying it did it again yesterday. I suggest even though the filter caps look and measure good, that we replace them. He agrees. This time I keep it for about a week. I do everything I can think of to try to make it go into failure mode, but it only does it twice after dozens of power cycles, and always clears up the second I probe something. And then it won't do it again. I can't afford to clog up my bench with trying to make this amp go into failure mode with such a seldom occurrence, so I send it back for him to try.
Again it fails at his house, and he brings it back. Same as other times, I cannot measure anything because as soon as I try it comes out of failure mode. Thinking it may be an intermittant arcing OT, I suggest we try that, and he agrees. I put in a ClassicTone replacement, and send it back to him. He texts me the next day that the amp sounds GREAT, better than ever, and that I am the best tech, etc. I tell him let's just wait a while before we take out bows.
Sure enough three weeks later after performing fine at his house, he turns it on one day and it is in failure mode. He brings it to me,and sure enough it is making that ratty, low output ringing crap sound, almost like a ring modulator in the mix. And when playing an open string, as it dies down, rather than going steadily into silence, at maybe the 10% point it abruptly cuts out. I pull the chassis and get it on the bench and it is still in fault mode. I put my scope on the speaker jack with a load and when I play an open string, I can hear the fault, and the scope shows a noisy and much busier trace than what I would expect to see. Putting a sine wave from my generator of the same note, I cannot hear the artifact, but I can see the wave is distorted. On the down slope side about halfway between the upper and lower peaks, the trace goes horizontal for a bit before resuming. It does not do this on the up slope.Crossover distortion?
Once again as I touch a probe to a plate of V2, it makes the usual pop, and snaps out of it. That was 1.5 days ago, and it hasn't failed since after about a dozen attempts of powering off and on for differing amounts of time. One thing I have failed to determine is if the failure is in both Normal and Rev channels. I would really like to know this. I can't afford to keep this on my bench endlessly, so I am going to send it back with instructions to find out if it is in both channels, I am also going to swap the reverb driver with the PI 12AT7, see what becomes of that.
So here is what I have done thusfar:
Probed to look for mechanical failure, none found
swapped out all tubes one at a time
replaced all electrolytics
freeze sprayed the entire board in sections
replaced OT
also:
replaced screen resistors (for another issue)
replaced a couple of plate resistors (for another issue)
verified grounds
Good gravy, this has been a challenge. It's hard to fix a problem when it only rears it's ugly head once in a while, and then runs away when you get your tools out.
http://ampwares.com/schematics/deluxe_reverb_ab763.pdf
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