I need up to date schematics on SVT Classic which include the updates based on service bulletins that were published a few years back.
The problem is that these two amplifiers kick out a fault the first time they're powered on. Shutting them down and back on, clears the fault and it doesn't recur.
My first thought was the old set of 6550s were not matched, as I verified on a curve tracer. Mismatched tubes can cause one in the bunch to share more current than the safe threshold and trip the fault. So I put in an Apex matched set of Tung-sol reissue 6550s. I ran it into a dummy load for a half hour and then re-biased it again. However, the fault still occurs after initial power on, just less often. The fault will not recur unless the amp has been off for many hours (overnight) and is initially powered on for one minute, then the standby switch flipped. Sometimes, but not every time, the fault will trip.
All of the research I found online talked about changing screen resistors, adding a cap to the protection op amp to slow it's response time, and removing diodes that were across screen resistors. All of these changes are incorporated already into the design of these PC boards on these two amplifiers.
The maddening thing is it only happens once, and the amp is heavy and disassembly makes live testing near impossible, if not very dangerous. Many screws have to come out just to get the top metal plate off, and that gives a small window into the main PCB and limited access to the power tube PCB, which is constrained by many wires. After all this disassembly, I find out the circuit updates are already present.
I have this hunch that the inrush current on a FULLY DISCHARGED power supply may be tripping this fault circuit, as it doesn't trip if the power is cycled within a few minutes of the initial turn on. But with one-time events like this, it's not easy to repeat and test because of the long time to reset the condition.
Have I missed any other bulletins that detail circuit changes to mitigate the false fault tripping? If I had the right schematic for this s/n range, I could at least analyze it and determine a way to lower the sensitivity threshold a bit more. And perhaps there is new data on this problem which my search did not pick up.
The problem is that these two amplifiers kick out a fault the first time they're powered on. Shutting them down and back on, clears the fault and it doesn't recur.
My first thought was the old set of 6550s were not matched, as I verified on a curve tracer. Mismatched tubes can cause one in the bunch to share more current than the safe threshold and trip the fault. So I put in an Apex matched set of Tung-sol reissue 6550s. I ran it into a dummy load for a half hour and then re-biased it again. However, the fault still occurs after initial power on, just less often. The fault will not recur unless the amp has been off for many hours (overnight) and is initially powered on for one minute, then the standby switch flipped. Sometimes, but not every time, the fault will trip.
All of the research I found online talked about changing screen resistors, adding a cap to the protection op amp to slow it's response time, and removing diodes that were across screen resistors. All of these changes are incorporated already into the design of these PC boards on these two amplifiers.
The maddening thing is it only happens once, and the amp is heavy and disassembly makes live testing near impossible, if not very dangerous. Many screws have to come out just to get the top metal plate off, and that gives a small window into the main PCB and limited access to the power tube PCB, which is constrained by many wires. After all this disassembly, I find out the circuit updates are already present.
I have this hunch that the inrush current on a FULLY DISCHARGED power supply may be tripping this fault circuit, as it doesn't trip if the power is cycled within a few minutes of the initial turn on. But with one-time events like this, it's not easy to repeat and test because of the long time to reset the condition.
Have I missed any other bulletins that detail circuit changes to mitigate the false fault tripping? If I had the right schematic for this s/n range, I could at least analyze it and determine a way to lower the sensitivity threshold a bit more. And perhaps there is new data on this problem which my search did not pick up.
I feel your pain, the NEW ONES are a PITA to work on, the old ones are a breeze. You can't do anything with it unless it's completely apart and then you can't do anything with it, Catch 22. Maybe check the low voltage PS/regs that supply the opamps in the protection circuit. Call LOUD TECHNOLOGY on Monday and ask for everything regarding those amps, they're usually pretty helpful.
Craig
Craig
Now I have an additional problem that I didn't have earlier: the filaments don't get power after reassembly. I partially disassembled and checked that the filament wiring is attached to the power amp PCB and it is. I read secondary winding DCR across pins 2 and 7 also. Neither the 6550s OR the 12AX7/U7s are getting filament power. So I checked the fuse on the filament transformer and it is good.
Bad angle to work from under the chassis. I'm tempted to turn the whole thing upside down, but am concerned that the two wooden 'rails' that hold the chassis might not be strong enough to hold all that weight. But I can't work on it by squatting under the bench and trying to look up through a narrow slit between the chassis and the cabinet. Was trying to avoid having this turn into a four hour job, but it looks like it's going to be six hours now..
Bad angle to work from under the chassis. I'm tempted to turn the whole thing upside down, but am concerned that the two wooden 'rails' that hold the chassis might not be strong enough to hold all that weight. But I can't work on it by squatting under the bench and trying to look up through a narrow slit between the chassis and the cabinet. Was trying to avoid having this turn into a four hour job, but it looks like it's going to be six hours now..
After yanking the chassis and putting the whole cabinet on it's end, I found one of the brown wires that terminates in a shovel pin was loose. The connector wasn't gripping the pin on the board. So I crimped it closed and pushed it on. It is now tight.
Got filament power again. Put it all together and powered it on. Came out of standby and a minute later, it faulted again. <sigh> What's frustrating is that I won't be able to reproduce this fault for another 24 hours. I have a hunch it has something to do with inrush current on totally discharged capacitors, because the only condition that's different after 24 hours is the voltage is at zero. I can power it on and toggle standby multiple times and not have another fault trip. What a design..
Got filament power again. Put it all together and powered it on. Came out of standby and a minute later, it faulted again. <sigh> What's frustrating is that I won't be able to reproduce this fault for another 24 hours. I have a hunch it has something to do with inrush current on totally discharged capacitors, because the only condition that's different after 24 hours is the voltage is at zero. I can power it on and toggle standby multiple times and not have another fault trip. What a design..
All of the output cathode circuits are summed together to provide a signal for the input of the protect opamp, is there a way to monitor that summed signal? I'm not sure if you get to it or not. At least verify it is the output section causing the trip. There are several small electrolytics in the the cathode circuits maybe there's one with high ESR?
Craig
Craig
That's the hitch.. I would have to invest two hours taking the whole thing apart just to get it arranged so that I can get a probe in to those places. And to test a one-time event that only happens once in 24 hours and won't reproduce again til the next day. Merely shutting it off for several hours and testing again does not manifest the fault. Those odds don't really inspire me to invest that much time.
After fixing a loose shovel pin connector on the power PCB, I was hopeful, but nah... it faulted one time upon startup.
I can say one thing: the condition of the driver tube determines the power output of this amplifier. Any degradation of the driver produces drastic power reduction in the output.
A check of every resistor on the power amp PCB showed all good values. Whether it's a high ESR cap somewhere in the voltage compariter circuit, or just an overly sensitive to any glitch design, I'm not sure. I think the circuit is just trip-happy and perhaps there's a spike soon after power on that trips it. But it ONLY happens after the amp has been fully discharged over night.
After fixing a loose shovel pin connector on the power PCB, I was hopeful, but nah... it faulted one time upon startup.
I can say one thing: the condition of the driver tube determines the power output of this amplifier. Any degradation of the driver produces drastic power reduction in the output.
A check of every resistor on the power amp PCB showed all good values. Whether it's a high ESR cap somewhere in the voltage compariter circuit, or just an overly sensitive to any glitch design, I'm not sure. I think the circuit is just trip-happy and perhaps there's a spike soon after power on that trips it. But it ONLY happens after the amp has been fully discharged over night.
The strange thing is that you TWO amps doing the same thing? I have repaired several of these but have never experienced this problem. The last one dealt with strange biasing LED behavior. Your heater voltage could have caused a problem as that voltage is also used to bias the cathode/protect circuit. Have you tried draining the filter capacitors manually to speed up the failure rate?
Craig
Craig
Yeah, it is pretty strange. But both came from the same venue, and both have the same symptom noted on the trouble ticket.
They appear to be a late (recent) and very much up to date revision, in that all of the service bulletins are incorporated into the amp PCBs.
I've also inspected the solder joints especially the tube socket joints, but all are shiny and new looking.
Access to the high voltage requires taking both the preamp and the power amp out of the box. The preamp can only come out the front, as a riveted on lip on the bottom prevents it from pushing in. So the only way to get them both out is to slide the preamp out the front, disconnect several cables and one ground wire that's glyptoled real thick over the nut that secures the wire to chassis. Then remove five bolts under neath and slide out the amp chassis.
Today I observed it trip the fault after warm up, but only when coming out of standby. IOW, if it trips once, and I reset the power to clear the fault, it won't happen again. But this second time today now invalidates my original theory about the cold start inrush.
It's starting to look more like a protection circuit that's simply too sensitive.
They appear to be a late (recent) and very much up to date revision, in that all of the service bulletins are incorporated into the amp PCBs.
I've also inspected the solder joints especially the tube socket joints, but all are shiny and new looking.
Access to the high voltage requires taking both the preamp and the power amp out of the box. The preamp can only come out the front, as a riveted on lip on the bottom prevents it from pushing in. So the only way to get them both out is to slide the preamp out the front, disconnect several cables and one ground wire that's glyptoled real thick over the nut that secures the wire to chassis. Then remove five bolts under neath and slide out the amp chassis.
Today I observed it trip the fault after warm up, but only when coming out of standby. IOW, if it trips once, and I reset the power to clear the fault, it won't happen again. But this second time today now invalidates my original theory about the cold start inrush.
It's starting to look more like a protection circuit that's simply too sensitive.
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