Trying to fix a broken amp I bought. The issue is intermittent, but it's happening with increasing frequency. On power up or after a few minutes, the power amp module for one of the two channels quickly gets extremely hot. Sometimes it's the channel 1 module, other times it's the channel 2, never both simultaneously; tried swapping the modules, but it didn't make a difference. I rarely keep it on for more than a few seconds once the problem starts, but that's enough to get it to where you can barely touch the heat sink. Sometimes powercycling the amp resolves the issue, other times not.
The bias on the module experiencing the thermal problems, as it is experiencing them, is all over the place, from 200 to 600mV! I didn't adjust it. On the occasions when powercycling gets the module to operate normally, the bias is a few mV, which is correct, per the specs.
Any suggestions for things to try, or how to narrow down the problem? Perhaps this is a common failure mode? Amp is '97 vintage.
I've tried disconnecting the input board from the main board, to no effect on the thermal issue. Measuring the input DC voltage on the main board fingers where the power modules plug in, it is +/- 61.3V when the amp is on and the power modules are disconnected. When they are connected and working properly, it is +/-59V. When the thermal problem starts, the voltage drops to +/-45V. That could just be because of the power draw of the power transistors.
Schematic attached.
The bias on the module experiencing the thermal problems, as it is experiencing them, is all over the place, from 200 to 600mV! I didn't adjust it. On the occasions when powercycling gets the module to operate normally, the bias is a few mV, which is correct, per the specs.
Any suggestions for things to try, or how to narrow down the problem? Perhaps this is a common failure mode? Amp is '97 vintage.
I've tried disconnecting the input board from the main board, to no effect on the thermal issue. Measuring the input DC voltage on the main board fingers where the power modules plug in, it is +/- 61.3V when the amp is on and the power modules are disconnected. When they are connected and working properly, it is +/-59V. When the thermal problem starts, the voltage drops to +/-45V. That could just be because of the power draw of the power transistors.
Schematic attached.
It is odd that it happens to one or the other. That would imply that both channels have a problem but that one channel is more sensitive at a given moment to the trigger. Without knowing anything else, I would suspect an oscillation in the particular amplifier that gets hot and I would blame a similar electrolytic capacitor that has changed value over time and temperature that is present in both channels. Inspect all the electrolytic capacitors on the amplifier boards and look for bloated capacitors. If they appear fine then you will need to check to see if all of them are actually filtering by looking at the ripple voltage on the capacitors. If the big capacitors seem all right then a small capacitor involved in the compensation loop may have changed its value. I think that you likely have a parametric shift in some components that all installed in both channels and some small difference between the channels triggers one or the other to start oscillating.
How old is the amp. If 20 yr or less it is still under warranty and i believe follows the amp not the owner.
Theynwill have records if you need to find out.
dave
Theynwill have records if you need to find out.
dave
Amp was produced in 1997, according to the serial number on the back.
Definitely no visible damage to any of the electrolytic caps on any of the boards.
I checked the large caps with an LCR meter in circuit, and they seem ok. All the smaller caps on both the main board and the power amp boards are out of spec, but this may be because I'm measuring in circuit.
Definitely no visible damage to any of the electrolytic caps on any of the boards.
Just to make sure I understand correctly, are you referring to the 2200uF caps at the negative feedback to the power amp? Can you talk a little bit about why these caps are the likely problem and not some other electrolytics on the power supply board or the power amp board? Would be helpful to understand this and to get some intuition about where to look next time.
I checked the large caps with an LCR meter in circuit, and they seem ok. All the smaller caps on both the main board and the power amp boards are out of spec, but this may be because I'm measuring in circuit.
I am referring to power supply capacitors. I doubt that a faulty 2200uf input capacitor would cause an oscillation but I could be surprised. If a power supply capacitor becomes open or loses value then the ac ripple will increase which will make the loop less stable because the loop has to compensate for this changing voltage. It would be best if you would scope your power supply rails to see if they look good. If you don't have a scope then an AC measurement could be useful. I would say that no more than a few volts of ac voltage should be on your power rails. If you are able to locate the main loop compensation capacitor then you could replace it or tack another one across it. You could also suspect a faulty capacitor in a current limit protection circuit if it has one. If you have a schematic then you should post it. It could eliminate a lot of guessing. There may be more than one compensating capacitor in the loop that could trigger an oscillation. Look for bad power supply voltages first and then for changed loop stabilizing components.
Thank you. I posted the schematic at the bottom of my original message. There is no protection circuit in these amps, just some fuses.
I desoldered some caps to check their value out of circuit. The ESR on most of the small ones is very high (5-15 Ohm), even though the capacitance on all but a few is still within spec. The input capacitor's ESR is under 0.5 Ohm and it is also within spec. I'm going to replace the putatively faulty caps next.
I desoldered some caps to check their value out of circuit. The ESR on most of the small ones is very high (5-15 Ohm), even though the capacitance on all but a few is still within spec. The input capacitor's ESR is under 0.5 Ohm and it is also within spec. I'm going to replace the putatively faulty caps next.