I`m sure this has been discussed here before, so if you know where, please let me know.
The bias and offset is correct, but still, the green light does not come on and something is holding back the output relays from kicking in.
The bias and offset is correct, but still, the green light does not come on and something is holding back the output relays from kicking in.
Given low offset and good bias, it seems that the amps themselves are likely OK and that something in the protection circuitry has failed.
I would disconnect any valuable speakers as a precaution, and test with speakers that are expendable and/or install 100R in series for protection.
IC101 pin 7 appears to monitor for excessive DC bias at the input. Ideally, it should be 0V, but 100 mV or less would be OK. It's probably not the culprit, but I would check it in both channels to rule it out.
There's a smallish power supply in the lower right corner of the schematic. There's no voltage noted, but it's probably anywhere between 10V and 20VDC across both C011 and C012. Would you measure these voltages and report just to know what they are?
I believe R002 and C013 establish the turn-on delay. If all were well, C013 voltage would rise to more than about 5V and drive and Z001 and Q104 into conduction. It you don't find C013 rising, the Q001/Q002 network may be intervening. Try installing a temporary short across C014/R005. This will disable fault sensing paths from both channels. The relays might activate. If this is the case, next we'll try to find the offending path. But let's pause here to see what you observe.
Good luck!
I would disconnect any valuable speakers as a precaution, and test with speakers that are expendable and/or install 100R in series for protection.
IC101 pin 7 appears to monitor for excessive DC bias at the input. Ideally, it should be 0V, but 100 mV or less would be OK. It's probably not the culprit, but I would check it in both channels to rule it out.
There's a smallish power supply in the lower right corner of the schematic. There's no voltage noted, but it's probably anywhere between 10V and 20VDC across both C011 and C012. Would you measure these voltages and report just to know what they are?
I believe R002 and C013 establish the turn-on delay. If all were well, C013 voltage would rise to more than about 5V and drive and Z001 and Q104 into conduction. It you don't find C013 rising, the Q001/Q002 network may be intervening. Try installing a temporary short across C014/R005. This will disable fault sensing paths from both channels. The relays might activate. If this is the case, next we'll try to find the offending path. But let's pause here to see what you observe.
Good luck!
Hello and thanks for the information. I did in fact change all the capacitors in the smallish power supply as I thought that's where the problem would be.
And in fact the green light came on for some time, 3 - 5 minutes. And then shut down the relays. THEN I noticed that the right channel wasn't as warm as the left channel. So I checked the bias on that channel and now it was gone. 😳 So I guess thats probebly where the real problem is. 🤔 Some sort of unstable whatever.... The voltage supply is ok.
And in fact the green light came on for some time, 3 - 5 minutes. And then shut down the relays. THEN I noticed that the right channel wasn't as warm as the left channel. So I checked the bias on that channel and now it was gone. 😳 So I guess thats probebly where the real problem is. 🤔 Some sort of unstable whatever.... The voltage supply is ok.
BTW, what are the main rail supply voltages?
The amp is basically a giant opamp on steroids. To get a quick look at bias, I suggest reporting the voltages at + input (gate of Q103), - input (gate of Q104), and the output at R144,R133, and the voltages at all pins of IC102.
Do you have a scope? If so looks for possible oscillation using x10 probes.
If you have any symptoms of intermittencies, try tapping/prodding with a wooden chopstick or similar.
The amp is basically a giant opamp on steroids. To get a quick look at bias, I suggest reporting the voltages at + input (gate of Q103), - input (gate of Q104), and the output at R144,R133, and the voltages at all pins of IC102.
Do you have a scope? If so looks for possible oscillation using x10 probes.
If you have any symptoms of intermittencies, try tapping/prodding with a wooden chopstick or similar.
IC102
1 = 1,38V
2 = 297mV
3 = 403mV
4 = 714mV
8 = 15,5V
Gate of Q103 = 2,38V
Gate of Q104 = 3,14V
Output at R144, R133 = 75V
All measured to ground. Main rail supply voltages +/ - 76V
1 = 1,38V
2 = 297mV
3 = 403mV
4 = 714mV
8 = 15,5V
Gate of Q103 = 2,38V
Gate of Q104 = 3,14V
Output at R144, R133 = 75V
All measured to ground. Main rail supply voltages +/ - 76V
Pin 4 of IC102 should be about -15VDC. That's probably not the only problem, but maybe fixing that issue will have other happy results. Look for presence of ~ -76VDC at R148. It should likely be warm. Might be open resistor, open trace, etc.
Good luck.
Good luck.
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