Sorry it appears I got which pin is which backwards again... I was measuring the voltage on the square tab. The round tab (pin 3) is reading -11.45 volts.
On this amp, there is a mix or right and wrong transistor layouts on the board. The square pad is always supposed to be pin 1 but that's not the case with this board. For this board, look at the face of the component with the text upright and the left pin is pin 1.
All of the readings for the points you measured seem to be OK. Reinstall the 6488 to see if it still powers up.
All of the readings for the points you measured seem to be OK. Reinstall the 6488 to see if it still powers up.
Rail voltage did not drop after installing 6488.
My power supply is not adjustable for voltage.
Voltage across that diode is +20.94 volts
My power supply is not adjustable for voltage.
Voltage across that diode is +20.94 volts
Can you read the number on the diode?
On the other board?
I think the regulated voltage for these amps is about 18v. If the diode in your amp is the original and is a 21v diode, there may have been different versions.
On the other board?
I think the regulated voltage for these amps is about 18v. If the diode in your amp is the original and is a 21v diode, there may have been different versions.
Had to pull it to read it. It is a 1N4748A. It is a 22v diode. It is the original diode. The other board has the same diode in it.
OK.
So, what's the difference from the amp as it is now and when it would not power up and pin1 of the 494 was pulsing. Is it only that the two protection circuit transistors are out of the circuit?
So, what's the difference from the amp as it is now and when it would not power up and pin1 of the 494 was pulsing. Is it only that the two protection circuit transistors are out of the circuit?
Yes, the two protection transistors are out of the circuit and the jumper on pins 4 and 7 on the 494 is still in place.
See what happens if you remove the 4-7 jumper with the protection circuit transistors out of the circuit.
Reinstall the protection circuit transistors, one at a time to see if either prevents the amp from powering up normally.
I'm getting ready to put it back in the heat sink and clamp it down. I'll let now how it preforms shortly.
Amp worked perfectly for about 8 minutes then shut down. Monitored it for anything heating up. Nothing seemed to be getting hot. Warmest thing on the board was the Transformer at 102 degrees Fahrenheit.
Took the following readings after it shut down:
Opto:
Pin 1: +1.16
Pin 2: -0.00
Pin 3: -0.019 to +0.147 pulsing
Pin 4: +12.14
Pin 5: +12.32
Pin 6: +12.80
Small transistor (MPSA06 behind opto):
leg 1: +11.49
leg 2: +12.14
leg 3: +12.32
Thermostat:
+ 11.48 on both
494:
Pin 1: 0.00
Pin 2: +0.49
Pin 3: +0.49
Pin 4: +4.32
Pin 5: +1.48
Pin 6: +3.42
Pin 7: -0.00
Pin 8: +11.49
Pin 9: 0.00
Pin 10: 0.00
Pin 11: +11.49
Pin 12: +11.49
Pin 13: +4.92
Pin 14: +4.93
Pin 15: +0.49
Pin 16: 0.00
Just checked it again. Amp is intermittently powering up now.
Took the following readings after it shut down:
Opto:
Pin 1: +1.16
Pin 2: -0.00
Pin 3: -0.019 to +0.147 pulsing
Pin 4: +12.14
Pin 5: +12.32
Pin 6: +12.80
Small transistor (MPSA06 behind opto):
leg 1: +11.49
leg 2: +12.14
leg 3: +12.32
Thermostat:
+ 11.48 on both
494:
Pin 1: 0.00
Pin 2: +0.49
Pin 3: +0.49
Pin 4: +4.32
Pin 5: +1.48
Pin 6: +3.42
Pin 7: -0.00
Pin 8: +11.49
Pin 9: 0.00
Pin 10: 0.00
Pin 11: +11.49
Pin 12: +11.49
Pin 13: +4.92
Pin 14: +4.93
Pin 15: +0.49
Pin 16: 0.00
Just checked it again. Amp is intermittently powering up now.
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