I don't see anything that can drive it other than Q42.
What's the DC voltage on it's pads with it out of the circuit?
What's the DC voltage on it's pads with it out of the circuit?
2.2v at the led pads when bench supply is at 14.4v.
So assuming 494 pin 3 would turn off the IC off at 3.3 volts - what can I do to turn that led on when pin 3 reaches 3.3 volts. I tried increasing R34 but that results in a gradual turn on not a switched on and off which is what I need.
Or, I just disconnect it and say, "your amp works fine but your protection led is not going to work."
So assuming 494 pin 3 would turn off the IC off at 3.3 volts - what can I do to turn that led on when pin 3 reaches 3.3 volts. I tried increasing R34 but that results in a gradual turn on not a switched on and off which is what I need.
Or, I just disconnect it and say, "your amp works fine but your protection led is not going to work."
Desolder the striped end of D11 and confirm that the collector of Q30 goes to very near 0v.
Desolder one terminal of R56 and measure the resistance across the thermistor. What's the value.
Reconnect R56 and heat the thermistor to confirm that the voltage on its low-voltage leg (connected to R56) rises as it's heated.
Desolder one terminal of R56 and measure the resistance across the thermistor. What's the value.
Reconnect R56 and heat the thermistor to confirm that the voltage on its low-voltage leg (connected to R56) rises as it's heated.
Desolder the striped end of D11 and confirm that the collector of Q30 goes to very near 0v.
Stays at 5v
Desolder one terminal of R56 and measure the resistance across the thermistor. What's the value.
103K
Reconnect R56 and heat the thermistor to confirm that the voltage on its low-voltage leg (connected to R56) rises as it's heated.
Yes voltage increases with heat
Stays at 5v
Desolder one terminal of R56 and measure the resistance across the thermistor. What's the value.
103K
Reconnect R56 and heat the thermistor to confirm that the voltage on its low-voltage leg (connected to R56) rises as it's heated.
Yes voltage increases with heat
With C33 out Q30 collector:
12v on bench supply it is at 1.7v
14.4 on bench supply it is at 3.7
The protection light is not lighting. It lights if I put it back in of course.
12v on bench supply it is at 1.7v
14.4 on bench supply it is at 3.7
The protection light is not lighting. It lights if I put it back in of course.
Confirm that R57 is within tolerance.
C33 may be exhibiting dielectric absorption. Replace it and confirm that the voltage on the collector of C30 not longer remains at 5v.
What's the DC voltage on the terminal of the thermistor that varies with temperature (while it's at room temperature).
C33 may be exhibiting dielectric absorption. Replace it and confirm that the voltage on the collector of C30 not longer remains at 5v.
What's the DC voltage on the terminal of the thermistor that varies with temperature (while it's at room temperature).
R57 is good.
THermistor at 1V at room temp.
Replaced C33 with new one from donor board. Light back on.
THermistor at 1V at room temp.
Replaced C33 with new one from donor board. Light back on.
I don't see what's driving voltage to that part of the circuit. Pin 1 of the 494 shouldn't be able to do it.
Is that a new IC?
Pull the 494 and see if the voltage on the collector of Q30 goes to 0v.
Is that a new IC?
Pull the 494 and see if the voltage on the collector of Q30 goes to 0v.
It goes to .5V with the chip out.
494 back in, at remote pin1 slowly builds to 5v. Takes about 20 seconds.
494 back in, at remote pin1 slowly builds to 5v. Takes about 20 seconds.
Is that a new 494?
The input impedance of the error amp on pin 1 is so high that it shouldn't be able to drive voltage to the circuit.
Reinstall Q30. Does it affect the voltage on its collector?
The input impedance of the error amp on pin 1 is so high that it shouldn't be able to drive voltage to the circuit.
Reinstall Q30. Does it affect the voltage on its collector?