Why was the original 494 replaced?
Reinstall D11 and Q42 and confirm that the Q30 collector voltage is still 0v. 494 out.
Are these 494s from a reputable distributor (not ebay, not amazon...)?
Reinstall D11 and Q42 and confirm that the Q30 collector voltage is still 0v. 494 out.
Are these 494s from a reputable distributor (not ebay, not amazon...)?
Replaced due to this protection led issue. I started this process trying to figure out why I had the voltage on Pin 1 (Q30 Collector).
Digikey sourced 494's.
Still 0v with items re-installed.
Digikey sourced 494's.
Still 0v with items re-installed.
Lift pin 1 so it doesn't go back into the socket and plug the 494 back in.
Carefully power up to make sure that the amp doesn't do anything strange. If the amp doesn't like pin 1 lifted, solder a high-value resistor (100k) between the lifted pin and a ground VERY close to pin 7 of the 494.
Do you still have 0v on the collector of Q30?
What's the voltage on the lifted pin 1.
Carefully power up to make sure that the amp doesn't do anything strange. If the amp doesn't like pin 1 lifted, solder a high-value resistor (100k) between the lifted pin and a ground VERY close to pin 7 of the 494.
Do you still have 0v on the collector of Q30?
What's the voltage on the lifted pin 1.
This appears to show that the 5v isn't coming through pin 1. The meter likely has a 10M ohm input impedance and that's enough to pull the DC voltage down to 1v.
Is there any leaked electrolyte or any other contaminant on the board?
Is there any leaked electrolyte or any other contaminant on the board?
What's the DC voltage on terminal 3 and on the collector of Q30?
You can't always see contamination. Take your scope probe (scope set to 1v/div and 2ms) and touch it to the board and move it around. Does the trace ever deflect? It shouldn't.
Do this on both side of the board.
You can't always see contamination. Take your scope probe (scope set to 1v/div and 2ms) and touch it to the board and move it around. Does the trace ever deflect? It shouldn't.
Do this on both side of the board.