Ok from Perry's diagram I was totally numbering TL494 wrong. I thought a bit of glue from the nearby transformer was the pin 1 designation. It appears that this chip only has a band to indicate top which puts pin 3 as NC.
Here's the measurements corrected.
Uo1 TL494
pin 1 0v
pin 2 4.9v
pin 3
pin 4 0V -transformer hiss
pin 5 1.4v
pin 6 3.6v
pin 7 0v
pin 8 11.8v
pin 9 4.7v
pin 10 4.7v
pin 11 11.8v
pin 12 12.7v
pin 13 4.9v
pin 14 4.9v
pin 15 4.9v
pin 16 0v
The soft start board has 5 pins
pin 1 10.9v (this pin actuates the relay)
pin 2 11.8v
pin 3 0v (transformer hiss)
pin 4 0v -gnd
pin 5 12.7v
Here's the measurements corrected.
Uo1 TL494
pin 1 0v
pin 2 4.9v
pin 3
pin 4 0V -transformer hiss
pin 5 1.4v
pin 6 3.6v
pin 7 0v
pin 8 11.8v
pin 9 4.7v
pin 10 4.7v
pin 11 11.8v
pin 12 12.7v
pin 13 4.9v
pin 14 4.9v
pin 15 4.9v
pin 16 0v
The soft start board has 5 pins
pin 1 10.9v (this pin actuates the relay)
pin 2 11.8v
pin 3 0v (transformer hiss)
pin 4 0v -gnd
pin 5 12.7v
With the NPN installed at Q75 the circuit started to make more sense. I noted that the base of Q75 is driven from D74 but I wasn't getting the signal through it. With the rising voltage fed into D74 I figured that it was a zener that allows Q75 to activate above a certain threshold.
I pulled D74 and swapped a though hole part 9V zener and voila amp back working again.
9V seems like a good value but if you could tell me the proper value I'd appreciate it. I've got a good selection of through hole zeners and its easy enough to use them as a SMT part.
I pulled D74 and swapped a though hole part 9V zener and voila amp back working again.
9V seems like a good value but if you could tell me the proper value I'd appreciate it. I've got a good selection of through hole zeners and its easy enough to use them as a SMT part.
