The customer decided to call it quits on this one. I'd still like to figure it out, just for the knowledge.
If you have a current limiter, clamp all transistors and remove the two SMD transistors that burned. Power it up. Does the amp draw excessive current?
Does the amp produce audio?
Does the amp produce audio?
I went back to this one. I powered it up like normal and the short light came on. I removed Q419 and powered it through a series resistor. It came on. I did not try sending audio through it. Q419 is tied to Q417. Q417 is connected to the output of the rectifier diode. 417 is tied to R442 and 419 to R447. Both resistors are marked 1001. But their resistance reading are wildly different. 442 measures 1000 ohms while 447 is around 6500 ohms. The last thing I checked was for a short between the speaker terminals and from each speaker terminal to B+ and B- terminals. There were no shorts.
I really don't have a good excuse, to be truthful. So I went back and tried it. It won't power up with a speaker connected. I'm getting 10.8 volts DC Offset with the speaker disconnected and no audio source. I noticed that the cap next to the speaker terminal block is warm after just a few minutes at idle.
Pin 4 should be -15v. I'd expect pin 8 to be nearer to +15v.
What is the output voltage from the two voltage regulators?
What is the output voltage from the two voltage regulators?
It will only power up through the series resistor. 10.8 volts input. The output of the rectifiers and voltage regulators is 10.8v. No rail voltage increase, either. Everywhere I check, it's 10.8 or less.
Yes. There is no negative voltage anywhere. I don't understand that, either. Both power supply FETs have 10 volts on the source and drain. Same with both output FETs. If I probe the positive speaker terminal with the meter grounded to B-, I get 10 volts. Do the same with the speaker negative, 0v. Even though the 'negative rail' has positive 10 volts on it. ??? I'm lost like a ball in high weeds!
Yes. On the power supply FETs center leg and on the 494(I don't remember which pin). But nowhere else that I've seen.
Do you have a drive signal going to the power supply FETs?
Are they driving the power transformer?
Are they driving the power transformer?
My scope was at work, so I didn't check the drive signal. I will tonight when I bring it back home.
Okay, I have a saw tooth signal from the 494 and a drive signal on the gate of the power supply FETs. There is a signal on the input to both rectifiers and DC coming out of them. But today, I'm seeing 8.5 volts on the B+ terminal. But about 11 volts on the rails. Both rails are positive voltage. I still have 11 volts offset on the speaker terminals.
Is the square wave on the center leg of the power supply FETs swinging from ground to 2x the B+ input voltage?