Seems my process is try to fix one thing, break another. I was probing a transistor for another issue and shorted two leads. Big pop and my 5V rail blew. I am still new to this and making mistakes. Fortunately, this is not my day job.
After replacing the fuse, the power was still off. Can't remember how "off"; that was a few fixes ago. I followed the power supply troubleshooting guide in the SM and disconnected everything it said to disconnect except the calibrator circuit. Figured that was not the source of my problem and didn't want to desolder more parts than necessary as each one is a future possible problem.
All other power rails are good, in case that matters. It is only 5V rail that is off.
I have replaced Q4312. Twice. (I forgot the mylar insulator between the transistor "tab" and the mounting post, shorting collector to ground and blowing a fuse, so I suspected the transistor also might be bad and replaced again.) I have also temporarily swapped out Q4311 and U4206 (see parts below in case it matters). Since the new parts did not change result, I reverted to the originals.
With Q4312 removed, I verified resistance of R4422 and R4325.
Also, with Q4312 removed, I also removed Q4311, turned unit on and measured voltage at 5V rail. It was 0.7V. Just wanted to make sure some other circuit was not affecting the power rail.
Right now the 5V rail is 8V, not 5V.
My DC voltage readings are as follows:
Part No Expected Actual Comment
Q4312 collector 9.3V 10.2V This is input from filter cap. The higher value is probably due to the 110V vs. 120V line in. All the other power rail inputs from filter caps are high, too.
U4206 pin 3 5.0V 5.01V V+ input
U4206 pin 4 -8.0V -8.01V Vee
U4206 pin 8 15.0V 15.0V Vcc
U4206 pin 1 6.7V 1.8V Channel 1 out (bad)
U4206 pin 2 5.0V 8.05V V- input and also the 5V rail output (bad)
With Q4312 collector at 10.2V and base at 1.8V, it should turn on. However, with emitter at 8V (after R4325), it seems that it can't be turned on. This would seem like an open between collector and emitter and voltage drop is just due to the two resistors.
Having replaced this transistor twice , though, it doesn't seem like the transistor is the issue. But if it isn't this transistor, what else could cause the voltage to be too high?
Finally, in case it matters, my replacement/swap parts are:
Part no Original Replacement
Q4312 SPS3414/MPS-U45 CEN-U45
Q4311 S038487 2N2222
U4206 MC1458N NE5532
Any suggestions for what check, likely suspects, etc.?