Good evening,
I was hoping someone could tell me why Q41 would be missing it's base voltage? See attachment. If I short emitter/collector, the PS and it's rails work as expected. I thought it might be standby pulling the base down to ground, but I bypassed that and the ~5V still does not appear. Thanks!
I was hoping someone could tell me why Q41 would be missing it's base voltage? See attachment. If I short emitter/collector, the PS and it's rails work as expected. I thought it might be standby pulling the base down to ground, but I bypassed that and the ~5V still does not appear. Thanks!
Attachments
Where does it come from? R41 R43 R47. Trace along there and see where voltage is lost.
5V? Why do you expect 5 V rather than about 18.7 V?
I hope you didn't blow up anything when you shorted the collector and emitter.
I hope you didn't blow up anything when you shorted the collector and emitter.
It's not clear to me what 'missing the base voltage' means. Is it 0V? does it float? Is it the same as Ve?
Jan
Jan
Good afternoon,
I ended up getting this supply working by removing Q43 (tests good, not shorted). I'm going to need to investigate the protection/standby circuit which may be faulty or actually functioning depending on how you look at it. lol. My bypass, apparently, was ineffective. I was reluctant to remove Q43 because this PCB has been cooked and some of the pads are lifting very, very easily. Not a very high quality PCB unfortunately. This whole circuit has seen a lot of action, it runs pre-amp circuitry set in class A mode.
The protection/standby "circuit" uses a UPC1237HA. I don't know how reliable these are long term, but they seem to be selling well on that auction site.
I ended up getting this supply working by removing Q43 (tests good, not shorted). I'm going to need to investigate the protection/standby circuit which may be faulty or actually functioning depending on how you look at it. lol. My bypass, apparently, was ineffective. I was reluctant to remove Q43 because this PCB has been cooked and some of the pads are lifting very, very easily. Not a very high quality PCB unfortunately. This whole circuit has seen a lot of action, it runs pre-amp circuitry set in class A mode.
The protection/standby "circuit" uses a UPC1237HA. I don't know how reliable these are long term, but they seem to be selling well on that auction site.
Yes, I did check along those and the voltages were present. Regardless, Q41 wouldn't go high.
I got my specs mixed up - I was hoping to see at least 5V in order for Q41 to go high. You're right, it should be much higher. No damage, the supply is now functioning actually.
Yes, let me re-phrase. Base voltage is low or at near zero volts, thus not enabling Q41.
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Sometimes you just have to laugh.
The cathode of D49 was shorted to the junction of R411 and R49 from a very tiny solder blob. This was keeping the base of Q43 high and thus the PSU in direct "standby" so to speak.
This PSU section was from a NAD C 720BEE. Pretty sure somebody else was in there before. Whoops!
Anyway, she sings lovely now! Another piece of junk out of the landfill!
Cheers
The cathode of D49 was shorted to the junction of R411 and R49 from a very tiny solder blob. This was keeping the base of Q43 high and thus the PSU in direct "standby" so to speak.
This PSU section was from a NAD C 720BEE. Pretty sure somebody else was in there before. Whoops!
Anyway, she sings lovely now! Another piece of junk out of the landfill!
Cheers
