Not my board, it's @richardpp
Sansui BA-3000 blowing mains fuse following restoration
- Thread starter BradWil
- Start date
So to recap:
-Unit was working after restoration for a couple of weeks.
-Main fuse blew.
-Found R01 (3.9Ω 7W) had gone high - replaced.
-Transformer and bridge rectifiers checked out fine.
-Outputs TR708, TR702, TR710 failed on right channel (front heatsink).
-Now unable to bias either driver board, even with one having only a recap and all original semis
I will get the +/- 35V regulated measurements soonest.
-Unit was working after restoration for a couple of weeks.
-Main fuse blew.
-Found R01 (3.9Ω 7W) had gone high - replaced.
-Transformer and bridge rectifiers checked out fine.
-Outputs TR708, TR702, TR710 failed on right channel (front heatsink).
-Now unable to bias either driver board, even with one having only a recap and all original semis
I will get the +/- 35V regulated measurements soonest.
richardpp
Well-Known Member
Ah, yes, sorry about that! I try and use thumbnails instead if not my thread! And yes, you're right the solder is removed, it was a work in progress!
Gents, you've been a real help, thanks
Sorry this one's a headscratcher!
Plan for tomorrow:
Measure voltages on all regulated PS connections to the connect board with regulated PS in place.
Remove the protection board and unsolder and test each transistor. Replace any failed ones.
Sorry this one's a headscratcher!
Plan for tomorrow:
Measure voltages on all regulated PS connections to the connect board with regulated PS in place.
Remove the protection board and unsolder and test each transistor. Replace any failed ones.
A little math: ~65V across R16 which measures 10 ohms --> over 400W dissipated by R16. I think you would have noticed this
The implication is there's more happening here than meets the eye.Agreed, it's not always a direct path to the solution.
I would spend some time reviewing your assumptions and any work that's been done in repairing the -65V supply.
Among other things you could check for the regulator:
- Is ZD01 oriented correctly? Is there 15V across the zener when the regulator is in operation?
- What's the voltage at the base of TR08 (should be about -15.7V)? Also check the node voltage where R22, R26 and R27 meet, should be a little lower than -15.7V.
- Are you comfortable that the transistor replacements are installed correctly?
- There appears to be a pad missing in the middle of the board in the photo. Is the fix recent, it looks close to another pad but that may be just the angle?
- C09 seems like it should be replaced with a higher voltage rated part. The schematic shows 16V rating and it operates at 15V 100% of the time. Probably worth testing this cap.
All was working for weeks following restoration. Very comfortable that any replacement semis are installed correctly. I had only replaced known problematic...
I appreciate your feedback and I will indeed measure the zener voltage drops and other areas you suggested.
Yes, that pad was repaired during resto. Comfortable will all joints and pads on all boards.
I think testing each transistor on the regulated PS and protection boards may reveal some failures.
Firstly, I'm going to measure the outputs of the regulated PS, to see where we are.
Continuing digging
I appreciate your feedback and I will indeed measure the zener voltage drops and other areas you suggested.
Yes, that pad was repaired during resto. Comfortable will all joints and pads on all boards.
I think testing each transistor on the regulated PS and protection boards may reveal some failures.
Firstly, I'm going to measure the outputs of the regulated PS, to see where we are.
Continuing digging
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Just a note to say that after refreshing with new, the installed semiconductors in that design of regulator (previously working), I have found they sometimes simply won't work (just won't start up). I make small changes to the selection of transistors used and bingo - working again. I retest removed semi's = all good. I have no idea why that happens. 
Thanks for that. Not at my bench but from memory the only transistors replaced were 2SA726, 2SC1313 and one of the 2SA850's had a questionable unknown replacement in the past. The zeners and the evil VD1212's were replaced too. I do have a feeling that this is the board to focus on until I can get some sensible voltages to the drivers.
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Will do. Think I'll disconnect the outputs to be safe too
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richardpp
Well-Known Member
I notice that your F-2508 board doesn't have the protection diodes for TR09 and TR10 often seen on the conductor side of the board as shown below. Were these removed at restoration? They would help protect the transistors for any voltage spikes that might have happened when the output transistors shorted.
very cool, thanks. Unknown origin unfortunately, apart from QLD, Australia but I'd say it was imported from the EU given the 220V mains setting when it hit the bench.
24607 indicates July 1976 production, which was basically the end of production of the BA-3000, so a very late model. Thanks!
Here are the voltages from F-2508 regulated power supply. Taken from the back of F-2512 connector, very carefully using a needle lead. All readings on full power to GND.
Seems like the F-2508 is failing to produce any voltage on the last connector, which traces to both drivers on F-2512.
Will investigate TR08 and surrounding resistors first.
At least the protection module can remain in situ (for now) - pain to remove and reinstall multiple times!
EDIT: I had incorrectly listed the first voltage as + when it should have been - oops!
Seems like the F-2508 is failing to produce any voltage on the last connector, which traces to both drivers on F-2512.
Will investigate TR08 and surrounding resistors first.
At least the protection module can remain in situ (for now) - pain to remove and reinstall multiple times!
EDIT: I had incorrectly listed the first voltage as + when it should have been - oops!
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