Sansui BA-3000 blowing mains fuse following restoration

[jmjrcv]

Missing a -80V rail, one of your +82.8V values is a typo?

The +0.5V reading is on the pin where we're expecting -65V, correct?

 

[BradWil]

ugh darn - yes a typo. First reading should be -82.8VDC - will change the image now. Yes, expecting -65V at the +0.5V reading

EDIT: image changed on #80 - sorry folks! Thanks for pointing this out @jmjrcv

 

[BradWil]

Voltage drop across the F-2508 Zeners as follows - full power

ZD01 14.46VDC
ZD02 14.52VDC

 

[BradWil]

Failed TR04 - reading as diode B-E with C not recognised. Looks like the case has cracked too. This, along with all the other transistors were removed and tested as part of the restoration. I do like to stick to the known problematic only when choosing to replace them, however in this case, I'm going to repalce all the small transistors on the F-2508. I no longer trust them - all have the black silver oxide leads. Shame as the originals are a lot faster but it is what it is. I'll go through them all and research replacements. Will update soonest.

 

[BradWil]

Planned repalcements

2SA850 > KSA1013
2SC1735 > KSC2383
2SC1708 > KSC1845

Agreed?

 

[richardpp]

Failed TR04 - reading as diode B-E with C not recognised.

Good find!

 

[richardpp]

No, definitely not removed. They were not present on my board. Can you clarify what pins they connect to and the orientation please?

Looks like 3 diodes across 2 transistors.

Will 1N4004 suffice?

Here are the diode locations, 1N4004 should be fine.

 

[BradWil]

Here are the diode locations, 1N4004 should be fine.

View attachment 3832470View attachment 3832482

Fantastic! Thank you so much. If they are going to add some resilience to the regulated PS, I'm all in

 

[richardpp]

No worries, I think you found your problem. It should be smooth sailing from here on in.

 

[sregor]

Been following - one thing is bugging me. To get the +66 volts you have to have -66 volts at the end of R16 (10 ohms). That point is a continuous trace to the -66 volt output pin with the missing voltage. Either there is a crack in the trace, or a bad connection at the -66 volt output pin.
If it's been resolved, I must have missed it.

 

[BradWil]

F-2508 reg PS with outputs disconnected and both driver amps removed, passed DBT.

Full power voltages below. Seems the supply to the driver amps is +67.3VDC, sound right?



F-2508 transistors as follows. The only failure was TR04 2SC1735 but I went ahead and replaced all the small BJTs with 'black legs' with modern items



Now to test 1 driver amp at a time, with outputs connected and of course on DBT ...

 

[richardpp]

Correct your last reading to -67.3V??

 

[BradWil]

problem is, it's +67.3V and getting no bias again on the power amp

 

[richardpp]

!! it should be negative reading. Recheck your transistor orientation.

 

[richardpp]

Did you test TR10?

Did you install the diodes, if so remove them.

 

[BradWil]

Didn't install the diodes. All small BJT's installed correctly, double-checked to SM layout and silkscreen. Will remove and test TR10 and TR05 again as the failure of TR04 may have knocked one of them out

 

[BradWil]

TR10 and TR05 test fine

 

[BradWil]

F-2508 small BJTs ...

 

[BradWil]

Will pull TR11 and TR16 tomorrow to test ... ugh

 

[richardpp]

Apologies for jumping to conclusions and thinking it was sorted. TR04 being in the negative line and faulty made sense to me.

A voltage reading at the collector and emitter of TR10 might help, when you're next at the bench.