Sansui BA-3000 blowing mains fuse following restoration

[richardpp]

but you still have negative 66V on the collector of TR10, correct?

 

[BradWil]

but you still have negative 66V on the collector of TR10, correct?

yes

 

[BradWil]

Readings F-2508 unloaded with drivers and outputs removed. This is before I repalce TR05, TR10, TR11 and TR16.
So the last pin that should be reading -67.2VDC, is now reading +VE milivolts. No amount of wiggling and proding seems to alter it.

 

[BradWil]

Discovered an invisible hairline fracture on the F-2508 connection to the last Molex pin. You can see the pad lifted. Repaired. Hey presto, -67.2VDC returns - no changes made to F-2508.
Now onto drivers ...

 

[BradWil]

WE HAVE BIAS!

 

[richardpp]

Well done!

 

[BradWil]

The hairline crack was really not obvious but I really should have looked harder at the Molex connections to F-2508. With so many removals and installations of that board, it's no wonder the joints weakened. Beer time

 

[BradWil]

Both driver amps dialled in perfectly

 

[BradWil]

thanks everyone for your help and perseverance. Gotta love AK

 

[richardpp]

Discovered an invisible hairline fracture on the F-2508 connection to the last Molex pin. You can see the pad lifted.

Just for my own sense of sanity, was that your test point when measuring voltage on R16 ?

 

[BradWil]

Just for my own sense of sanity, was that your test point when measuring voltage on R16 ?

Collector of TR10 to that last Molex pin

 

[Hyperion]

Well Done Brad! and Richard!

That is one peculiar fault, flips to a positive voltage when open circuit.

 

[richardpp]

Collector of TR10 to that last Molex pin

Ok thanks, that explains a lot.

 

[richardpp]

Well Done Brad! and Richard!

That is one peculiar fault, flips to a positive voltage when open circuit.

Thankyou John, that was some weird ride!

 

[BradWil]

Well Done Brad! and Richard!

That is one peculiar fault, flips to a positive voltage when open circuit.

Thanks John and Richard! So much going on tracing the source of output failure. Some silly errors on my behalf during the diagnosis ! Lack of sleep. Anyway, problems solved. Thanks for hanging in there with me

 

[BradWil]

Preventative maintenance

Removed the Molex connector from F-2508. Another pin was close to losing connectivity. Scraped back the solder mask added wire where needed, cleaned up and resoldered.

Next: need to replicate changes made to the right driver amp, over to the left. Will list all semi subs across the regulated PS and drivers.

 

[BradWil]

F-2508 semis

TR15 2SA850 was replaced with KSA1013 during the initial restoration, as it had a failed, unknown transistor in that position.

TR04 2SC1735 was the root cause of the recent issue. It looks like it had failed (reading as a diode), case cracked, causing the negative 35V rail to fail on the regulated PS. It likely caused a spike to the drivers, knocking out 2 x 2SC1735 and 2SA908 Sanken outputs.

The negative 66V rail issue was down to a cracked trace to the Molex on F-2508. It happened during trobleshooting of the above issue. Over 50 odd years, these Molex connections get a lot of strain at every service and the tracks and pads weaken. It's an area I will spend more time on in future.

All outputs were replaced with robust On Semi MJ21195G and MJ21196G's.

I decided to replace all the small transistors that were suffering from silver oxide leads as per TR04 2SC1735, to be safe.

Location DES CAT Type Value Volt/LS PWR/hFE RPL Value RPL Volt RPL PWR/hFE
F-2508 PS D01 DIO DUAL VD1212 2 x x1N4148
F-2508 PS D02 DIO DUAL VD1212 2 x x1N4148
F-2508 PS ZD01 DIO ZENER RD-15E 15 0.5 1N4744 15 1W
F-2508 PS ZD02 DIO ZENER RD-15E 15 0.5 1N4744 15 1W
F-2508 PS TR01 TRA PNP 2SA850 BCE 132 KSA1013 238
F-2508 PS TR02 TRA NPN 2SC1735 BCE 76 KSC2383 200
F-2508 PS TR03 TRA PNP 2SA850 BCE 99 KSA1013 241
F-2508 PS TR04 TRA NPN 2SC1735 BCE FAIL KSC2383 213
F-2508 PS TR05 TRA NPN 2SD381 BCE 44 RETAIN
F-2508 PS TR06 TRA NPN 2SC1735 BCE 50 KSC2383 210
F-2508 PS TR07 TRA NPN 2SC1708 BCE 328 KSC1845 398
F-2508 PS TR08 TRA PNP 2SA847 BCE 269 KSA992 509
F-2508 PS TR09 TRA PNP 2SA850 BCE 137 KSA1013 246
F-2508 PS TR10 TRA NPN 2SD381 BCE 54 RETAIN
F-2508 PS TR11 TRA NPN 2SD382 BCE 55 RETAIN
F-2508 PS TR12 TRA NPN 2SC1735 BCE 87 KSC2383 205
F-2508 PS TR13 TRA NPN 2SC1313 BCE 524 KSC1815 168
F-2508 PS TR14 TRA PNP 2SA726 BCE 390 KSA992 335
F-2508 PS TR15 TRA PNP 2SA850 BCE FAIL KSA1013 241
F-2508 PS TR16 TRA NPN 2SD382 BCE 58 RETAIN

 

[richardpp]

TR04 2SC1735 was the root cause of the recent issue. It looks like it had failed (reading as a diode), case cracked, causing the negative 35V rail to fail on the regulated PS. It likely caused a spike to the drivers, knocking out 2 x 2SC1735 and 2SA908 Sanken outputs.

There were a couple of faults on F-2508, i.e., the faulty transistor at TR04 and the broken trace at the connector block.

We can't be sure what symptoms TR04 was causing because it was replaced before the broken trace was repaired. The fact we got voltage after it was replaced may have been a coincidence to do with removing and refitting the board. Regardless, the reason the there was no voltage on F-2512 was because of the broken trace on F-2508.

Edit: So even though there were a couple of identifiable faults I don't think it's definitive.

Anyway, onward and upward.

 

[BradWil]

Understand. Pre-drivers being replaced but I'm reluctant to replace the high speed drivers that test fine, as the replacements are much slower. What are your thoughts on this?

 

[BradWil]

Actually, I have some NOS KSC2690, KSA1220 that I could use for high speed drivers. They are 175mhz