A-1000 Rehab: DC voltage in signal path SOLVED

You're going to slap me (I just kicked myself anyway). I think it's those diodes, I totally forgot / neglected to replace them. Adding them to next order. To be continued.
 
Well, new diodes are in at D105 & D105, new zener in at D109 & D111 ... still in protection, now I'm seeing -17V on one channel and -24V on the other (through DBT). Dangit.
 
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D105 is an 1N4148 (yes, it is, I just put it in, this is confusing)
D105A: -23.5
D105C -13.3

D109 is a 9.1V zener (yes, it is, I just put it in, this is confusing)
D109A: -23.5
D109C -24

D107 is an 1N4148
D107A: -23.5
D107C: -24.2

D111 is a 9.1V zener
D111A: -23.5
D111C:-14.2

Anyway, D105A should be 2.4V and comes from TR111C; D111C should be 1.1V and comes from TR113C. Time to look at TR111 & TR113.

(This is through the DBT btw so high voltage rail is at ~42V.)
 
Hmmm ... looking at TR115

TR115E: -12.7
TR115C: -6
TR115B: -13.3

Following TR11C upstream ...
C191+: 41V
C191-: 41V

R349: 1.9k in circuit

D171 tests ok in circuit

Hmmm, what are the voltages at C193?
C193+: -13.5
C193-: -41.2

Edit: Hmm. They don't match. But TR127/TR129 should be seeing close to the high rail power supply at C ... so C193 shouldn't be dropping that much voltage I don't think.
 
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... I wonder if C127 popped. It's got 50V across it! It's only rated at 6.3V. C129 only has 7.5V across it (hmm, still a hair high).

Edit: Pulled it out, the bottom exploded lol yeah this is definitely what popped
 
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Looking for reasons why there might be 50V across C127 ... TR127 looks way off

TR127E: 41.2
TR127C: 40.9
TR127B: 40.4

It's counterpart TR129 also looks wacky

TR129E: -13.3
TR129C: -13.4
TR129B: -1.9
 
avionic said:
If its bulging any its bad.50v is definitely a problem.
Click to expand...

So the Nichicon FG that went in popped first, was bulging on the top. This was plugged into the wall. I put the original cap back in before I started diagnosing through the DBT. This one has also popped, the bottom exploded! So I just pulled it out. I guess I'll leave C127 empty until I can get rid of that 50V.
 
That 50V difference is the result of TR115E being at -12.7V and TR127B being at 40.4V ... that's 53.1V across C127. Bad.

With C127 out, TR127B is still at 40V. I now suspect TR127 of a C-B short, so I will pull and test.
 
avionic said:
TR127 looks shorted C-E. What voltages are you seeing around TR115?
Click to expand...

Right on. Pulled TR127, it's all kinds of shorted, including C-E. (Checking to see if I have any KSC2383 or KSC3503 around.)

With C127 in place, I measured TR115 earlier:

TR115E: -12.7
TR115C: -6
TR115B: -13.3
 
Put in a KSC2383 ...

TR127E: 41.4
TR127C: 41.0
TR127B: 31.5

Re-measured TR115 (2SC1815) ... (and diode-tested B-E, it is dropping voltage either way ... I have KSC945 on hand, will replace)

Edit: Pulled, tests fine. Going back in.

TR115E: -12.8
TR115C: -5.8
TR115B: -12.8
 
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avionic said:
Do you have a 100µf at 50 or more volts for trouble shooting

KSC945C would be a better fit.
Click to expand...

Thanks, yeah they are KSC945C. I tried one for the heck of it, no significant change to voltages. Put the original 2SC1815 back in.

Will see if I have a 100uF / 50V cap I can use. Edit: nope. Closest I have is 35V.

Edit: Attached a pic of the popped cap, for giggles.

IMG_20191005_120238.jpg
 
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Probably last longer than a 6.3 . Make sure your KSC945C is still good.
 
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The new KSC945C good.

However I pulled and tested TR111, it's open B to C and shorted E to C. I pulled TR113 too, same thing! Open B to C and shorted E to C.

They are a 2SA1144/2SC2704 pair, and I happen to have an MJE15030G/MJE15031G pair on the bench, so I'll toss them in.
 
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Hmm. It's drawing way more current now (bulb much brighter), power supply is down to 23V high rail, 13V low rail.

With that in mind ...

TR111B: 20
TR111C: -3.5
TR111E: 21

TR113B: -23
TR113C: -3.5
TR113E: -21

I'm down to 15V across the solder pads of C127. I tossed that 100uF / 35V cap in, still have 13.5V on + side, -1.8 on - side.
 
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Ok wow I pulled TR129 (2SA1145) and it's open B to E and B to C, but shorted C to E. Put in a KSA1013.

Now ...

TR129E: -1.4
TR129C: -23
TR129B:-1.4

THAT seems ok.

Back to TR127 ...

TR127E: 23.2
TR127C: 23
TR127B: 13.4

NOT ok.

D175 tests OK (in circuit).
 
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