Need some help fixing vintage RCA amp

[Pityocamptes]

Q41a -
E 1
B 7
C 0


Q41b -
E .9
B 6.9
C 0

[eccerr0r]

something wrong with emitter voltage but the schematic's too confusing for me to read. "145" or port 9 (probably the subassembly pin 9)? goes to both but I don't see where it goes, must be something tying this transistor to the positive rail but haven't found it yet... If your measurements are right, the transistor's in deep cutoff and no sound is going through.

[redwire]

OP, you've made a mess by adding some mistake replacing the caps, that killed the working channel. Again, do a close visual of your soldering and the new cap's polarity etc - look for mistakes.
If you can't find any, then carefully measure the voltages at each transistor and post here.

Q41a, Q41b voltages show there is a problem after them. They are sitting there getting nothing from the output stage Q403, and their E-B junction is in zener avalanche.

I'm wondering about the use of Os-Con's, if they are a concern. C406, C407 were odd black plastic 200uF 10V electrolytics in the feedback path which is important.
In 1974 they did not have polymer caps, which have high leakage current and sound not great. They are not an audio coupling cap.
What parts did you use there and why don't you ask us first at the forums for Capacitor Experts known as Badcaps lol.

[Pityocamptes]

I now I f''d up replacing all the caps at once. The funky black caps says 1068-7336 / 1470494-28 / 200uf 10v cap has + and - markings. I received my oscope and have these readings, I dont know if they help or if you can help me where to start. Thanks again.

oscope set on AC, 10x, 100ms

147 - sine wave 1.31v
135 - sine wave 1.8v
149 - half sine .12v
137 - half since .24v

coll Q41a - .01v no apparent wave
coll Q41b - .05v no apparent wave

coll Q42a - .07v no apparent wave
base Q42a - .05v no apparent wave

coll Q42b - .05v no apparent wave
base Q42b - .05v no apparent wave

base Q404 - .01v
base Q402 - .05v

pos C416 - .03v
neg C416 - .04v

pos C417 - .03v
neg C417 - .02 v



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[eccerr0r]

Well actually the problem was showing with the DMM only and no oscope was needed because we measured Q41a/b were deep cutoff/reverse breakdown and this needs to be fixed. As far as I can tell, again, I don't understand node 145. and how it's pulled up to the high side, and this is necessary to get proper biasing to work...

Actually do you have a clear track photograph somewhere? Are the two 145's for both channels hooked up together on the board, and what else is it hooked up to, other than R41a R41b R412 R443 R411 and R442? Could the only source of the bias depend on the midpoint voltage of the output drivers?

[redwire]

SAM'S Photofact have "CircuiTrace" where they've added label nodes on the circuit board pictures. To help people trace things and charge more for their schematics.
The black background square "145" is just point 145 for the pictures, it's on the little board. The round circles are I think pin numbers for the little board. Not seeing much for wire-wrapped terminals.

What about Q44 (driver) DC voltages? Node 153/142 is up?

[eccerr0r]

Correct, but I would assume that 145 should be somehow tied to the positive rail in order for Q41a/b emitter to be at a higher potential allowing the transistor to function in common emitter mode and allow for gain. As mentioned the B-E junction of these transistors are in deep cutoff or reverse breakdown which shouldn't be affected by the downstream Q44a/b. Perhaps something with Q42a/b could cause a problem with Q41A/b... don't know...

[Pityocamptes]

Here are pics of the Transistors between B to E.

[Pityocamptes]

[Pityocamptes]

[eccerr0r]

Don't worry about the scope now. Not helpful because the biasing on Q41a/b is wrong, really wrong - if you measured the voltages properly. Besides, I'm not sure how you hooked up the probes and the waveforms mean nothing without that point of reference.

[Pityocamptes]

Ok so where do I need to start, and do I use the scope or a DVM? Thank you.

[redwire]

A scope is not helpful right now. First, get the amplifier(s) right when it is idling.
OP if you can take DCV measurement on the six transistors, then it can be figured out. You already did for one on both channels Q41a,b. Need to go further downstream.

The amp is sitting there in cutoff. Q41 gets power at (145) from the output stage and it's turned off or shorted. Should be 15.43V at (152).
I asked for DCV on Q44 to troubleshoot further. With Q41 C at 0V, it's off and so is Q42 which should have Q44 full on, Q404 on and some voltage thus at (152 and 145).
Something might be dead at (146 or 153) or even (1).

[Pityocamptes]

Don't worry about the scope now. Not helpful because the biasing on Q41a/b is wrong, really wrong - if you measured the voltages properly. Besides, I'm not sure how you hooked up the probes and the waveforms mean nothing without that point of reference.

To measure from B to E, I put the oscope ground on B and probe tip to E. Is that not correct? Thanks

[Pityocamptes]

Ok let me re-run those values with the VM tool on oscope I got. I will post the values on what you indicate above. Thx

[eccerr0r]

Actually now reading the other forum, that's what he did with the scopemeter though it wasn't clear since the wording wasn't in the same post as the photos... Still not clear whether they are probed the same way (which probe is on B and which is on E) - by the looks of it, all 8 transistors are cutoff... minus Q41b which seems to be have noise on it.

Ideally measure B-E with red on B, black on E with DVM. A properly biased transistor will have around .7V when you measure a NPN and -.7V when you measure a PNP.

Also unclear if you got the formerly working channel working again, so we're looking for two gremlins?

[Pityocamptes]

Thanks. I will probe those with a dvm with the leads you indicated. No, I never got the other channel working again after i replaced all the caps. So yes, 2 gremlins, though I did prefer the Pinto... lol. Thanks again.

[redwire]

I find it easier to just ground the multimeter (-) and then move around and take measurements with the (+) probe. If you use two probes say across E and B I think it's too easy to slip and short out something.
You can also put some black electrical tape around the (+) probe tip if it has a longggg pin like some of them have, to make it less prone to short.