Help Needed: Excessive DC Offset/Imbalance - Rotel RB-990BX Power Amp

[dtm1962]

Thanks guys ...it is strangest thing I ever seen in an amp.

I will do more measurements tonight and update.....

Thanks again....

[Mooly]

No problem. We don't give up that easily

[jitter]

Indeed, I had to persist to find the cause of very strange fault in my CD-player, but eventually (2 years later), I did.

[Mooly]

Originally Posted by jitter

Indeed, I had to persist to find the cause of very strange fault in my CD-player, but eventually (2 years later), I did.

That ones going back a bit. Must have missed it first time around so well done Faults like that are real odd balls but its really satisfying to stick with them and fix them.

Reminds of a guy at work (this was years ago and... and he was one of our best bench techs) who had an Hitachi D980 ? top of the range cassette deck and it would always throw a wobbler at the most inconvenient time by shutting off or going into some wrong mode. He'd checked absolutely everything you could think of so in frustration brought it into work for a joint effort.

Long story short, and I said as a joke that all that was left were the fuses on the PCB. Reply "if that fixes it you can have it" Hitachi x*7"3&^* junk. Guess what We replaced them and it never missed a beat afterward. The wire must have parted at the end caps or something. It was so funny... he kept the deck though.

so if dtm1962 is reading all this. Well we'll get a result with this one too

[dtm1962]

Originally Posted by Mooly

so if dtm1962 is reading all this. Well we'll get a result with this one too

Yes I am home now and I am going to do all measurements and plot on a marked up schematic. I will measure the frontend right thru to the Q617...

Stay tuned...

Thanks,

[dtm1962]

Hi Guys : Please review the 3 (actually 4 sets of measurements) attachments

Observations & Notes:

1) Lifted one lead of all 3 Polystyrene pF range caps - no effect....
2) Measured Q613 (Black on C & Red on E) in Mohm range = measures OPEN
3) Measured Q615 (Red on C & Black on E) in Mohm range = measures OPEN
4) All measurements on 3 attachments is with Q617 shorted (C & E)

Attachment # 1 Q613 BASE lifted
Attachment #2 Q613 IN CIRCUIT
Attachment #3A Q613 REMOVED
Attachment #3B Q613 & Q615 REMOVED


My feeling is that Q613 is SHORTED & Q615 IS OPEN.

Let me know what you think.

Thanks in advance,

[dtm1962]

Even with the BASE of Q613 LIFTED from the circuit, I measured +51.3 VDC on the BASE PIN which was "floating above the PCB". So does that say the EMITTER (@ +56.7VDC) was leaking to the BASE?

Let me know.

Thanks,

[tauro0221]

Hi,
At this point the only left it is to replace all 3 transistors with new one and leave Q619 and Q619 out. Maybe somebody have a better suggestion.

[tauro0221]

OOOps Should read leave Q619 and Q621 out.

[jitter]

What I find remarkable is the difference in currents through R623 with each measurement.

First set: 52 mA;
Second set: 15 mA;
Third set A: 66.7 mA ???? With Q613 removed, both ends of R623 should read the same voltage (i.e. 0 mA current). This looks like an error in measurement, please repeat.
Third set B: unknown, I'd like to know the value in this situation too, just to rule out leakage across the PCB (unlikely but not impossible).

Q613 sure looks questionable to me.
Edit: Q615 cannot be open. If that were the case, the voltage across R629 would be 0 V.
In the first two sets of measurements, it's open and conducting 38 mA, in the last it's not and the current drops to near 0, as expected.