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

[dtm1962]

Hi Guys:
Right now the DC offset has settled at 65 mv
with an actual speaker load....
If in the future it actually becomes audible in any
way ... I will build a simple Vbe tester using
a +/- PS with a 4 resistors/ diode circuit
and match the Vbe to get under 10 mV..

So guys I appreciate all the help to repair the amp!
It was an adventure for sure .... Lesson learned
..... ensure the parts that you put in are actually what
they are tagged.... Measure & confirm and
Don't make assumptions!

 

[Mooly]

Your welcome and its great to see you achieve a good result. In fact I've learnt quite a bit about that particular configuration by the simulations too.

65 mv, that's quite acceptable and it won't be and won't ever become audible. It will never change or drift with age either. Nothing to worry over.

 

[east electronics]

wait ... do you say that you have 65mv of offset with a load connected ??? if so that is a very wrong practice and you should measure the offset without a load ...

 

[wahab]

That would be a logical assumption but it doesn't seem to work like that for this type of configuration. Different input bias current don't seem to alter the offset. I tried 10k and 100k (now thats unbalanced) yet the offset is still in single mv figures. It does seem to purely rely on the matching of the semiconductors.

You cant draw conclusions from a simplified schematic..

What i did is to simulate the complete amplifier schematic since
i had it on a simulation file and equaling the bias resistors yielded
100mV output DC voltage variation.

 

[dtm1962]

I left the unit for 3 hours without load and it @ 75 mV. I felt
Confident that it was safe to connect to speakers hence testing
With actual speakers at an average 5 watt output as per voltmeter
Calculation via AC setting and also measured Dc Offset via DC
Setting which settled at 65 mV.

 

[Mooly]

You cant draw conclusions from a simplified schematic..

What i did is to simulate the complete amplifier schematic since
i had it on a simulation file and equaling the bias resistors yielded
100mV output DC voltage variation.

Appreciate that and that simulation is only part of the story (really must get a new signature ) I never tried equalising the bias resistors... and I'm on another PC so haven't got the file to hand to try. I'll try it when I get chance

 

[srinath]

Just bitten

So guess what?

The PNP 2SA1142 measures as a NPN!!! Via Diode test and hfe test... D*MN!

Since the original NEC 2SA1142 is a green body unit.....and this was a black body unit....I should have noted...

I just got bitten by this too.
On my Marantz 2325 the a818 I was replacing with the A1142 actually is fine.
Its matched pair the c1628 was bad.

Can I run the a818 and the c2682 (replacement for the c1628) in the amp - will it hurt it ?

Cool.
Srinath.

 

[Scott Munks]

Rotel RB-990BX amplifier staying in protection mode

Hello Everyone,

I NEED HELP PLEASE !

I recently bought a Rotel RB-990BX amplifier. I tested the amplifier with a DVM on the speaker terminals to make sure I had good A/C voltage and I did. I had no isues with the amp going into protection, so I shut it off. Next day I hookup my speakers and the amp stays in protection mode and has no output. So I disconnect the speakers and it still stays in protection mode with no output. No fuses have blown. ANY SUGGESTIONS ???