ROTEL RA-820BX3 not working

[sangeet2]

Thanks so much much for your input!

(Besides, the +/- 0.53 V indications on the schematic are in the wrong spot.)

what you mean by wrong spot?

First of all, your supply voltages seem rather low now. Lower than they were in the beginning, actually. (IIRC they were about +/-37 V.) There may be some oscillation going on - do you think you can get a scope somewhere? Does the amplifier get unusually warm? How much AC voltage do you measure between the two fuses? Which DC levels do you measure on R901/902?

I am getting a scope today
Drivers are at 51 degree,Q617 Has More Then 66 Degree (and the have no heatsink...), after the amp is on for about 10 minutes
Thats the hottest spot in the amp.

AC = 2x25,5
DC = 17,5/33,5

Voltage offset at Q603 base looks better now. It's not as low as I'd like to see, but a lot lower than it was. (I think the rest is from output DC offset, which should contribute about 19 mV on its own via the feedback. It's an inverting amp...) About 60 mV of difference between Q603/605 also lines up with simulation.

And you say you still can't adjust DC offset properly? What kind of range do you get? What is the minimum resistance you can obtain between the wiper and each end of VR603? Maybe it's just a dirty pot that's playing tricks on you.

VR603 is measuring ok, range is smooth (also on other channel!)
With R671 at 270k I measure DC offset from 0,48-1,22 volt.
When I reduce it to 180k, like you suggested it looks better: 0,158-0,8 volt.
With 100k I am able to adjust it to the desired range - cool hack
BUT: still hum from the speaker...
Maybe thats because of the old electrolytics in the power supply?

I did a bit of simulation again. It could be that the kind of voltages you saw on Q605's base (90-97 mV) are indeed too high to be compensated via the DC offset adjustment. (BTW, an extra 25 mV already results from having 5 megohms(!) in parallel to Q605's B-E junction. The circuit board better be clean as a whistle, with no flux residue in sight.) You may have to decrease R671 in value, to about 180 kOhms.
Seems like a bit of a hack, but it's the best idea I can offer for now.

Quite possibly the value of R671 was chosen empirically at the time, with all-new transistors and little reserve left for aging.

Hmm the board doesn't look so clean...

 

[sgrossklass]

what you mean by wrong spot?

If the output is at 0.00 V, where would you expect Q623's base to be, approximately?

I am getting a scope today
Drivers are at 51 degree,Q617 Has More Then 66 Degree (and the have no heatsink...), after the amp is on for about 10 minutes
Thats the hottest spot in the amp.

Q617 has to dissipate over 1 W, so that's not surprising.

AC = 2x25,5
DC = 17,5/33,5

AC is only a hint lower than it was originally, so I'm not sure why voltages should have sagged that much.

Can you repeat the ripple measurement with the coupling cap on the supplies? (It'll be more comfortable on the scope in AC mode, obviously, but I'm not sure how much sense comparing apples to oranges makes.) That one looked good last time, but that was before the hum appeared.

What was the last thing you did before this happened?

VR603 is measuring ok, range is smooth (also on other channel!)
With R671 at 270k I measure DC offset from 0,48-1,22 volt.
When I reduce it to 180k, like you suggested it looks better: 0,158-0,8 volt.
With 100k I am able to adjust it to the desired range - cool hack

So maybe clean the board area around the inputs with the usual isoprop alcohol / distilled (or demineralised) water mix, both sides as far as possible. (Kontakt WL, being a spray, may be more useful on the top side.)

BUT: still hum from the speaker...
Maybe thats because of the old electrolytics in the power supply?

Then you'd have had hum all the time, and at first you didn't. It rather seems the power supply is dragged down, resulting in a lot of ripple which then becomes audible as any amplifier's PSRR is finite.

Looking at your previous measurements, I think I know why. There's about a volt of difference between the emitters of Q627/629 and Q631/633, similarly on the other side. That's like >2 A of Iq! In other words, looks like you goofed when adjusting quiescent current.
Are you sure you measured between the two testpoints TP1/TP3 and not from either to ground?

At least now I'm not surprised about the hum any more. The power supply is not exactly designed for Class A current levels.

EDIT: The odd thing is that at the same time, Q621 C-E voltage only measures about 1.6 V, when I would expect about 2.2..2.3 V at normal bias levels. Maybe the output stage is so far underbiased that things start to oscillate. Try attaching clip leads to the non-grounded ends of R681 and R683 and adjusting the voltage difference to about this range.

 

[sgrossklass]

Oh, and do check for bad solder joints around Q621 C and E first.

In addition, the measurement results at Q623/627/629 and their complements were odd to say the least. Normally you'd expect output transistor bases to be +/- 0.55 V from the output and driver transistor bases +/- 1.1..1.2 V from the output.

 

[sangeet2]

If the output is at 0.00 V, where would you expect Q623's base to be, approximately?

around 1,2 V?

Q617 has to dissipate over 1 W, so that's not surprising.

Ok, but why Rotel didnt put heatsink?

AC is only a hint lower than it was originally, so I'm not sure why voltages should have sagged that much.

Can you repeat the ripple measurement with the coupling cap on the supplies? (It'll be more comfortable on the scope in AC mode, obviously, but I'm not sure how much sense comparing apples to oranges makes.) That one looked good last time, but that was before the hum appeared.

My multimeter reads a ripple of around 60 mv, and around 50Hz. Is this AC not a bit too much? I have no idea

What was the last thing you did before this happened?

Ahem, I have to say that I dont know when the hum started, sorry.

So maybe clean the board area around the inputs with the usual isoprop alcohol / distilled (or demineralised) water mix, both sides as far as possible. (Kontakt WL, being a spray, may be more useful on the top side.)

The inputs looks ok to me. There is some flux at the transistors which I exchanged of course...

Then you'd have had hum all the time, and at first you didn't. It rather seems the power supply is dragged down, resulting in a lot of ripple which then becomes audible as any amplifier's PSRR is finite.

Looking at your previous measurements, I think I know why. There's about a volt of difference between the emitters of Q627/629 and Q631/633, similarly on the other side. That's like >2 A of Iq! In other words, looks like you goofed when adjusting quiescent current.

Sorry, I dont understand this especially what you mean by difference. Difference to where? Here are new measurements:
Q627
Base -18,6 mv
Collector 33,9 v
Emitter 0,53 v

Q629
Base -20,2 mv
Collector 33,8 v
Emitter 0,52 v

Q631
Base -0,26 mv
Collector -33,6 v
Emitter -0,57 v

Q633
Base -27,5 mv
Collector -33,7 v
Emitter -0,57 v

They all look better cause the DC offset is much smaller now

Are you sure you measured between the two testpoints TP1/TP3 and not from either to ground?

Absolutely.

At least now I'm not surprised about the hum any more. The power supply is not exactly designed for Class A current levels.

That makes me smile, but I dont understand it quiet right...

EDIT: The odd thing is that at the same time, Q621 C-E voltage only measures about 1.6 V, when I would expect about 2.2..2.3 V at normal bias levels. Maybe the output stage is so far underbiased that things start to oscillate.

Q621
Base -1,15v
Collector 1,1 v
Emitter -0,53 v
Hmm still not enough volts from C-E...

Try attaching clip leads to the non-grounded ends of R681 and R683 and adjusting the voltage difference to about this range.

That would mean that my multimeter, with one end connected to ground should read 1,1 v at base of Q623 and Q625, right?

Oh, and by the way, after the amp is running for about an hour:
DC offset rises up till 42 mv (no load at speaker output)

 

[sangeet2]

Oh, and do check for bad solder joints around Q621 C and E first.

All solder joints look ok to me, but I could resolder all transistors...

In addition, the measurement results at Q623/627/629 and their complements were odd to say the least. Normally you'd expect output transistor bases to be +/- 0.55 V from the output and driver transistor bases +/- 1.1..1.2 V from the output.

Good info, I will try to keep this in my little mind

Drivers:
Q623
Base 1,09 v
Collector 33,44 v
Emitter 0,51 v

Q625
Base 1,16 v
Collector 33,66 v
Emitter -0,59 v

Ok with this new info you gave me my drivers look good. Power transistors measured in the post before not so good...

 

[sgrossklass]

I think you got B and E mixed up with the power transistors. Then it would line up.

How can Q621 emitter be at -0.53 V when Q625's base is at (+?!)1.16 V? That should be the exact same node. Please verify that Q621 emitter, Q619 collector and Q625 base are in fact all connected.

 

[sangeet2]

I think you got B and E mixed up with the power transistors. Then it would line up.

Hrmpfh, you are of course right It was to early this morning. Now when I look again to the layout of the board it makes sense.

As I have exchanged Q621 I can confirm that all pins are connected.
Q619 collector and Q625 base are in fact all connected. Also I exchanged electrolytics from power supply now, lets see if the DC offset still drifts after an hour.

BUT: hum is still there...

 

[sangeet2]

BUT: hum is still there...

Ok, ok, on my Nad 3020 there is almost no hum.
On my Denon Receiver DRA-585Rd is almost no hum.
On my other Denon receiver in the living room is hum, maybe same like on the Rotel...


So I think its ok.

Time to say another thank you very much to especially:

http://www.diyaudio.com/forums/members/sgrossklass.html and
http://www.diyaudio.com/forums/members/goranb.html

 

[GoranB]

Ok, ok, on my Nad 3020 there is almost no hum.
On my Denon Receiver DRA-585Rd is almost no hum.
On my other Denon receiver in the living room is hum, maybe same like on the Rotel...


So I think its ok.

Time to say another thank you very much to especially:

http://www.diyaudio.com/forums/members/sgrossklass.html and
http://www.diyaudio.com/forums/members/goranb.html

You are very welcomed.