Help Needed: Excessive DC Offset/Imbalance - Rotel RB-990BX Power Amp - Page 2 - diyAudio
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Old 2nd October 2012, 02:29 PM   #11
dtm1962 is online now dtm1962  Canada
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Default Update

Quote:
Originally Posted by sajti View Post
I would try to use 100W bulb, or 2x60W parallel, to let a bit more voltage for the amplifier. And I would set the bias to 0 on the faulty channel, to reduce the power consumption.

Sajti
I have taken new measurements and I will post later today.... To make it work with a 60 Watt bulb, I disconnected other channel while measuring.

I got rail voltages near 60V versus 70V so we should be more accurate.

The good channel worked as expected with the correct voltages with a Bias of 4.5mV versus the recommended 7mV, while the bad channel had +45VDC on the output. The bulb stayed very dim so I believe I could have just bypassed the bulb for the measurements as I believe the outputs are fine....

Stay tuned.
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Old 2nd October 2012, 02:33 PM   #12
sajti is offline sajti  Hungary
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What about the bias on the faulty channel?

Sajti
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Old 2nd October 2012, 02:47 PM   #13
dtm1962 is online now dtm1962  Canada
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Quote:
Originally Posted by sajti View Post
What about the bias on the faulty channel?

Sajti
I can set the bias voltage to 4.5mV on the faulty channel and the DC offset goes down to 45VDC versus 58VDC when the bias is set at minimum 0.1mV.

Stay tuned...
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Old 2nd October 2012, 04:58 PM   #14
jitter is offline jitter  Netherlands
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Looking at the voltages written in de attachment of post #3, I can't help but think that things go wrong around Q615, R627, R629, VR601.
It's as if there isn't enough current flowing from the negative rail through these components wich causes high positive voltages to appear on the base of Q617.
Being in the middle of almost identical voltage dividers between positive and negative rails, shouldn't this measure ~0 V-ish?

Please compare measurements around these components with those of the good channel.

Last edited by jitter; 2nd October 2012 at 05:02 PM.
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Old 2nd October 2012, 05:04 PM   #15
dtm1962 is online now dtm1962  Canada
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Default Measurements

Here are the measurements for the faulty & good channels.

Once the bias is increased on the faulty channel from 0.1mV to 4.5mV the DC Offset decreases from 58VDC to 45VDC.
The voltages in the input change very slightly if I increase Bias from 0.1mV to 4.5mV but the output decreases.


Please review the attachment and post any comments.


Thanks in advance,
Attached Files
File Type: pdf rotel rb 990 voltage measurement.pdf (185.7 KB, 87 views)
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Old 2nd October 2012, 06:37 PM   #16
Mooly is offline Mooly  United Kingdom
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Very quickly going off your readings Q619 appears open circuit B-E as there is 62.3 on base and 60.5 on emitter.

When faced with faults like this it can be much more revealing to measure B-E volt drops actually on the device rather than from ground.

Check that result out
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Old 2nd October 2012, 09:22 PM   #17
dtm1962 is online now dtm1962  Canada
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Default Observed Changes from 1st Set to 2nd Set of Measurements

Between the 1st & 2nd set of measurements you will notice an improvement in the input stage.

I changed 3 resistors and resoldered all the jumpers for peace of mind.

So looking at the 2nd set of measurements we have + DC voltage at the output only and the input is reasonably good for now.

So you guys are saying that the 3 transistors in the pre-driver stage (Q613 619 & 635) that are on the + V Rail may be the cultprit? So if we are getting a Positive Offset then I should be looking at these 3 as per the measurents?

If it was a Negative offset I would be looking at the transistors being fed from the Negative V Rail?

Is this a general rule or am I misinterpreting your comments?

Let me know.

Thanks again for all your input!
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Old 3rd October 2012, 03:51 AM   #18
dtm1962 is online now dtm1962  Canada
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Default Parts Replacement - No Change

Replaced Q613, Q615, Q617 with new spare replacements..... no change.

Then replaced Q619 & Q621 (with used units- Hfe & Diode measurements are good) and no change.

Note Is this a clue but : Q615 (2SC2682) on heatsink does get hot but there is +57VDC on collector , -63.6VDC on Emiiter & -62.7VDC on the Base while its partner Q613 (2SA1142) also on the heatsink is barely warm as their is 64.5VDC on Emitter, 62.3VDC on Collector & 62VDC on Base... probably not conducting much current.

The Input Stage voltages are still reasonable on the +/- ~0.5V ranges on their transmitter emitters (diff pairs & current sources).

I have reach a crossroad as I don't know what to do next..... lol!

The only parts that haven't been changed out are the 160V Polystyrene pico range capacitors and the 2.2kohm pot.

The board has no cracks as I was going to desolder all connections to look for separated/disconnected solder terminal rings. But the Rotel rings are pretty beefy..

Any ideas.

I will buy another pair of Q619 & Q621 as the ones I just pulled out seem to measure good via Hfe & diode test as well as the pair I put back in its place.

Thanks in advance,
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Old 3rd October 2012, 05:29 AM   #19
jitter is offline jitter  Netherlands
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In my experience in over 12 years in industrial electronics manufacturing, cracks in boards can be so small that a microscope is needed to see them. Failing that, you might check continuity of traces with your DMM.

Do you have access to a scope? I ask this because oscillations could register as DC voltage on a DMM while they're actually AC. A faulty cap could be the cause of oscillations. Failing a scope, replace those you haven't yet too.

VR601 is among the resistors in the negative leg of the circuit that drives Q617. A wrong voltage on its base will affect the circuit to the right of it, I'd replace VR601 to make sure it's OK.

If I really couldn't find the fault, I'd probably resort to removing Q617, Q619 and Q621. That way any connection to the circuit to the right of Q613 and Q615 is broken. In between VR601 and R625 should be ~0 V. If it isn't, there must be something wrong there. If it there is ~0 V there, then reinsert Q617 and measure again.

Edit: did you check the caps that have not been replaced for short-circuit?

Last edited by jitter; 3rd October 2012 at 05:51 AM.
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Old 3rd October 2012, 07:22 AM   #20
Mooly is offline Mooly  United Kingdom
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I don't mean this to sound critical so please don't take this the wrong way but you have to be much much more methodical in this. Replacing components in hope is a recipe for disaster.

All the evidence is there if you can find it.

For example,
"Is this a clue but : Q615 (2SC2682) on heatsink does get hot but there is +57VDC on collector , -63.6VDC on Emiiter & -62.7VDC on the Base

That gives a B-E drop (and this is where the main clues will lie) of 0.9 volts which should turn the transistor fully on. (In fact it shouldn't really be possible to reach 0.9v across a forward biased junction at low currents). So if we say "fully on" then why is the C-E differential 120 volts or so ?

I'm sure all these semiconductors aren't faulty, its just you are not measuring in the correct way. You MUST do a measurement across the B-E junction to get a true reading and determine from that if there is a problem. ALL the B-E drops irrespective of NPN or PNP type should be around 0.7 if forward biased. If any are reverse biased due to the fault then that will limit out at around 8 or 9 volts (of the opposite polarity) give or take. Rmemember for the NPN's the base is the higher voltage with respect to the emitter and vice versa for PNP.

If needed you can break this amp into stages and with 10 minutes work get a good idea of where the fault is.

Did you short out the vbe multiplier as I suggested earlier ?

If you remove Q619 and Q621 you isolate the output stage completely.

Do you see component marked "C657" which is a 22.1K RESISTOR that is the main feedback return. Its to the right of the output transistors and connects to the amp output. If that is removed and the right hand end reconnected so that it now connects to our "shorted out vbe multiplier" i.e. collector of Q614 or Q616 and the amp powered up then the voltage at Q614/616 should settle to near zero. If it does then you are looking at a problem to the right of this. If the offset (at this point in the circuit) is still present then you have a problem in the front end.
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