Help Needed: Excessive DC Offset/Imbalance - Rotel RB-990BX Power Amp - Page 3 - diyAudio
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Old 3rd October 2012, 11:38 AM   #21
dtm1962 is offline dtm1962  Canada
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I agree Mooly that I am approaching the repair totelly wrong and out of frustration.

I will follow your methology and plan and give you an update once complete.

Failing that as well I will look at your ideas as well Jitter to see if its an odd condition with the board etc.

Thank you guys for sticking with me during these frustrating time.

As you can tell this is not my forte hence "component replacement" without a plan. lol!

Stay tuned...

Thanks again,
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Old 3rd October 2012, 12:04 PM   #22
Mooly is offline Mooly  United Kingdom
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Take it slowly and be careful measuring... no slips
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Old 3rd October 2012, 05:02 PM   #23
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by dtm1962 View Post
If it was a Negative offset I would be looking at the transistors being fed from the Negative V Rail?
Not necessarily.
A positive offset could be caused by not enough current from the negative side or vice versa. Apply Ohm's Law U=I*R.

Imagine yourself a positive rail of +1 V and a negative rail of -1 V. Connected in between these two rails are two 1 Ohm resistors in series. Ohm's Law calculates a current of 1 A going from positive to negative rail (the difference between -1 V and +1 V is 2 V, and 2 V across 2 Ohm resistance equates to 1 A of current).
The voltage across each individual resistor is 1 V (1 A * 1), so where the two resistors interconnect, the voltage is 0V (pos rail -1 V or neg rail + 1 V).

Now imagine that the resistor on the negative rail has burnt through partially and has become 9 Ohms. As a result, total resistance between positive and negative rail has become 10 Ohms (five times higher) and the current between the two rails is only 1/5 th, or 0.2 A.
The voltage across the 1 Ohm resistor is now only 0.2 V, but the voltage across the damaged (now 9 Ohm) resistor is 1.8 V.
The voltage where the two resitors interconnect rises from 0 V to 0.8 V (pos rail - 0.2 V or neg rail + 1.8 V). This is a positive offset caused by a malfunction in components connected to the negative rail.

If Q615 doesn't turn on or the resisitors around it have failed, a similar situation as in above example could be present in your amp. Hence my advice to check them out.

Last edited by jitter; 3rd October 2012 at 05:06 PM.
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Old 3rd October 2012, 06:03 PM   #24
steveu is offline steveu  United States
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Probably the PNP voltage driver is shorted or the NPN is open, or both. It's hard to read the refs but I think they are Q513 2SA114D and Q613 2SC2582. Given that there is any voltage accross the bias transistor, that means there is current so the NPN is probably OK. I think the PNP is shorted. Like many amps, this circuit needs a currenty limit on the voltage drivers although I don't see any output protection iether. They are related cause often the output protection fights with the voltage drivers.
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Old 4th October 2012, 11:21 PM   #25
dtm1962 is offline dtm1962  Canada
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Default Update

I took your advice Jitter & Mooly and isolated the Output stage via removing Q619 & Q621 as well as the Vbe multiplier Q617.

With all 3 above removed from the circuit the voltage between VR1 & R625 was a -1V maximum.

So I put back Q617 (2SD1953) in and powered up a got +59VDC like before.

So I have identified that the addition of Q617 will create the offset.

SO how do I measurre 2SD1953 as it is a Darlington (with integrated 2 transistors + a diode & couple resistors). The gain for this device is >1000 up to 30000.
My DMM doesn't seem to register it properly (nominally a NPN).

Is it possible to use a DMM to measure this as I have used the standard diode setting and I measure the 2 out of 6 that have ~.625V and 3 - infinity but I do measure ~1.2V in the 6th measurement for all 3 units I have.

Let me know.

Thanks,
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Old 4th October 2012, 11:44 PM   #26
dtm1962 is offline dtm1962  Canada
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Quote:
Originally Posted by dtm1962 View Post
I took your advice Jitter & Mooly and isolated the Output stage via removing Q619 & Q621 as well as the Vbe multiplier Q617.

With all 3 above removed from the circuit the voltage between VR1 & R625 was a -1V maximum.

So I put back Q617 (2SD1953) in and powered up a got +59VDC like before.

So I have identified that the addition of Q617 will create the offset.

SO how do I measurre 2SD1953 as it is a Darlington (with integrated 2 transistors + a diode & couple resistors). The gain for this device is >1000 up to 30000.
My DMM doesn't seem to register it properly (nominally a NPN).

Is it possible to use a DMM to measure this as I have used the standard diode setting and I measure the 2 out of 6 that have ~.625V and 3 - infinity but I do measure ~1.2V in the 6th measurement for all 3 units I have.
So I measured the 3 2SD1953 (NPN Darlington) transistors out of the circuit and the following are the actual measurements:
E (Red) to C (BLK) = 554
E (Red) to B (BLK) = Infinity
C (Red) to B (BLK) = Infinity
E (BLK) to C (RED) = Infinity
E (BLK) to B (RED) = 1075
C (BLK) to B (RED) = 640

All 3 transistors measured with approximately the same measurements.

Let me know if these are good.

Thanks in advance,
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Old 5th October 2012, 02:52 AM   #27
dtm1962 is offline dtm1962  Canada
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Default 2nd Update

So I assuming the problem was associated with the Vbe Multiplier (2SD1953) so I did the following:
I removed the 3 Pico-range PS caps and measured them with my LCR meter and they proved to be within 2%.
Replaced the resistors (6.19k & 8.2k) + the 2kohm pot as well. Also replaced the the 10uF 50V with a quality 10uF 63V Silmic II as well as the 100uF 25V cap. ( I did change out the current sources 1016 & 2362 for the input diff. transistors since I had them available. Checked their hfe prior to installing to insure they are good.

So powered up WITHOUT the Vbe Multiplier (Q617) transistor (2SD1953) and the Outputs still isolated via removal of Q619 & Q621, I could adjust the pot to get 0.00VDC at the VR1 & R625 junction. So powered down and added the good channel's 2SD1953 and powered up and I'm back to +59VDC at VR1 & R625 junction.

So for reference the 4 (qty) 2SD1953 all measure exactly (including the good channel's 2SD1953 before and after the insertion in the bad channel's amp.

SO any ideas?

I am really don't know where to go from here.

The board was fully checked and cleaned with alcohol at all terminations. I desoldered all the components that were replaced. I checked for "lifted" solder rings and could not find any. All solder joints from components to components were checked for continuity.

So basically all the component replacements have resulted in no change from the 1st day I started repairing the unit. Initially the 221 ohm reistor on the +71VDC in the front end, got hot & failed as well as a few transistors (1016's) that seem to been a little flakey).


Let me know what I could as I am really stuck....


Thanks,
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Old 5th October 2012, 02:55 AM   #28
wahab is offline wahab  Algeria
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If you remove a VBE multiplier be careful to short the collector and emitter
traces on the PCB.

The amp will have no iddle current but it will prevent catastrophic failures.
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Old 5th October 2012, 04:30 AM   #29
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Hi,
In you thread #3 in the schematic you show 22 volts at the base of the 2 input transistors. That it is wrong. You should have zero volts since it is the input. I can not read the transistors number they look like Q401 and Q407. I think the transistor npn Q401 it is shorted. You have 22 volts at the input.
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Old 5th October 2012, 04:44 AM   #30
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Hi,
Correction I think it is Q601 & Q607. Also you have 30 volts coming from the speaker to the base of Q603 and Q605.
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