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

[dtm1962]

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,

 

[Mooly]

Take it slowly and be careful measuring... no slips

 

[jitter]

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.

 

[steveu]

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.

 

[dtm1962]

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,

 

[dtm1962]

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,

 

[dtm1962]

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,

 

[wahab]

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.

 

[tauro0221]

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.

 

[tauro0221]

Hi,
Correction I think it is Q601 & Q607. Also you have 30 volts coming from the speaker to the base of Q603 and Q605.

 

[jitter]

tauro0221, the attachment of post #3 is obsolete, please look at the attachment of post #15. On the input is -0.17 V.

This may seem hopeless, but we're not there yet. As per the suggestion of Mooly, the feedback resistor must be removed as a fault at the output can find its way back to the input and get you to bark up the wrong tree.
Shorting BE of the VBE multiplier was advised in an earlier post too and repeated by wahab, it will prevent possible runaway and destruction of the output stage.

I can't find in your recent posts that you did any of the above, so it's time to sit back, and carefully reread the thread as it seems like you're rushing things and missing out on some of the advice given.

In a complicated repair, it might be wise to stop for a while, and come back to it later. At the stage you're in now, you might not be seeing the woods for the trees (please don't take this as criticism but as advice from someone's who's been in the same situation).
At work I don't have the luxury of time with repairs, but a night's sleep often helps. The next day, when I've cooled down, I often spot the fault quite quickly that seemed to elude me the day before.

 

[tauro0221]

Hi,
Please accept my apologies the problem I didn't have anything to do and check the site Diyaudio to see if somebody was having problem and found your problem. Immediately By looking at your schematic I saw a problem at the base of the transistors but I didn't follow up the end of the thread.

Now I check your new reading at post #15 and see that you have 59.7 volts at the base Q617 and it should be .5 volts. Means that Q615 it is not conducting. So please check the voltage at the emitter of Q615. It should read about -65 volts. If not resistor R629 it is open. If you have -65 volts then Q615 it is open. Replace it. You need Q615 to conduct to balance the voltage at the base of Q617.

 

[Mooly]

Dan... you are swapping to many parts without following the basic tests

Make sure the amp is correctly put back together with all the right devices fitted in all the right places and then follow this exactly and report back with results,

1. Remove Q619 and Q621

2. Add a wire link across Q617 collector and emitter.

3. Remove the 22.1K feedback resistor that is marked "C657"

4. Fit a 22K (or use the 22.1K just removed) from base of Q609 to the wire link now shorting the Vbe multiplier.

5. Check that the eight resistors R607/609/619/623/615/617/611 and 629 are OK.

6. Switch on and measure the voltage on the wire link across the Vbe multiplier.

What is it reading ? Also the bulb tester should be dim. If it is not then we need to fix that first.

 

[dtm1962]

Thanks again for more input to help me fix this "stubborn" amp

I will follow your exact plan Mooly...

I wasn't very clear but I did do it more systematically then I let on in my last posting. I actually changed out parts "one system at a time" and then powered up to check its changeout influence on the outcome. (ie 3 pF PS caps, then checked; 2 Current sources then checked; 2 resistors & VR1 then checked; 2 EE caps then checked; good channel's 2SD1953 then checked etc.).
It was a time consuming process but I was trying to improve my troubleshooting plan to what Mooly & Jitter and others have recommended.

I will update you later in the weekend once I implement Mooly plan exactly.

Thanks again guys I appreciate it

 

[Mooly]

No problem, we'll look forward to it

 

[jitter]

Indeed.

 

[dtm1962]

Measurements Updates

Dan... you are swapping to many parts without following the basic tests

Make sure the amp is correctly put back together with all the right devices fitted in all the right places and then follow this exactly and report back with results,

1. Remove Q619 and Q621

2. Add a wire link across Q617 collector and emitter.

3. Remove the 22.1K feedback resistor that is marked "C657"

4. Fit a 22K (or use the 22.1K just removed) from base of Q609 to the wire link now shorting the Vbe multiplier.

5. Check that the eight resistors R607/609/619/623/615/617/611 and 629 are OK.

6. Switch on and measure the voltage on the wire link across the Vbe multiplier.

What is it reading ? Also the bulb tester should be dim. If it is not then we need to fix that first.

Here are the readings:

Resistors all proved to be correct.

Bulb is very DIM.

Rail Voltage is +63.1V & -62.8V for reference

+59.4VDC at Q617 CE jumper
+59.2VDC at Q617 Base


Q613 (PNP) measurements :
+59.4V E
+60.1V B
+59.2V C

Q615 (NPN) measurements :
-59.1V E
-58.6V B
+55.9V C

Front End Measurements
Q601
+44V E
+36.5V B
+59.6V C

Q603
+44V E
+36.5V B
+56.3V C

Q607
+37V E
+45V B
-58.5V C

Q609
+45V E
+45V B
-62.5V C

Q611
+44V E
36.5V B
+37V C

Q605
-62V E
-61.7V B
+44.2V C

If I remove the jumper from across Q617 C & E (VBE Multiplier), then the values in the front end will approach less than +/-1 VDC on both B & E of Q601 Q603 & Q607 Q609.

As well the measurements for Q613 & Q615 when Q617 C & E Jumper IS REMOVED are as follows:

Q613 (PNP)
+62.4V E
+60.8V B
+60.0V C

Q615 (NPN)
-62.5V E
-61.8V B
+60.0V C

Let me know what you think of these measurements.

Thanks,

 

[tauro0221]

Hi,
Q615 still it is not conducting. The base of Q615 it is more positive than the collector consequently Q615 should be ON making the collector positive and the base of Q617 should read almost zero voltage. If you compare both channel voltages they almost compare with the exemption of Q617 that it is 65 volts at the base.

 

[Mooly]

I said we'd look forward to it

Now Q607 and Q609 readings seem haywire so just recheck these

The emitters should connect together but you have 37 and 45 volts ? here (check for lifted pads and open circuit print). Remember I said a reverse biased B-E junction will level out at around 8 or 9 volts. Thats what you are showing here with Q607. You have 45 volts on the base and 37 on the emitter. Its not open circuit, its reverse biased.

Have you got the 22k feedback resistor connected as I described ? although that would not give the different emitter volt readings.

Edit... and keep Q617 shorted out for now.

 

[dtm1962]

Errors in Documenting Measurement Readings

I said we'd look forward to it

Now Q607 and Q609 readings seem haywire so just recheck these

The emitters should connect together but you have 37 and 45 volts ? here (check for lifted pads and open circuit print). Remember I said a reverse biased B-E junction will level out at around 8 or 9 volts. Thats what you are showing here with Q607. You have 45 volts on the base and 37 on the emitter. Its not open circuit, its reverse biased.

Have you got the 22k feedback resistor connected as I described ? although that would not give the different emitter volt readings.

Edit... and keep Q617 shorted out for now.

2 Errors in my documentation Q603 B: +44V; Q611 E +59.5

Yes for all the inital measurements 22.1kohm resistor is tied between Q617 C/E & Base of Q609.

I will add new attachment(s) showing schematic's measurements.

These results are repeatable and not intermittent.

Note : I will show the measurements when the Q617 C/E Jumper & 22.1kohm resistor is removed as the Front end Q602/603/607/609 B & E measurements become more normal and the Q613/Q615 measurements change where Q613 goes from being reversed biased (across B/E) to normal biased but with a larger than normal voltage drop (~1.6V)

Sorry for the confusion.

Stay tuned