Help Needed: Excessive DC Offset/Imbalance - Rotel RB-990BX Power Amp - Page 4 - diyAudio
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Old 5th October 2012, 05:10 AM   #31
jitter is offline jitter  Netherlands
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tauro0221, the attachment of post #3 is obsolete, please look at the attachment of post #15.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.

Last edited by jitter; 5th October 2012 at 05:25 AM.
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Old 5th October 2012, 06:35 AM   #32
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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.
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Old 5th October 2012, 07:39 AM   #33
Mooly is online now Mooly  United Kingdom
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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.
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Old 5th October 2012, 04:27 PM   #34
dtm1962 is offline dtm1962  Canada
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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
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Old 5th October 2012, 04:33 PM   #35
Mooly is online now Mooly  United Kingdom
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No problem, we'll look forward to it
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Old 5th October 2012, 05:11 PM   #36
jitter is offline jitter  Netherlands
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Indeed.
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Old 7th October 2012, 12:13 AM   #37
dtm1962 is offline dtm1962  Canada
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Default Measurements Updates

Quote:
Originally Posted by Mooly View Post
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,
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Old 7th October 2012, 02:01 AM   #38
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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.
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Old 7th October 2012, 06:49 AM   #39
Mooly is online now Mooly  United Kingdom
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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.
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Old 7th October 2012, 01:46 PM   #40
dtm1962 is offline dtm1962  Canada
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Default Errors in Documenting Measurement Readings :(

Quote:
Originally Posted by Mooly View Post
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
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