Help Needed: Excessive DC Offset/Imbalance - Rotel RB-990BX Power Amp - Page 6 - diyAudio
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Old 7th October 2012, 08:34 PM   #51
Mooly is offline Mooly  United Kingdom
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Typo above... I did a quick edit but you might have the wrong info

Should read " If it doesn't then Q613 is faulty (leaky).
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Old 8th October 2012, 04:52 PM   #52
jitter is offline jitter  Netherlands
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I agree with Mooly that Q613 might be leaky.

Looking at the voltage drop across R623 (in the first pdf of post #41 ), it seems that a lot of current is flowing through it, i.e. 36 mA, despite Q613 supposedly being off (B is more positive than E).
Q615 seems to be turned on and there's -41 mA flowing through it at quite a large voltage differential between C and E, which might explain why it gets hot.

Last edited by jitter; 8th October 2012 at 04:58 PM.
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Old 8th October 2012, 05:08 PM   #53
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Quote:
Originally Posted by sakis View Post
Just last night i had to fix a HK 870 with 7 volts of offset in the one out ...Nightmare to remove piggy board of drive to measure and/or exchange transistors with no success but while looking equipped with a second pair of eyes i noticed a good crack on the pcb solder it fixed in seconds

you may inspect that also

As Ian said the best approach is to measure always looking at the working Chanel for reference ,

Often there is a case where a transistor measures quite properly with a DVM or even a hfe tester but fails under 50-60 volts you must observe that also

Finally if the offset is positive you may start swapping parts of the +rail from the working channel but always with caution .

starting swap point should be transistors Q613 -619 - 635 ( this will be of course after verifying proper voltage conditions )

NOTE :
( at the specific amp consumption is fairly low also bias is fairly low .... 60 W bulb may drop the mains a few volts but not half of it .Behavior like that means that one of your transistors is conductive fully open when powered and that brings us to the point where a semi looks ok outside the amp but not when powered with 50-70 volts )

Kind regards
sakis

well ...
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Old 8th October 2012, 06:11 PM   #54
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by sakis View Post
well ...
well indeed

I always like using my old AVO to test transistors. The 15 volt battery on high ohms range soon shows any leakage. DVM's just don't cut like the gear Mind you, you have to do the mental juggling of reversed polarity on an analogue meter, the red lead is negative on ohms... just to confuse you.
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Old 9th October 2012, 02:56 AM   #55
dtm1962 is offline dtm1962  Canada
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Default Update

Thanks for the input Mooly, Jitter, Tauro0221 & Sakis....

I will test the Q613 out tomorrow (as Mooly has suggested) as I had a busy last couple of days doing family things!

I will update you on the outcome....

Thanks again guys!
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Old 10th October 2012, 12:58 AM   #56
dtm1962 is offline dtm1962  Canada
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Default Measurements Update

Quote:
Originally Posted by dtm1962 View Post
Thanks for the input Mooly, Jitter, Tauro0221 & Sakis....

I will test the Q613 out tomorrow (as Mooly has suggested) as I had a busy last couple of days doing family things!

I will update you on the outcome....
This is the tests as suggested by Mooly.....

A quick and dirty test... remove Q613 and see if the voltage on Q617 (still shorted) falls to near the minus supply rail.
If it does replace Q613 but leave the BASE unsoldered and floating. Again see if the Q617 volts falls away. If it doesn't then Q613 is faulty (leaky).



Here is the results of the above tests:

Q617 C & E shorted.... and Q613 removed from board
E = -0.4V
C = -0.4V
B = -0.4V

Q617 C & E NOT shorted.... and Q613 removed from board
E = -65.4V
C = -65.4V
B = -64.7V

Q617 C & E shorted.... and Q613 ON BOARD but BASE Disconnected
E = +49.6V
C = +49.6V
B = +49.6V

Q617 C & E NOT shorted.... and Q613 ON BOARD but BASE Disconnected
E = +46.3V
C = +49.6V
B = +47.0V

Does this mean that Q613 is SHORTED & Q615 IS OPEN?

Let me know what you think....

Thanks in advance,
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Old 10th October 2012, 05:37 AM   #57
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by Mooly View Post
A quick and dirty test... remove Q613 and see if the voltage on Q617 (still shorted) falls to near the minus supply rail.
Quote:
Originally Posted by dtm1962 View Post
Q617 C & E shorted.... and Q613 removed from board
E = -0.4V
C = -0.4V
B = -0.4V
It didn't, so I'm not going to take any conclusions from the two below.

Quote:
Originally Posted by Mooly View Post
If it does replace Q613 but leave the BASE unsoldered and floating. Again see if the Q617 volts falls away. If it doesn't then Q613 is faulty (leaky).
Quote:
Originally Posted by dtm1962 View Post
Q617 C & E shorted.... and Q613 ON BOARD but BASE Disconnected
E = +49.6V
C = +49.6V
B = +49.6V
I'd like to know more voltages such as:
- the voltages on the bases of Q613 and Q615.
Please also report voltages in above situations of:
- positive and negative supply rail;
- on E of Q613;
- on E of Q615.
With the above three, you can calculate the voltage drops across the resistors and then with I = U/R the current through them. The currents through these resistors inherently also flow through the emitters of the transistors under investigation.
Please write them down in the schematics like in post #41, it helps visualizing things.

Last edited by jitter; 10th October 2012 at 05:39 AM.
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Old 10th October 2012, 06:38 AM   #58
Mooly is offline Mooly  United Kingdom
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From the latest readings Q613 appears faulty.

With the base disconnected it should appear to the circuit as if it has been removed and it doesn't. In both cases the voltage on Q617 comes back to around +50 volts. So that is one problem. Based on those reading Q613 is faulty.

I can't reconcile the different results of Q617 being shorted and not shorted although that does not alter the above.

For interest have you measured Q613 out of circuit and on a high ohms range. Thats the range that would measure resistors in the Meg ohm region and the range that shows a reading with you just gripping the leads. Assuming a normal DVM then with the black lead on C and the red lead on E there should be no reading. It should read as if the leads were just open.
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Old 10th October 2012, 06:53 AM   #59
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Hi,

Q617 C & E shorted.... and Q613 removed from board
E = -0.4V
C = -0.4V
B = -0.4V

Q619 and Q621 removed from board

The result if this test I can not follow it. If you shorted Q617 collector to emitter then the E and the C should read negative but it read -0.4V. Where the positive voltage it is coming from to read -0.4V. If you removed the jumper then everything read negative.

Q617 C & E NOT shorted.... and Q613 removed from board
E = -65.4V
C = -65.4V
B = -64.7V

The only way to feed a positive voltage is from C613 leaking.

dtm1962 can you remove C613 and do the test again.
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Old 10th October 2012, 07:00 AM   #60
Mooly is offline Mooly  United Kingdom
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It is a strange result for sure. Certainly its worth checking the cap for leakage.
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