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

[dtm1962]

Schematic Measurements With Q617 CE Shorted & Without

Here are measurements that are made today as well.

1st Attachment is with Q617 C & E Shorted and
2nd Attachment is with Q617 C & E not shorted

Note: Q619 & Q621 are removed to isolate the outputs from the measurements....

Note the +/- Rail Voltage does dip by ~3VDC when Q617 C & E is shorted.

If you want higher resolution pdf's of the measurements...

please email me dmullins at cogeco dot ca

Let me know what you think

Thanks in advance,

 

[dtm1962]

Update Revision 1

Here are measurements that are made today as well.

1st Attachment is with Q617 C & E Shorted and
2nd Attachment is with Q617 C & E not shorted

Note: Q619 & Q621 are removed to isolate the outputs from the measurements....

Note the +/- Rail Voltage does dip by ~3VDC when Q617 C & E is shorted.

If you want higher resolution pdf's of the measurements...

please email me dmullins at cogeco dot ca

Let me know what you think

Thanks in advance,

A few revisions was worth redoing the attachment.

For some reason +/- Rail voltage would vary enough (+/-3Volts) to make some measurements invalid, hence my reissuing the attachment for "the non-shorted Q617 C & E version". The Shorted version is accurate as is.

The 3 updated voltages are "back circled" for reference.

Let me know your thoughts.

Again Higher resolution copy of marked-up schematics are available if you email me at: dmullins at cogeco dot ca

Thanks in advance,

 

[Mooly]

Its easily done and no problem

I've just amended the circuit to show two your new readings so I'll have look and see if I can make any sense out of them.

If you want to do a new set of readings that's fine and is a good double check.

 

[Mooly]

Lol, I didn't turn the page... I''ll study your results

 

[Mooly]

Heres what I'm thinking so far...

Two of your results show Q613 with 62.6v on the emitter and 60v on the base. Something amiss there. Can you recheck that measurement under the same conditions and also check by measuring across the B-E junction as well. Thats very important. That B-E drop must not exceed around 0.8volts with the emitter more positive than the base. If it does we might have a problem with that device.
The other result shows Q613 with the base positive with respect to the emitter (so Q613 should be off) and yet there is still nearly 60 volts on the collector. So again that reading also points at Q613

Now there may be other problems too but this seems like one identifiable issue that we must correct first.

I would still leave Q617 shorted as it takes all the circuitry associated with it out of the equation.

 

[dtm1962]

Measurements Update

Heres what I'm thinking so far...

Two of your results show Q613 with 62.6v on the emitter and 60v on the base. Something amiss there. Can you recheck that measurement under the same conditions and also check by measuring across the B-E junction as well. Thats very important. That B-E drop must not exceed around 0.8volts with the emitter more positive than the base. If it does we might have a problem with that device.
The other result shows Q613 with the base positive with respect to the emitter (so Q613 should be off) and yet there is still nearly 60 volts on the collector. So again that reading also points at Q613

Now there may be other problems too but this seems like one identifiable issue that we must correct first.

I would still leave Q617 shorted as it takes all the circuitry associated with it out of the equation.

Hi Mooly:

Here are the results:

A) Q617 C & E shorted...

Q613 BE voltage drop = -0.6V drop with Red lead on E & black lead on B
=+0.6V drop with Red lead on B & black lead on E

Q613 voltages ref to ground = E = +56.5V; +57.3V; +56.7V


B) Q617 C & E NOT shorted...

Q613 BE voltage drop = -1.5V drop with Red lead on E & black lead on B
= +1.5V drop with Red lead on B & black lead on E

Q613 voltages ref to ground = E = +60.2V; +58.5V; +58.1V

Note: When Q617 C & E is shorted that the rail voltages goes down....

I hope this helps.

Thanks in advance,

 

[tauro0221]

Molly the voltage at the R627 one side read -58.8 volts and the other side +59 volts. The plus side should read +1.2 volts. No? Can he check the resistor R627 for open.

 

[dtm1962]

Hi Mooly:

Q613 voltages ref to ground = E = +56.5V; +57.3V; +56.7V


B) Q617 C & E NOT shorted...

Q613 voltages ref to ground = E = +60.2V; +58.5V; +58.1V

QUOTE]

So it should read as I forgot to identify the last 2 voltages for each...

A) Q617 C & E shorted...

Q613 voltages ref to ground = E = +56.5V; BASE +57.3V; COLLECTOR +56.7V


B) Q617 C & E NOT shorted...

Q613 voltages ref to ground = E = +60.2V; BASE +58.5V; COLLECTOR +58.1V

 

[dtm1962]

Molly the voltage at the R627 one side read -58.8 volts and the other side +59 volts. The plus side should read +1.2 volts. No? Can he check the resistor R627 for open.

R627 measures correct as it is 1/2W MF Dale versus 1/4W generic MF type as I have changed it out a couple of times. The ones I remove always mesures within 1% as well...

 

[Mooly]

Hi Dan,

I'm on a different PC so haven't all your info to immediate hand... but this might help 'till tomorrow. I'm still not really happy with Q613 readings and I know this all gets confusing too. The devil is in the detail with these numbers

1. With your red lead on E and black on B you measure minus 0.6 volts. That means the base is higher than the emitter and so the device should be OFF. That appears not to be happening as you have plus 60ish on the collector.

2. The 1.5 volts that you seem to be able to get across this junction with Q617 not shorted again should turn this transistor off.

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).

I'll be back on usual PC tomorrow... and maybe that will give you something to go on.

 

[Mooly]

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).

 

[jitter]

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.

 

[east electronics]

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 ...

 

[Mooly]

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.

 

[dtm1962]

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!

 

[dtm1962]

Measurements 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....

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,

 

[jitter]

A quick and dirty test... remove Q613 and see if the voltage on Q617 (still shorted) falls to near the minus supply rail.

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.

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).

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.

 

[Mooly]

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.

 

[tauro0221]

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.

 

[Mooly]

It is a strange result for sure. Certainly its worth checking the cap for leakage.