Amp In Protection Mode

[Rick79]

Rick,
I would go with Edd's and Sunnysky's suggestion at this stage:
replacing the Trs from the bad channel and inserting them in the good one, after they are cleaned well(photo?).
but first inspect Q601 visually for corrosion etc.
and please measure the voltages relative to Gnd on the output (junction of R646 and R648 ,if it is easier measure the voltages on both sides of R628 instead),and the voltages on Q608.Q606

Do I still leave the leg of R627 off and Q605 off the board before I swap Q603 and Q604?

 

[dorke]

When testing the "good" channel it has no effect,you can leave it not connected
when putting Q603 in the place of Q604.
Have you cleaned it well?
When you take out Q604 ,please take a photo of it's bottom.

 

[Rick79]

Rick,
I would go with Edd's and Sunnysky's suggestion at this stage:
replacing the Trs from the bad channel and inserting them in the good one, after they are cleaned well(photo?).
but first inspect Q601 visually for corrosion etc.
and please measure the voltages relative to Gnd on the output (junction of R646 and R648 ,if it is easier measure the voltages on both sides of R628 instead),and the voltages on Q608.Q606

OK I cleaned the transistors.
With Meter set on 200 V, R628 reads 0.71 V on one side and on the other side it reads nothing.

Voltage Readings for Q608
01.6 V ==>E
46.0 V ==>C
02.1 V==>B

Voltage Readings for Q606
46.0 V ==>E
02.1 V ==>C
45.4 V ==>B

Then I took out Q604 & installed Q603 in its place, R628 still read 0.71 V & nothing.
So I put Q604 back in its place.
Then I took out Q602 and installed Q601 in its place. R628 still read 0.71 V & nothing.

Photo is of Q604

 

[dorke]

I assume that "reads nothing" you mean zero volts?

Q606/Q608 readings are exactly as they should be.

All that means Q603 and Q601 are both fine!
Did you have to clean Q601 as well?

 

[Rick79]

I assume that "reads nothing" you mean zero volts?

Q606/Q608 readings are exactly as they should be.

All that means Q603 and Q601 are both fine!
Did you have to clean Q601 as well?

Sorry, Yeah nothing means 0V
No, Q601 looked ok

 

[dorke]

O.K

Lets put Q602 back in it's original place.
Please measure the voltages on:
R622
R646,R648(should be in the mV range).
R624(should be zero).
What do we get?

Then,
Please put back Q601 and Q603 in their original places,
re-connect the lifted leg of R627 to GND .
and measure the voltages on:
R621
R623
Q603
What do we get?

 

[Sunnysky]

When testing the "good" channel it has no effect,you can leave it not connected
When putting Q603 in the place of Q604.
Have you cleaned it well?
When you take out Q604 ,please take a photo of it's bottom.

Do I still leave the leg of R627 off and Q605 off the board before I swap Q603 and Q604?

R627 must be reconnected properly in order for the Q603<>Q604 swap test to work!
@dorke The loop must be closed.

Your test method is at fault.

 

[dorke]

Sunnysky,
The note meant R627 is part of the left(bad channel) and has no effect on the right(good) channel-in which both Trs 603 /604 are tested.
On the right channel R628 is connected normally.
Both Q603(cleaned) and Q604 work properly on the right(good) channel.,hence they are both fine

 

[Sunnysky]

Was the left channel tested with good parts and offset adjustment done?

 

[dorke]

No,
It will be tested with the original Q601/603(they are confirmed good after cleaning in the good channel) gradually,first without Q605/Q607 , then with them, and then adjusted.
Let's see what more surprises await us...

 

[Sunnysky]

Sim for DC offset. Switch and 100k leakage pot with Bias and offset adjust included.

 

[Rick79]

O.K

Lets put Q602 back in it's original place.
Please measure the voltages on:
R622
R646,R648(should be in the mV range).
R624(should be zero).
What do we get?

Then,
Please put back Q601 and Q603 in their original places,
re-connect the lifted leg of R627 to GND .
and measure the voltages on:
R621
R623
Q603
What do we get?

OK, Q602 back in its place.
R622 reads 47.5 V & 47.8 V
R646 reads -43.0 V & -43.0 V

With meter set on 2 V, R648 reads .005 V & .003 V
R624 reads .724 V & .724 V

Then with Q601 & Q603 back in their place & R627 leg connected to GND...
R621 reads 47.7 V & 47.1 V
With meter on 2 V setting, R623 reads .074 V & .005 V

Meter on 2 V setting, Q603 reads...
.074 V ==>E
.735 V ==>B
.177 V ==>C

I still have Q605 & Q607 off the board.

 

[dorke]

Rick,

The reading you get on R623 in #92 are wrong,
could you please confirm them(relative to GND).

Please measure DCV ,in the lowest range possible with both probes on both resistor legs:
R623,R624
R621,R622
R607,R608
R617,R618
a total of 8 measurements.

 

[Rick79]

Rick,

The reading you get on R623 in #92 are wrong,
could you please confirm them(relative to GND).

Please measure DCV ,in the lowest range possible with both probes on both resistor legs:
R623,R624
R621,R622
R607,R608
R617,R618
a total of 8 measurements.

R627 leg still connected to chassis Ground & black probe on chassis Ground, Meter set on the lowest range 200mV, R623 reads 72.8 mV & 05.1 mV
Then with both probes on resistors legs...
R623 reads 67.6 mV
R624 reads 00.0 mV
R621 reads .601 V with meter set on 2 V
R622 reads .286 V with meter set on 2 V
R607 reads .59.2 mV
R608 reads 10.2 mV
R617 reads 01.9 mV
R618 reads 01.2 mV

I don't get a reading for R621 & R622 unless I move the meter range up to 2 V

 

[Sunnysky]

R623 reads 67.6 mV. ...... 39Ω ~2mA implies leakage across C613 if stable. bad cap?


R621 reads .601 V ... shud be similar to R622 but depends on offset adjustment VR601

If not adjustable , something is still pulling more current on Q603 that forces its collector lower and output to speakers +ve high DC offset in original condition. Initially VR601 could not null the final output offset. I suspect it is still true that R621cannot be trimmed down near same as R622.

 

[dorke]

O.K
As I suspected ,the reading on R623 is indeed wrong,
there should be no DC current through it (hence no voltage on it).

Please take a good photo of of R623,C613,C617 and vicinity.
Please take a good photo, of the solder side of R623,C613,C617 and vicinity.

Could be dirt oxidation or corrosion .
If dirty clean it well.

If clean, take out C613(note it's polarization) and C617.
Power up and re-measure voltage between both legs of R623 if we get DCV of 0.0mv we have a bad cap ,
most probably the electrolytic C613.

At this stage,measure the voltage on both legs of R621 and trim it with VR601 to about DC 300mv.

 

[Rick79]

O.K
As I suspected ,the reading on R623 is indeed wrong,
there should be no DC current through it (hence no voltage on it).

Please take a good photo of of R623,C613,C617 and vicinity.
Please take a good photo, of the solder side of R623,C613,C617 and vicinity.

Could be dirt oxidation or corrosion .
If dirty clean it well.

If clean, take out C613(note it's polarization) and C617.
Power up and re-measure voltage between both legs of R623 if we get DCV of 0.0mv we have a bad cap ,
most probably the electrolytic C613.

At this stage,measure the voltage on both legs of R621 and trim it with VR601 to about DC 300mv.

Not sure I'll have time to pull out the capacitors and do the readings today, but here are some photos.
Could I use C614 in C613 place for testing like how I did with the transistors?

 

[dorke]

You can,
but there is no need to,
simply pull out C613 alone (leave C617 in for starters) and check without a cap in it's location.

A few notes:

1.The Print-Side looks very nice,no visible problems.

2.R627(Blue) is lifted and not connected,at this stage it should be connected to GND.

3.The pot(Red) has some glue residue which may cause problems(wiper not touching well),
but lets leave that for now.

4. C613 and another cap (both Green) have some stuff around them ,
that may be glue or the guts of the caps spilled out.
What is the other cap(C619)?
Does C614 have the same stuff around it?

 

[Rick79]

You can,
but there is no need to,
simply pull out C613 alone (leave C617 in for starters) and check without a cap in it's location.

A few notes:

1.The Print-Side looks very nice,no visible problems.

2.R627(Blue) is lifted and not connected,at this stage it should be connected to GND.

3.The pot(Red) has some glue residue which may cause problems(wiper not touching well),
but lets leave that for now.

4. C613 and another cap (both Green) have some stuff around them ,that may be old glue or the guts of the caps spilled out.
What is the other cap(C641)?
Does C614 have the same stuff around it?


View attachment 28515

Oh yeah, I had the ground wire connected, but i disconnected for the photos.
The other capacitor is C619. I think it is old glue, the same brown stuff is on C614 as well.

 

[Sunnysky]

The glue is added by factory to prevent vibration on all big radial parts is ok and broken leads or moving pot position.

I would still swap Q603.

RV601 looks like it is already adjusted to minimum CCW with possible no contact. Check voltage on center tap when adjusting for contact test.

We're still looking for 0.5 to 1mA DC excess sink thru/around Q603.
If C613 was mysteriously in backwards that would explain it

FWIW.
If a large signal 10Hz input is applied C613 only see 2Vpp. An alum. e-cap can only handle reverse voltage up to 10% of rated forward voltage. After this damage may occur and of course too much then they heatup. and go pop with no current limit, but this has current limit for bass low end attenuation.