@mjaa
Thanks! I couldn't have done this if it wasn't for Mr.Chips.
But i think it's really interesting.

 

Finally my transistors arrived.

I got only one of Q420, C3311..
But ten pcs. of C2631 and eight of A1123

What is our next move, Mr.Chips?

 

Ok.

Install new Q406 - 2SC2631
Install Q408

Power ON and check for smoke, especially at R414.

Measure voltage at
R428
R430

 

So i installed Q408, and a new one 2SC2631 at Q406.

Power on.

R414 smoked instantly.

Measured voltage:

R428: - 56,8 / -54 Volt
R430: - 59 / -64 Volt

 

That's a bummer!

Remove Q408 and R414.
Install new R414, 100Ω.
Leave out Q408.

Power ON and measure voltage at R414.

Edit: That was an error. I meant Q406. At this point, it doesn't matter any more.

 

Removed Q408, replaced R414 100Ω.

Power on, no smoke.

Measured voltage at R414: -64 / -64 Volt

 

Thanks.

There is a short or bad component bringing -64V to R414. We've got some digging to do in order to get to the bottom of this.

The first thing would be a close, careful visual inspection of both sides of the board, top and bottom around the following components looking for shorts or bad solder bridges:

R414
R416
C410
D412
C414
Q412
Q416

Next step would be to set you DMM to the lowest resistance range.
With the power OFF and disconnected, see if you can measure resistance across
C410
D412
These two are in parallel. Hence you will get the same reading.
Measure resistance one way and again with the leads reversed.

Later, we may have to pull some components and measure.

(I will look at the photos already posted to show you the area we need to investigate.)

 

Can we zoom in on this section of the solder side of the board?

 

I understand, i have now inspected the board on the underside and overside carefully at these components you have mentioned, i cannot find any shorts or bad solder bridges.
I took a picture of the area you wanted to look closer on, it is not a very good picture as i didn't have the best light available.
Notice the PCB beginning to get a little worn at R414.. it doesn't take the heat very well always. I had to scrape off a little of the green "paint" to get a new point to solder R414, but that shouldn't be a problem.
Just to mention why it looks like it does.

I measured D412 and C410, and they have the same values as you mentioned.

So i set my DMM to lowest resistance range:

D412: With red nearest the heat sink: 80Ω
With black nearest the heat sink: 79,5Ω

C410: same as D412

 

Please remove C410 and measure its resistance out of circuit.

Also, remove Q406 and test both Q406 and Q408.

After doing that, set the meter to diode test.
Test the diodes both ways, forward and reverse, while still in circuit, power OFF:

D409
D411
D410
D412

For your convenience,
J119 is the connection where D409 and D411 are joined.
J124 is the connection where D410 and D412 are joined.

 

Ok, so i've done what you've said.

I removed C410; and i get open circuit on any resistance measuring on this capacitor.

So i measured the transistors again, out of circuit of course... has my new Q406 got killed?
Q406:
RED probe to B.
BLACK probe connected to E: 753 mV
BLACK probe connected to C. 764 mV
Reverse the measurements as follows:
BLACK probe to B.
RED probe connected to E. 1053 mV
RED probe connected to C. Open circuit
Connect RED probe to E.
Record DMM reading with BLACK probe connected to C. 1015 mV
Reverse the procedure as follows:
Connect BLACK probe to E.
Record DMM reading with RED probe connected to C. Open circuit

Q408:
Connect RED probe to B.
Record DMM reading with BLACK probe connected to E. Open circuit.
Record DMM reading with BLACK probe connected to C. Open circuit.
Reverse the measurements as follows:
Connect BLACK probe to B.
Record DMM reading with RED probe connected to E. 884 mV
Record DMM reading with RED probe connected to C. 852 mV
Connect RED probe to E.
Record DMM reading with BLACK probe connected to C. Open circuit.
Reverse the procedure as follows:
Connect BLACK probe to E.
Record DMM reading with RED probe connected to C. Open Circuit.

So the diode measurements:

D409: 953 one way and open the other
D411: 955 one way and open the other
D410: 947 one way and open the other
D412: This measures 084 both ways !

 

Yes, Q406 gets killed along with R414 when this happens.

See if you can test Q412 and Q416 while they are still in-circuit.
If we get uncertain results we might have to pull them and measure out of circuit.

Looks like D412 is bad. We will have to replace both D410 and D412 at the same time.
Part # MA4082MTA 8.2V zener diode

Substitutions are:
NTE5016A 8.2V 500mW
1N5237

 

I stopped by the local service shop and asked if he had the right type of diode.

He got these Motorola 8,2 V zener diode, but they are a bit bigger then the ones in the amp.
They are marked with C8 and V2, he didn't know the exact wattage but he said sure i could use them...
I have googled the numbers and i believe they "are" 1300 mW ...

What do you think MrChips? Could we use these?

 

Ok. Since you have them on hand we can try it and see if it will work. But before we do anything, please check Q412.

 

I removed both Q412 and Q416 and measured out of circuit.


Q412:
Red to B:
Black to E: 842 mV
Black to C: 824 mV
Other measurments are open circuit. So this is ok.

Q416 has short in every measurement.

 

Leave out Q406 and Q408 for now.
Install Q412.
Install new Q416.
Instead of installing the D412 replacement, just solder-tack it on the solder-side of the the PCB. Watch the direction of the diode very carefully. The cathode (black band) of D412 goes towards J124. (The black band of D410 goes away from J124. We are not going to replace D410 at this stage.)
Install C410.

Power ON and hope for the best.
Switch the DMM back to voltage measurement.
Measure voltages at
R414
R416
D410
D412 (on back of PCB)
R428
R430

 

MrChips,

May I recommend the measurement of the Q416 emitter to ground as well as Q414. The shorting C-E of Q416 is the primary cause of R414 dissipating more than 0.25 W. A Q414 C-E short will test R416's ability to dissipate more than 0.25W. If the readings are similar, I would eliminate the loading as a cause of the C-E short.

 

MrChips,

May I recommend the measurement of the Q416 emitter to ground as well as Q414. The shorting C-E of Q416 is the primary cause of R414 dissipating more than 0.25 W. A Q414 C-E short will test R416's ability to dissipate more than 0.25W. If the readings are similar, I would eliminate the loading as a cause of the C-E short.

Thanks for the tip. As far as I can tell, Q406, D412, and Q416 all shorting would cause R414 to blow. I cannot tell what was the cause and what was the after-effect.

I'm also concerned that it took out dual opamp IC402 (M5218AP) in the process. We have to contemplate having to replace IC402 before going any farther.

 

I agree. Individually Q406 C-E short or Q416 C-E short can cause R414 to smoke. If only D412 were shorted, R414 and R416 both would smoke.

I'll get back to reviewing the preceding and post circuitry and get back later.

 

@JoeJester - I'm afraid that when Q416 shorted, it put -60V on IC402 (M5218AP). I think we should go ahead and replace it anyway.

Take a look at the M5218 Datasheet.
Possible replacements are:

RC4558
RC5532
LM4562
MC1458
MC1558

Any suggestions?