I would have said, when the replacement transistors arrive, remove Q331 and Q333. Do not replace.

Replace the blown fuse.
Do not connect speakers.
With volume level set to 0.
Power on. Wait to see if fuse stays ok. Listen for relay click.

Measure voltage at B pad/connection of each of the four power transistors with respect to ground (DMM set to read DC volts).

Edit: You can do this while waiting for the replacement transistors.

 

How do you know that it is bad?

50nF is acceptable for 47nF.
OL is the expected reading on an Ohmmeter. You cannot expect to read 10Ω because it is in series with a capacitor.

so it is still good uh?! I don’t know, it is all burnt on the side and started to melt the brown wire next to it... I was convinced that it was dead... I'll put it back then!!

 

I would have said, when the replacement transistors arrive, remove Q331 and Q333. Do not replace.

Replace the blown fuse.
Do not connect speakers.
With volume level set to 0.
Power on. Wait to see if fuse stays ok. Listen for relay click.

Measure voltage at B pad/connection of each of the four power transistors with respect to ground (DMM set to read DC volts).

Edit: You can do this while waiting for the replacement transistors.

All righ, so I put Z304 back on , I removed Q331 and Q333. I changed the fuse, put vol to minimum, powered the amp on, and no fuse flown . I could hear the relay click just like before.

Now, I'm not too sure about the measures you want me to do. Is it still with Q331 and Q333 removed, and the amp powered on?
And I measure DC voltage between leg B of each of the 4 power transistors to ground. Where do I take ground? On the frame of the amp?
Thanks!

 

Keep Q331 and Q333 removed.
Disconnect all speakers.
Set DMM to 200VDC range.
Connect DMM black lead to chassis (frame).
Measure voltage at E C B at all four output transistor connections Q331-Q334.

 

Keep Q331 and Q333 removed.
Disconnect all speakers.
Set DMM to 200VDC range.
Connect DMM black lead to chassis (frame).
Measure voltage at E C B at all four output transistor connections Q331-Q334.

In ECB order, Q331 I got 0;55V;0. Q333 0;55V;0. Q334 0;55V;0 and on Q332 I got a spark when measuring C, the fuse blew. I replaced it, tried to turn the amp on and it blew again... so with amp off, I measures EC resistance like before. Q334 is a s before (0 Ohms, then 2 MOhms increasing in both directions) but Q332 gives 0.4 Ohms... I think it died...
Shoud I remove it, change fuse, and measure voltage from ground to ECB again?

 

1) You got a spark because because your probe slipped and shorted out something on Q332.
Now you have to remove Q332 and Q334.

2) When your DMM indicates a negative sign it is important and essential that you report that as well. Otherwise we are missing vital information.

3) If there are digits after the decimal point, please report those as well. For example 55.0 must be reported as such.

4) Please remove Q332 and Q334 and repeat all measurements.
Measure all the 55V points on the 200VDC range.
Go back and measure all the 0V points on the 2VDC range.

 

1) You got a spark because because your probe slipped and shorted out something on Q332.
Now you have to remove Q332 and Q334.

2) When your DMM indicates a negative sign it is important and essential that you report that as well. Otherwise we are missing vital information.

3) If there are digits after the decimal point, please report those as well. For example 55.0 must be reported as such.

4) Please remove Q332 and Q334 and repeat all measurements.
Measure all the 55V points on the 200VDC range.
Go back and measure all the 0V points on the 2VDC range.

Here are the measures. Also I received 2x 2SA1106 and 2x 2SC2581 transistors.

 

Something not right.
Collector of Q333 and Q334 should be at negative 53V.
Check the voltages at C601 and C602.
Also at R383, R384.

 

Something not right.
Collector of Q333 and Q334 should be at negative 53V.
Check the voltages at C601 and C602.
Also at R383, R384.

Yes, I checked again and the values for Q333 and Q334 are negative. I didn’t see the negarive sign on DMM that is just under the DC sign, and they look like each other!! Needless to say that I'm not used of my DMM!
So are those value ok with the negative sign?

 

Yes, I checked again and the values for Q333 and Q334 are negative. I didn’t see the negarive sign on DMM that is just under the DC sign, and they look like each other!! Needless to say that I'm not used of my DMM!
So are those value ok with the negative sign?

Yes, there should be about -52V on the collectors of Q333 and Q334.
0V on all of the emitters is a good sign.

Where are we now?
Are all four transistors Q331-Q334 removed?

 

Yes, there should be about -52V on the collectors of Q333 and Q334.
0V on all of the emitters is a good sign.

Where are we now?
Are all four transistors Q331-Q334 removed?

Yes, all 4 are removed

 

Install new transistors Q331 and Q333.

Turn on power. If the fuse doesn't blow measure the voltages again.

 

Install new transistors Q331 and Q333.

Turn on power. If the fuse doesn't blow measure the voltages again.

I installed Q331 and Q333, the fuse didn't blow. Here are the voltages.

 

Voltages on Q331 and Q332 should be similar.
Voltages on Q333 and Q334 should be similar.

Q332 base voltage seems out of whack compared to the others.

Are you able to measure the voltages on Q330 and Q328 (without blowing anymore parts)?

 

Voltages on Q331 and Q332 should be similar.
Voltages on Q333 and Q334 should be similar.

Q332 base voltage seems out of whack compared to the others.

Are you able to measure the voltages on Q330 and Q328 (without blowing anymore parts)?

No spark this time!

 

Voltages on Q331 and Q333 appear to be reasonable.

According to the schematics,
Q330
b = 1.1V
e = 0.6V

Q328
b = -1.1V
e = -0.6V

Looks like you will have to replace both Q330 and Q328, along with Q332 and Q334.

 

Voltages on Q331 and Q333 appear to be reasonable.

According to the schematics,
Q330
b = 1.1V
e = 0.6V

Q328
b = -1.1V
e = -0.6V

Looks like you will have to replace both Q330 and Q328, along with Q332 and Q334.


View attachment 262660

I contacted a shop to procure the Q330 that is 2SD415 and Q328 that is 2SB549.
They told me that they don't have those but they have replacement ones:
2SB549 = 2SB649
2SD415 = 2SD600

Do you think that would be acceptable?

 

Yes.
The parameters you certainly want to match are:
TO-126 package
Polarity: NPN and PNP types
max voltage 100V
max current 0.8A
wattage 10W

next items to match are hFE and ft.

NPN
You can replace the 2SD415-R with
2SC2481, 2SC2481-R, 2SC2690, 2SC2690-R, 2SC2690A, 2SC2690A-R, 2SC3621, 2SC3621-R, 2SD600, 2SD600-D, 2SD600K, 2SD600K-D, 2SD669, 2SD669-B, 2SD669A, 2SD669A-B, BD791, KSC2690, KSC2690-R, KSC2690A, KSC2690A-R, MJE243, MJE243G or MJE244.

PNP
You can replace the 2SB549-R with
2SA1021, 2SA1021-R, 2SA1220, 2SA1220-R, 2SA1220A, 2SA1220A-R, 2SA1408, 2SA1408-R, 2SB631, 2SB631-D, 2SB631K, 2SB631K-D, 2SB649, 2SB649-B, 2SB649A, 2SB649A-B, BD792, KSA1220, KSA1220-R, KSA1220A, KSA1220A-R, MJE253, MJE253G or MJE254.

You want to select NPN and PNP complementary pairs.
2SD600 and 2SB631
2SD669 and 2SB649
MJE244 and MJE254

 

You want to select NPN and PNP complementary pairs

So 2SB649 and 2SD600 is not ideal?
2SD600 and 2SB631 is better?

 

I cannot say if one is better than the other or if it will work or not.
At this point you're trying to match the transistors' gain, hFE.
Now you also want to look at availability, price and delivery times.
Try to pair the transistors if you can.
2SD669 pairs with 2SB649.

For your reference have a look at these:

Complementary Transistor Pairs
https://www.smcelectronics.com/DOWNLOADS/COMPTRANS.PDF

2SD600 NPN power transistor
https://belchip.by/sitedocs/00014881.pdf

2SB631 PNP power transistor
https://www.electrokit.com/uploads/productfile/41012/2740.pdf

2SB649 PNP power transistor
http://www.secosgmbh.com/datasheet/products/SSMPTransistor/TO-126C/2SB649-649A.pdf