Z551 and Z552 are emitter bias resistors.
Their condition (good or bad) does not affect the assessment and evaluation of the circuit at this point in time.
It is possible that Z552 blew when Q558 and Q560 both shorted.

We are trying to establish correct supply voltages at IC501 and IC502.

We were measuring supply voltages and you encountered fluctuating readings. Has this been resolved?
Even if the voltages are now stable, we have not identified why you were getting unstable readings. It would be instructive to find out what was the cause of this.
What caused it?
What was done to resolve this?

 

We were measuring supply voltages and you encountered fluctuating readings. Has this been resolved?

check post no. 59
https://forum.allaboutcircuits.com/...epairing-technics-su-v90d.210443/post-2037415

Even if the voltages are now stable, we have not identified why you were getting unstable readings. It would be instructive to find out what was the cause of this.
What caused it?
What was done to resolve this?

I think we're at a standstill for now, apart from identifying a few damaged elements

 

I think we're at a standstill for now, apart from identifying a few damaged elements

I don't give up so easily.
The proper diagnostic approach is to test functionality, not to search for damaged elements.

I have steered you in the right direction. You need to stay on track and stay focused.

The first step is to verify supply voltages.

You should see the same stable V+ voltage at the collectors of Q501, Q502, Q553, Q557, Q554, Q559.
Similarly, you should see the same stable V- voltage at the collectors of Q503, Q504, Q555, Q556, Q560.

If that is not the case, then there is something wrong with your testing procedures.

 

If that is not the case, then there is something wrong with your testing procedures.

That's right, it turned out that the cause of the unstable voltages was the lack of a board with outputs

New measurements :

Q557 - B: -4.2v C: 52v E: 0v
Q559 - B: -5,4v C: -52v E: 0v
Q560 - B: 0v C: -52v E: 0v
Q558 - B: 0v C: 52v E: 0v

TP401, TP402 all pins 0v

IC501 - pin 8 : 7,7v
IC501 - pin 4 : -7,7v
IC502 - pin 8 : 0v
IC502 - pin 4 : 0v

Q504 - E: 0v C: -53v B: -8,4v
Q502 - E: 0v C: 52v B: 8,4v
Q503 - E: -7,7v C: -52v B: -8,3v
Q501 - E: 7,7v C: 52v B: 8,3V

 

Thanks for these measurements. We are back on track.

We are going to focus on the LEFT channel.

Measure the voltages at Q551, B C E.
Note that it may be easier to measure these at R553 and R555.

 

Q551 - B: -5,3v C: -3,7v E: -6v

 

In terms of creating a shopping list for possible replacements, here is what I would suggest, with minimum quantity in braces (QTY). You can buy more for spares.

(4) IC501, IC502 M5219P, try RC4558P
(2) Q502 2SC2631
(2) Q504 2SA1123
(2) Q551, Q552 2SC1815
(2) Q558 2SC3281
(2) Q560 2SA1302
(2) R560, R564 2.2Ω 2W
(2) R568 0.1Ω 0.25W
(2) Z552 0.22Ω 5W wire-wound

 

At this stage, are we still taking any measurements? Or are we focusing on finding replacements?

 

We don't yet know which components caused the catastrophic failure. Our focus is on getting the LEFT channel working so that we can get some real live voltages. I am contemplating replacing Q551 in the LEFT channel as a first stab at it.

Sooner or later you will need to buy components. Rather than placing multiple orders, it would be better to get them all at once in order to save time and shipping charges.

You will have a hard time replacing Z552. I am suggesting getting two 0.22Ω 5W resistors to replace the one dual resistor.

 

Here's a list of the components I need, along with my suggestions. Please confirm compatibility and help me select the remaining components:

1. IC501/IC502--- M5219P
https://www.digikey.com/en/products/detail/texas-instruments/RC4558P/277060

2. Q502------------- 2SC2631Q --?

3. Q504------------- 2SA1123R -- ?

4. Q551/Q552------- 2SC1815BG
https://www.digikey.com/en/products/detail/onsemi/KSC1815YTA/1047381

5. Q558/Q557------- 2SC3281 -- ?

6. Q560/Q559------- 2SA1302 -- ?

7. Q556-------------- 2SA1535AQRS -- ?

8. Q554-------------- 2SC3944AQRS -- ?

9. R560/R564-------- ERD25FJ2R2 2,2 1/4
https://www.digikey.com/en/products/detail/koa-speer-electronics-inc/CF1-4CT52R2R2J/13537675

10. R558/R557-------- ERD25FJ331 330 1/4
https://www.digikey.com/en/products/detail/koa-speer-electronics-inc/CF1-4CT52R331J/13537270

11. Z552-------------- ERF3GBKR22N
https://www.digikey.com/en/products/detail/yageo/SQP500JB-0R22/9167787

current basket :
https://www.digikey.com/short/bh73403z

If anyone has or knows where to find the originals, please let me know.

@abrsvc You wrote at the beginning that you have some components, please let me know how I can contact you because I can't send you a private message.

@MrChips In your proposal you provided higher resistor values, will these be better than the original values? Will my proposal, IC501, be better?

 

Q501, Q502 ---- KSP43BU
Q503, Q504 ---- KSA1013YBU
Q553, Q554 ---- TIP29CG
Q555, Q556 ---- TIP30CG
Q557, Q558 ---- 2SC5200
Q559, Q560 ---- 2SA1943

Which resistors are you referring to?

 

Get 8-pin DIP sockets for IC501, 502.
And you will need some heat-sink compound.

 

Which resistors are you referring to?

I was referring to the R560 resistor you mentioned in post #67. You gave a wattage of 2.2Ω 2W, while the original is 2.2Ω 0.25W.

I compared the values of your suggestions with the original parts:
Q501, Q502 ---- KSP43BU
Q503, Q504 ---- KSA1013YBU
Q553, Q554 ---- TIP29CG
Q555, Q556 ---- TIP30C

Are you sure they will be sufficient? They differ significantly from the original parameters.

KSP43BU which you provided as a replacement for Q501, Q502 has a different pinout, photo in attachment.

If we manage to get the amplifier working, I would like it to be rebuilt with good quality components


Will these solutions be better?

Q501, Q502 ---- https://www.digikey.com/en/products/detail/onsemi/KSC2383YTA/1048940

Q503, Q504 ---- https://www.digikey.com/en/products/detail/onsemi/KSA1013YTA/1048933

Q553, Q554 ---- https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TTC004B-Q/6600430

Q555, Q556 ----- https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TTA004B-Q/6600426

Final basket:
https://www.digikey.com/short/dc5z80hw

 

My mistake. R560 is 2.2Ω 0.25W.
I would use 2.2Ω 0.5W.

 

Should I also use a higher value of 330Ω 0.5W for the Q558/Q558 330Ω 1/4W resistor?

@MrChips Will the transistor suggestions I gave in post #73 be any good?

 

Yes. Use 0.5W for R557/R558 330Ω

I made a mistake with Q553-Q556. (I was confusing them with Q551-Q552).
I am surprised that Q553-Q556 are not mounted on heat sinks. There is room to put some cooling fins on them, something like this:

https://www.digikey.com/en/products/detail/boyd-laconia-llc/577500b00000g/240991

 

Yes, that's not a good solution. I'll order some heatsinks...

regarding Z552--- ERF3GBKR22N can I use something like this?
https://nl.aliexpress.com/item/1005...etail.0.0.5e216197EzQV4s&gatewayAdapt=pol2nld

 

Yes, that's not a good solution. I'll order some heatsinks...

regarding Z552--- ERF3GBKR22N can I use something like this?
https://nl.aliexpress.com/item/1005...etail.0.0.5e216197EzQV4s&gatewayAdapt=pol2nld

The problem there is that the resistors are not long enough and the leads will not reach the holes.
With single resistors, you can insert one resistor and connect the second one to the first.

 

You're right, it has completely different dimensions. I just checked it now.


Will two such resistors work as a replacement when connected?
https://www.digikey.com/short/z5h90fn3

https://www.digikey.com/short/588bvh8h

which one will be better 5w or 3 like in the original?

 

One is not better than the other. One can dissipate more power than the other.
One, you need to purchase 2000 pieces. The other, you can buy as many as you need.