FNIRSI DSO152 oscilloscope is ok up to 40 V with a standard x1 probe. If you use a x10 probe, it is good to 400 V.
Yes, you can use the DMM to check the +60 and -60 V supplies on the collectors.

SO, we have +63.6 V and -63.8 V on collectors of Q504 and Q506

this is the measurement on Emitter of Q506:


Reading on Q506 Base:


While Q504 Emitter:


And Base:


Checking here and there on PCB, I found another pad gone, connecting the base of Q426 to R430 and E Q418
Could it be the actual cause?
I will test it tomorrow.


Thanks!

 

I'm afraid this amplifier might have reached the end of its life.
Here is an update on my troubleshooting:

• The main PCB: I refurbished the main board, which brought the right channel back to life, though it's now noisier than before. However, the left channel was completely dead.
• The preamp section: I traced the issue to the preamp board located behind the front panel. The signal goes in but doesn't come out.
• The connectors: Digging deeper, I noticed that wiggling the connectors on the motherboard affected the behavior. Realizing they were faulty, I replaced them with standard pin headers (striplines), hoping they don't have to carry too much current.
• The new issue: After putting everything back together and powering it up, the front panel LEDs no longer turn on.
• The culprit: I checked the ribbon cables (flat cables) connecting the front panel to the motherboard. It turns out the trace carrying the +18V rail is broken.

I will post another update as soon as I fix or replace the ribbon cable.
Thanks!

 

Update II
we are back to best result reached,
both channels playing sound, left clean, right with HF signal at about 450Khz, but with 1 Khz signal it changes to
Thanks

 

FNIRSI DSO152 oscilloscope is ok up to 40 V with a standard x1 probe. If you use a x10 probe, it is good to 400 V.
Yes, you can use the DMM to check the +60 and -60 V supplies on the collectors.

Hi Mr. Chips,
I finally managed to fix the HF oscillation issue! It was due to a resistor R470
that was way out of spec—it measured 18 ohms instead of 1 ohm.
Now the signal is completely clean on the right channel as well.
Thank you so much for your invaluable support!

I am seeking a couple of final suggestions. I now need to buy new power transistors; what do you think about choosing the 2SC5200N and 2SA1943N to replace the OEM 2SC3182 and 2SA1265?
Also, I need to clean the heatsink and reapply thermal paste. What is your opinion on replacing the existing mica insulators with silicone pads?
Thanks!
Alberto

 

Glad to hear that you found the problem with R470.

You can try the power transistor replacements. Remove all four transistors and try adding one pair first.

Silicone pads are fine. Always check that the transistor is isolated from the heat sink.

 

Thank you @alprampo, @MrChips and @Pyrex for your collaboration to fix another one of these Technics DC nightmares. I've been working on a very similar SU-VX920 for some months now (on and off). It seems totally unpredictable. But now I have translated your process and suggestions to component numbers for my model, I hope I can make some headway again. If not, I know where the experts are!
Thank you again for all the time and effort put in to these forums.

 

Thank you @alprampo, @MrChips and @Pyrex for your collaboration to fix another one of these Technics DC nightmares. I've been working on a very similar SU-VX920 for some months now (on and off). It seems totally unpredictable. But now I have translated your process and suggestions to component numbers for my model, I hope I can make some headway again. If not, I know where the experts are!
Thank you again for all the time and effort put in to these forums.

you're welcome

 

@pumptronics

"You're entering a world of pain!"

Feel free to ask—I'm happy to share everything I've learned so far!

Tomorrow I should receive the new 100% genuine 2SA1943N and 2SC5200N power transistors from Mouser. I'll post an update with the results once they're mounted.
Bought 20 to choose hfe closer.
Alberto