Technics SUV4x Power Supply Issue Help

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi

Just out of interest does anybody have a theory about what caused the original chain of events that caused the 15v power supply failure? The previous owner (did not appear technical at all) said they used it for a while until it started going into protection from time to time until it stopped completely.

C523 blew up when first plugged in caused by R535 failing. It came out of protection after replacing these. Why did the diode fail? The fuse at the back had also been replaced as well I noticed. The main circuit board had also been patched due to broken tracks.

Plugged in at wrong voltage perhaps or just aging components?

Thanks
 

AlbertHall

Joined Jun 4, 2014
10,888
The only thing I can see which would kill D706 is either Q702 or Q705 being short but then I would expect R702 to be burnt.
As you don't know what work had been previously done, you don't know which if any of those had already been replaced.
Old age gets us all in the end.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi all. I got the diode and fitted it. The 15v rail is back but no change to actual operations in terms of source selection and STK voltages are not quite correct as well. I may shelve this at least for now as there could be so many things wrong with this amp and I have already spent some time on this.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi. AlbertHall. Plus and minus voltages are back.

Pin 1 and 16 of stk are 0.5v and 0v.
Pin 2 and 15 are around 50 v
Pin 5 and 12 are almost 0 v

STK is cold
Not in protection mode and DC at speaker terminals is less than 2mv

This is from memory. I have put it in the loft for now, out of sight as I need to focus on work for a few days and it is annoying me!
I will report back with accurate voltages in a few days.

Let me know your thoughts
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi. It is still in the loft for now but IC502 pins 1 and 16 should be 1.8v but one is a negative voltage and the other is 0v. Could there be damage to the diodes enroute to IC501 or is IC501 damaged (I have a spare but do not want to swap without good reason) .

I am leaving the input selector for now and trying to work out if IC502 is damaged in ewhich case the amplifier is kaput!

Any thoughts?
 

AlbertHall

Joined Jun 4, 2014
10,888
Those are inputs to IC502 so incorrect voltages there are more likely to be IC501 outputs being wrong. It gets complicated because IC501 and IC502 are in a feedback loop so you can't find the fault by measuring a single place. If one voltage is wrong they will all be wrong because of the feedback. You have to measure around the loop to find the inconsistency.
 
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