Sansui 517 - complete restororation

[Emanuele]

Hi all, i started a 517 complete restore, all electrolitics, zener, fuse resistor, 1212 diode, trimmers, killer glue etc...
During revision of 2721 and 2722 mono power module, i noticed a strange incongruence.
In one of the two modules, as you can see in attached pictures, diode d02 and d04 are soldered inversed compared to pcb serigraphy!
I can't see any rework on solders, so i think that diode was wrongly mounted originally.
What do you think about?

Thanks!
Emanuele

 

[Hyperion]

What do you think about?

I think you are right - those 2 diodes are mounted incorrectly, it is possible that someone turned them round after it left the factory, but unlikely especially as you say the solder appears untouched. They relate to the overcurrent sensing part of the protection circuit, which would not have worked for that channel.

 

[Emanuele]

So Hiperion, do you suggest me to restore correct diode position? It’s possible that there are different version of pcb?
Thanks
Emanuele

 

[Robisme]

Did you check the schematic. Could be the silk screening on the board is wrong.

Rob

 

[Hyperion]

Did you check the schematic. Could be the silk screening on the board is wrong.

Rob

I hear what you say Rob, based on my experience of restoring 2 (or was it 3) of these, and having just checked the board layout diagram against the schematic, I think those diodes are back to front.

So Hiperion, do you suggest me to restore correct diode position? It’s possible that there are different version of pcb?
Thanks
Emanuele

Sometimes there are anomalies like that especially with different PCB versions, but in this case I am not aware of any PCB revisions or other issues which might be in play here. I think they are installed the wrong way round and should be re-positioned to agree with the silk screening.

It is interesting that with the diodes reversed, no issues were apparently experienced. I have thought about what would happen if they were turned round, and can see that nothing would happen except no high current protection for that channel. The protection circuit high current protection is not often triggered in use, and may simply not have been noticed when the amplifier was tested at the factory.
Or there may be another story behind how that driver board got into your AU-517 - a Sansui Authorised Dealer servicing event perhaps?

 

[SansuiJavier]

In all the pics that I can find, they are according the board, in mine as well.

 

[Robisme]

Ya, looking at the connection to R40 they look backwards. Good catch.

Rob

 

[Hyperion]

Yes, a very good catch from the OP.

 

[Emanuele]

Here we go, all done!
Diode position restored, Bias and DC ok, sounds lovely!

Emanuele

 

[Hyperion]

Looks lovely too