Colin, thank you very much!!!
Btw. I never realized these PSU's are dated 1984. A piece of history. :-)
Also I always wondered why there is a zener diode (I never saw this kind of
computer power supply) instead of IO 7812.
And technically, why there is that R1 resistor? Is its purpose to limit maximum
current on 12V line (i.e. something like a cheap reversible fuse)?
Aley
______________________________________________________________
Od: "Colin Piggot"
Komu:
Datum: 15.01.2012 10:52
Předmět: Re: Power supply circuit diagram
Aley wrote:
You can clearly see that D4 is missing (right-bottom), and there is a very
large old zener diode put there instead. It is Tesla 6NZ70 - 12V 1.5W
Zener
diode which proudes a LOT of heat. (If I count correctly, a 2.5W diode
should be used instead.)
That's not the standard part... never seen that old type of Zener diode used
in the SAM power supply - perhaps that PSU has been repaired by someone else
in the past?
But yes, D4 is the Zener diode - the 1N5349B I suggested is a 5W diode which
I use as a replacement.
I am also suspicious about R1 (large grey resistor right beside D4) which
is a bit mechanically damaged (clearly visible on the second photo),
I would recommend replacing R1 - 33ohm if I remember correctly. I use a 10W
part for this - as I've repaired quite a few power supplies where the normal
wirewound resistor has charred the PSU and melted the inside of the power
supply case when the 12V line fails.
and also C4 (one of the large capacitors) which seems to me a bit
bloated.
With these power supplies being made around 1984 (going by date codes I've
seen on the transformers inside them) replacing the electrolytic capacitors
is a very good idea.
Colin
=====
Quazar : Hardware, Software, Spares and Repairs for the SAM Coupé
1995-2012 - Celebrating 18 Years of developing for the SAM Coupé
Website: http://www.samcoupe.com/
