8 Jun
2010
8 Jun
'10
12:02 a.m.
On Monday (06/07/2010 at 07:21PM +0100), Tony Duell wrote:
The 8V supply uses a bridge in a chassis mount package. The +/- 12V
supply uses a 24V CT secondary of the same transformer feeding a bridge
built from discrete diodes on a PCB.
Here's a picture of the whole shbang,
http://www.swtpc.com/mholley/MP_P/PowerSupply_Photo.jpg
as well as all the docs for the (trivially simple) supply, SWTPC MP-P,
http://www.swtpc.com/mholley/MP_P/MP_P_Index.htm
and Micheal Holley documents his modification to improve the supply here,
http://www.swtpc.com/mholley/MP_P/M6809/Power_Supply_Mods.htm
I have indentified two transformers that can be stacked in the same footprint
as the single original so I will not have to drill new holes to mount the
second transformer near the power distribution board nor will have I have
to mount the power distribution board on its side as Micheal did.
Well, there's only two other parts (a bridge and giant cap) to swap after
that so I will get there pretty quickly if it's not the transformer ;-)
Chris
--
Chris Elmquist
Ya... thanks
guys. The transformer is rated at 7.25V @ 10A. But it's
not even able to do 7.25V at 5A so I think it is the culprit.
Does it have just a simple 2-terminal secondary winding, or is it
centre-tapped? What sort of rectifier are you using (bridge, 2 diode
biphase, etc)? 
Does the thransformer 'look big enough'? If it
is 7.25V at 10A, that's
72.5VA. Is the core comparable in size to other 72VA transformers? No,
this isnot an exact thing, but in general the size of a small transformer
core (small being less than a couple of kVA...) gives a good idea as to
its VA rating.
It seems small to me but that's a very subjective measurement.
I am wondering just what sort of fault could cause
this other than a
mislabelled transformer.
I'd offer that it was an under designed supply and it could never
really source the kind of current they claimed. The transformer has no
markings on it so not possible to identify the manufacturer or actual
specifications. The specs I am operating against are those in the SWTPC
MP-P documentation, where the claim of 7.25V at 10A is made.
I think I have
converged on my solution. I have two new Triad
transformers that when bolted to a piece of aluminum angle stock,
suitably cut to size, will fit stacked in the same footprint as the
original transformer. Therefore, I won't need to drill any new holes
in the chassis, will have a "period" accurate modification and the thing
should work as was originally intended.
Sounds like a good modification. Totally reversable, and it keeps the
circuit design much the same.
I just hope the problem is just the transformer...