[sdiy] Matrix 12 PSU

[ASSI]

On Wednesday 14 April 2004 21:04, ASSI wrote:
[...]

For all those who inquired about the parts (and obviously some have 
missed part of the previous discussion, so I'll repeat): it is the 
primary transformer I intend to replace because it hums badly, not the 
one in the switching power supply itself. The faceplate says 60VA max. 
and I have measured the secondary current to be 2.75A @ 18V~, which 
seems to agree - and the primary current at 0.4mA @ 230V~, which not 
exactly does, as the iron and resistive losses should not be that high. 
But this explains nicely why the damn thing hums so loudly and gets so 
hot. Caution: a chap from Netherland has a Matrix-12 that takes 3.8A @ 
16V (the original transformer has burned, so he doesn't know what 
things would look like at 18V) - so measure first, cut later.

Selection criterias: toroids have much less iron loss and practically 
no stray magnetic fields and or hum, so I'm not going to put in a 
packed core transformer again(1). Since toroids have higher switch-on 
current (max. about 25x nominal) and the nominal voltage is given for 
full load and increasing by up to 7% at half load, I'm trying not to 
overdimension too much. Many toroid series give the choice between 50VA 
and 80VA, the first is uncomfortably tight and might overheat, the 
latter I deemed too much and might stress the switching PSU with too 
high input voltage. I found some 60VA types from other manufacturers. 
To keep the 110V/220V switch of the Matrix-12 functional and the 
original varistor snubbers in place one needs a transformer with split 
primary windings as well. Looking for distributors left me with a 
Multicomp TA060/18 (Farnell part no. 306-8754), which is rated at 3.33A 
secondary. The original transformer has only a single secondary, but 
the plug to the PSU has two pins per phase nevertheless (this is likely 
a remnant from the Xpander PSU which used a split secondary with 
half-bridge rectification), so the split secondary that seems to be 
standard for these trasnformers is easily accomodated.

I haven't ordered anything yet as I am still collecting information on 
a few other things to fill up the order to a good size. Also I haven't 
fully thought through the mechanical aspects and will need to take a 
few more measurements now that I know the dimensions of the 
transformer. My current plan is to replace the pins in the PSU plugs 
with new ones crimped to the secondary and new FastOn crimps on the 
primary. But since I'm having to accomodate the varistors (and can't 
solder them to the transformer like they are now) I might decide to 
leave the old leads in place and just connect the new transformer with 
solderless splices (Electrotap or similar). I'll make some photographs 
before and after once I get the parts and actually do the repair. Stay 
tuned, it has only been a year since I first recognized that I wouldn't 
tolerate the hum from the transformer... :-)

To clarify some other question: I have no information about the 
transformer used in the switching PSU other than the voltages it 
produces for the Xpander (the service manual for the Xpander can be 
downloaded from somewhere... just Google for it). If that one is broken 
things get interesting and I'd say the most promising way would be to 
take it apart, unwind (and count the windings!) and then rewind it. 


1) If you want to go that route, there are plenty models available. The 
core used in the Matrix-12 is good for about 90VA, but obviously the 
windings are dimensioned to be a bit less capable.

2) I've briefly pondered to replace the primary transformer with a 18V= 
wide-range input switching PSU. I buried the idea only because I 
couldn't source a suitable type at a reasonable price. I'm still 
looking, though...


Achim.
-- 
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