From: <(>
Date: Mon Jun 23 11:10:40 1997



> I was wrong. It's 17-0-17V, then. That's what I will use.

> > to ground, what else how can negative voltages can be generated so
> > it needs this ground. The both outside 2 pins for 17v ac lines goes
> Well, it could use a switching regulator. OK, I know it doesn't do so for
> the negative voltages - obviously I wasn't thinking straight earlier...
> > there. The card rectify it to generate 3 dc voltages, two is
> > positive and other one is negative. Bulk of the components is used
> > to generate 5vdc, and a wimpy 12vdc source for the floppy drive and
> The 5V regulator seems to be a switching one, which is what I'd have
> expected.
Huh? That would make sense because the linear regulator in T220 case
is limited to 1.5amp which it requires measly 3.6amp on that 5v

> Yes, is that suprising? After all, a switching supply is nothing magical.
> You still need to transform the AC output into DC and regulate it. Of
> course you can always perform some regulation on an SMPS by varying the
> drive waveforms on the primary side.
Hmmm...I fix PC powers and modern SMPS for the computer all the time
to save $. Around here in canada costs above $60cdn for a new
200watt PSU.

> Talking of bad design (I was earlier...) I've seen some _crazy_ SMPS's in
> my time. In a Zenith monitor there's a supply that combines the
> reliability of a switcher with the efficiency of a linear. It rectifies
> the mains, feeds it to a free-running chopper (no regulation applied
> here), shoves that into a little transformer, rectifies the output and
> feeds it (about 18V DC) into a linear regulator that uses the power-on LED
> as the reference. It's the only monitor that _requires_ a green power-on
> LED. When my blew up (chopper failed, etc), I replaced the entire mess
> with a little torroidal transformer. It was cheaper than a new chopper
> transistor and a lot simpler.
Ewwww.... that is bit overkill to design that! But closed loop
switchers is very efficient and compact but that transformer is
certainly larger than the orignals.
Are you talking about this beige tiny 12" green or amber monitor with
angular back? In this, chopper trans pops and kills one resistor.
They are usually either TTL or composite.
I have a working zenith Eazy PC with hd in it. Slow and buggy. :)
> Then there are the Boschert 2-stage supplies that are used in Sanders
> printers and PERQ 1's (and probably elsewhere). A shorted chopper in one
> of those (which is a very common failure mode) will blow up 2 more
> expensive power transistors and then take out a number of small
> transistors, the chopper control IC (a good old 723), a few passives and a
> couple of PCB tracks. I had to sort out such a mess once - I can provide
> the full story if anyone's interested.
Hmmm... you could email me privately with this account of yours. :)
I have a astec PSU switcher that one big transistor on masssive
heatsink driving BOTH switching transformers.
Is one of that odd design?

> Odd... That's not in my TechRef...

Well it was on different page I think, I did not have it as I was
looking through it and copying few vital items. But it did show the
pinouts only. Oh yeah, the power card does have a 2 pin connector
near the transformer input, that is wired directly to the center
tapped to the motherboard ground. It is required to make PCjr work

Check your techref, mine is different book is incorrect but my
analysis is correct and was used to power up PCjr via SMPS box: pin
1 is -5v, 2 and 3 is ground, 4 through 7 is 5v, 8 and 9 is ground
again, finally 10 is 12v. I guess it did not require -12v. oops. :)
Relation of pin A1 starts from front end works backwards to back for
pin A10.

For the transformer, it is 33watt fused, 17ac-0-17vac in same
order as power input for the power board.

By the way, where I could buy this real techref and how much?
Soooo, I could fix up it with addons to XT standards.

> But that is. The PSU card schematics (starting from the 3 pin connector)
> were included.
My source version didn't. :(


Jason D.
Received on Mon Jun 23 1997 - 11:10:40 BST

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