PET power supply questions

Ethan Dicks ethan.dicks at usap.gov
Sun Jan 13 16:22:09 CST 2008


On Sun, Jan 13, 2008 at 05:55:28PM +0000, Tony Duell wrote:
> Ethan Dicks wrote: 
> > Hi, all,
> > 
> > I've been collecting various classic bits to work on over the winter,
> > including a pre-CTRC 32K PET board (2001-N/3032) and a Static PET...

> I elieve the 8296D (internal floppy drives) PET used a switching supply. 
> Do any docs for that machine exist, I wonder...

Interesting.  I can check zimmers.net when I'm connected to the 'net
tonight.
 
> > Obviously, I'm unlikely to find a suitable mains transformer, so I was
> 
> Actually, I do have a 'spare' PET mains transformer somewhere, from 
> sombody who converterd a PET to the intenral drive model. The problems 
> would be (a) getting it to you and (b) it's the 240V-input version, and 
> (c) finding the darn thing.

Well... we are getting down to the end of the season here, so things
sent from this week onward have a decreasing chance of arriving before
the last plane. 

> You could consider winding the transformer. You can get 'transformer 
> kits' I believe that come with pre-wound primary windings to suit mains, 
> and a set of laminations. You wind the secodnaries (the instructions tell 
> you how many turns/volt to use) with suitable wire and put it together. 
> Said kits come in differnt total power ratings, either work out the total 
> power used by the PET, or estimate it from the size of the origjnal 
> transformer.

I have plenty of time to wind a transformer, but I probably don't have
time to get a kit like that in.  I don't happen to know the exact power
draw of a PET board - according to the 2001-N/3032 schematic on zimmers.net,
there are a pair of LM340s for +5V, a 7812 for +12V, and a 7905 for -5V,
but then off of the transformer, there's a tap for AC for the monitor
(which gets immediately regulated down to +12V with a local 7812), and
the cassette motor is powered by switched unregulated +9V, tapped off
between the bridge rectifier and the LM340s.  I don't plan on running a
_real_ cassette drive, but I think the latest versions of the C2N232
can use MOTOR-ON as another input line, so I'll need some small amount
of power for the motor circuit.

To get rolling, I was planning on building either the NAND-4066 or the
XOR video circuit that's been floating around for decades, then directly
connect a couple of bench supplies past the regulators which will give
me the benefit of being able to sample the current draws while
simultaneously running the board.  I don't have a keyboard here yet,
but I did just receive the C=Key, so I can build that up and get a
PET-friendly keymatrix downloaded to it.  After that, it's just a
packaging and storage-device issue.

-ethan

-- 
Ethan Dicks, A-333-S     Current South Pole Weather at 13-Jan-2008 at 22:10 Z
South Pole Station
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Ethan.Dicks at usap.gov            http://penguincentral.com/penguincentral.html



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