After reading all these posts, the modification is likely over my head. I
think I'll sell it and find a nice used variable supply. Thanks for all in
the input.
"Paul Keinanen" wrote in message
...
On Sun, 04 May 2008 23:51:08 -0700, geek
wrote:

On Mon, 05 May 2008 02:04:09 -0500, Ralph Mowery wrote:


"Toby" wrote in message
news:i8vTj.791$b%1.295@trndny04...
I have an Astron RS-20A, 20amp 12v fixed DC supply. I've heard that
people
have modified this model to become a variable voltage supply but have
not
found any schematics etc for this mod. Can anyone point me in the right
direction? Thanks!


Here is the schematic for a 35, it should ve similar to the 20 except
for
an extra pass transistor or two.

http://www.repeater-builder.com/astr...tron-vs35m.jpg

The diagram looks strange to me, with a direct connection from the
higher voltage driver supply (e.g. pin 12 of the 723 or collector of
Q2) to the base of the series pass transistors.

With a higher separate driver power supply the driver current should
flow through the Q2 driver transistor, through the small resistors
into the base connection of the series pass transistor. Thus in the
diagram the direct connection from the driver supply to the series
pass transistor base should be removed.

I modded one of those significantly. Too long ago to remember the exact
thing I did, but DO put that crowbar SCR anode on the collector of the
pass transistors. If it trips as is, you kiss the current meter and
expensive current sense resistor goodbye.

When using crowbars, there should be a some kind of fuse on the
feeding side, preferably immediately after the filter capacitor.

If the crowbar is before the series pass transistors, it is a good
idea to put a large reverse biased diode across the series transistors
(CR7 in the diagram), which would discharge any large capacitors on
the regulated side through the crowbar. Without the diode, the
collector side would be at 0 V after the crowbar tripped, but the
emitters could still be at +12 V due to any large capacitors on the
regulated side.

Paul OH3LWR