On Tue, 06 May 2008 16:25:50 -0700, jimmie68 wrote:

On May 5, 2:51*am, geek wrote:
On Mon, 05 May 2008 02:04:09 -0500, Ralph Mowery wrote:

"Toby" wrote in messagenews: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

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.

Cheers,
__
Gregg

Ive modified a couple of these(similar) power supplies by doubling the
the number of pass transisors. In the process I also double the value
of the emitter
resistors. If they were .05 I make them .1. Seemed like a reasonable
thing to do. Any opinions?

Yeah, go 0.1 ohms. The 0.05 ohm resistors are useless for practical
purposes.


Geez, I never thought about the SCR but you are right. While I have
added crowbar circuits and placed them before the pass transistors I
hadnt noticed that this connection on these power supplies needed to
be relocated.

Jimmie

And there should be a fuse in there to blow between the filter cap and
crowbar.

The supply was actually 3rd world designed and made I swear.

Cheers,
__
Gregg