"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

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

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

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?

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

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

In article ain,
geek wrote:

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

Better yet, put in a Resettable Circuit Breaker, as you can never find
the right fuse, and they ALWAYS blow at the worst possible moment.....

--
Bruce in alaska
add path after fast to reply

We had a thread recently on Astron power supplies and I thought I'd
mention that I have an RS-35A (35 amps intermittent) for VHF brick
amplifiers (main use) and I've had it for some 20 years. Recently, the PS
has started to "crowbar" off on me (voltage goes to zero) and not even
with high currents (maybe around 15 amps or so) and have to shut it off,
wait a few seconds, and turn it back on.

I also had another different regulated fairly high current DC PS go bad
long ago, in a similar way.

I'm wondering how many others of you have seen defective operation,
sooner or later, or other deterioration in your high current regulated DC
supplies?

And, if the overvoltage crowbar craps out, it blows up your rig? I'm
thinking of going exclusively back to deep cycle marine batteries, again.

"A" wrote in message
x.com...

We had a thread recently on Astron power supplies and I thought I'd
mention that I have an RS-35A (35 amps intermittent) for VHF brick
amplifiers (main use) and I've had it for some 20 years. Recently, the PS
has started to "crowbar" off on me (voltage goes to zero) and not even
with high currents (maybe around 15 amps or so) and have to shut it off,
wait a few seconds, and turn it back on.

I also had another different regulated fairly high current DC PS go bad
long ago, in a similar way.

I'm wondering how many others of you have seen defective operation, sooner
or later, or other deterioration in your high current regulated DC
supplies?

And, if the overvoltage crowbar craps out, it blows up your rig? I'm
thinking of going exclusively back to deep cycle marine batteries, again.


Sometimes adding a .1 or .22 uF cap on the gate of the SCR to ground
will stop nuisance crowbars. Some Astrons are more sensitive than others
in this regard. We had a similar problem with a few 50 amp Astrons in
repeater service.

Pete

I'm wondering how many others of you have seen defective operation,
sooner or later, or other deterioration in your high current regulated
DC supplies?

And, if the overvoltage crowbar craps out, it blows up your rig? I'm
thinking of going exclusively back to deep cycle marine batteries, again.
====================================
The crowbar is there to NOT blow up your equipment.

However operation of a crowbar should blow a fuse fitted 'upstream' of
the pass transistors. As already explained earlier in this thread ,the
crowbar should also be fitted 'upstream the pass transistors (but
'downstream' the fuse , while sensing the output voltage .

If the Astron voltage regulator would be suspect I would recommend a
very simple circuit with a 7812 voltage regulator and a PNP transistor
,its output 'steering' the pass transistors .

Ref : Experimental Methods in RF Design , by W7ZOI-KK7B- W7PUA
page 1.15


Instead of the 3 legged 7812 you can also use the similar LM317 enabling
output voltage adjustment
After all this is a homebrew NG

Frank GM0CSZ / KN6WH

A number of gremlins cause the crow-bar to mis-fire. Just depends on
your specific regulator board version and part values used. One of the
reasons I came up with my replacment regulator board option.

http://www.repeater-builder.com/astr...ron-index.html

I've not seen a newer generation crow-bar circuit fail causing
equipment damage.. but as always your results might vary.

cheers,
s.

: A wrote:

: We had a thread recently on Astron power supplies and I thought I'd
: mention that I have an RS-35A (35 amps intermittent) for VHF brick
: amplifiers (main use) and I've had it for some 20 years. Recently, the PS
: has started to "crowbar" off on me (voltage goes to zero) and not even
: with high currents (maybe around 15 amps or so) and have to shut it off,
: wait a few seconds, and turn it back on.

: I also had another different regulated fairly high current DC PS go bad
: long ago, in a similar way.

: I'm wondering how many others of you have seen defective operation,
: sooner or later, or other deterioration in your high current regulated DC
: supplies?

: And, if the overvoltage crowbar craps out, it blows up your rig? I'm
: thinking of going exclusively back to deep cycle marine batteries, again.

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

No, a really bad practice... don't do it.

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

Many parts/portions are done in Mexico...

: Better yet, put in a Resettable Circuit Breaker, as you can never find
: the right fuse, and they ALWAYS blow at the worst possible moment.....

No... you're asking for a serious problem and it's just a question of
when and not if.

cheers,
skipp