On Wed, 18 Jan 2006 22:43:07 -0600, Kerry Miller
wrote:

I've got a problem with my Astron RS-20 power supply. With no load,
it's fine, but anything above 4amps or so makes it drop way down and my
rig drops out of lock. I haven't measured it under load but the rig
works ok to 10 or 11 volts so it must be dropping below that.

I found a schematic and downloaded it, all the voltages are fairly close
to what it says on the schematic with no load, I haven't made any
measurements with a load on it. I'll pick up a headlight tomorrow to
put a load on it and make more measurements.

Does anybody have any ideas?

Thanks,
Kerry, WD5ABC



Hi Kerry, Since you have the voltage, and I assume that you checked it
at around 13.8 volts, and are not getting the current, there are
usually just a few simple things that usually crap out on these power
supplies. Please note, I have 3 or 4 RS-35s, (not an RS-20) but they
are built alike, enough so, that you should be able to figure out the
problem. First, providing the pass transisitors are good, and with the
power off, and AC unplugged, and filter caps discharged, look on the
backside of the supply, and you will see 4 sets of double paralelled 5
Watt .05 white, square inch long power resistors connected between the
4 pass transistors and BUS. Using a ohmmeter, check these for around
..025 Ohms resistance. If its a taste higher, OK... However, once you
get to around an Ohm or more, your power supply will exibit the
conditions you have mentioned. These resistors provide emitter pass
transistor protection as I recall, and many times will fry, and you
will not know it unless you measure them. If I find these resistors
have gone south, I further test the supply by jumping the resistors
with test jumpers. You'll know right away if the problem went away by
using a test rig to draw some current. PLEASE NOTE, I have fixed quite
a few Astron supplies, BUT if you blow your supply up (or test radio)
even more than it is, don't blame me for any info I have provided. If
it works, change out the resistors. I don't recommend you just jump
them out..Oh, BTW, since you have the LM723 regulator, look on the
bottom side of the board, and see if there is a resistor, hanging in
mid air, which has a wire connected to it that runs down to the bottom
floor of the supply, and is attached to a 3 prong transistor in a
T0-220 case. Sometimes, this resistor will fry, causing current
problems too.. One of my early supplies has the LM317 regulator, but,
you won't have to worry about this, as this requlator is not there,
however, the mounting holes (open of course) are still there on the
mainboard in the upper left corner.. Hopefully, this will help.. Best
Luck OM.. Tony, WB8MLA