I've had DMMs also go nuts at my site too. Luckily the Astron supply hasn't had
that problem yet.
The fuse usually blows when something catastrophic happens. One such thing is
when the output voltage goes too high and the built-in SCR crowbar fires. It
shorts the supply immediately, and the excess current usually causes the fuse
to blow. Sometimes it also causes the diodes to short out, and they end up
blowing the fuse. RF getting back into the supply can trip the SCR. Even RF
riding on the supply lines can cause the voltage that the SCR sees to be high
enough to trip it, even though it may not show up with a meter or even a scope.
You've probably done all you can with the ferrites unless you missed the wires
going to the SCR. On some supplies it's mounted to the chassis and has fairly
long wires (just waiting to pick up RF) running to it.
You'd be better off with ANY kind of unregulated power supply, such as what you
had with the MICOR supply. Ferro-resonant transformers usually aren't
susceptible to such RF problems, and there's nothing electronic such as a
crowbar inside to trip and blow the fuse. This doesn't explain why your MICOR
supply blew its fuse, unless you exceeded the output current capability. Most
MICORs were only rated up to 100 watts, and the supplies probably are good to
25-30 amps MAX; it seems that your PA is already exceeding that. Then you tack
on a receiver, exciter, etc, and you've gone past the limit for the MICOR
supply. Even an MSF5000 supply would be strained to handle that much current;
that's why the bigger stations have TWO supplies, one for each PA, and the VHF
stations have 28V supplies in them.
Solutions? You might consider a battery and a charger that's strong enough to
keep the PA happy, to run just the PA. Split everything else off and run that
on another smaller supply. Consider a switching regulator supply, rather than a
linear regulator supply, to run the rest of the equipment.
I know that some of the high power amps are now being built to run on 24-28VDC.
This cuts the current consumption in half and they can be run with switching
supplies.
Let's hope Skip comes aboard here. I know he's had experience with these units.
Bob M.
======
--- On Mon, 12/8/08, n9wys <[EMAIL PROTECTED]> wrote:
From: n9wys <[EMAIL PROTECTED]>
Subject: [Repeater-Builder] Astron P/S question
To: Repeater-Builder@yahoogroups.com
Date: Monday, December 8, 2008, 10:20 AM
For the learned group here…
I know there has been some discussion on one the list regarding Astron Power
supplies. Unfortunately, searching hasn’t revealed what I am looking for, so I
pose my query here and apologize in advance if this was a subject that was
discussed at length in the past… I will describe my problem in detail, so
forgive me for being so verbose right off the bat; but I figure if I provide a
lot of info now, it will avoid a lot of question and answer exchanges later in
order to get an understanding of my problem.
I have a UHF ham repeater system (TKR-820 as transmitter, MICOR SpectraTAC
receiver and comparator, Astron RM-70 Power Supply, and Crescend 150W P/A) that
is experiencing issues with the power supply. Seems that when the repeater is
on the air for any time (for example, over three minutes key-down) the power
supply blows a fuse. The first time this happened, I changed out the P/S with
a MICOR supply I had from a 100W continuous duty station. It also blew THAT
fuse… The Astron supply that blew the fuse had two bad diodes in the
rectifier, so that was repaired. There was nothing found wrong with the
Motorola supply, other than the main fuse had blown.
I took the PA back to Crescend, but they found nothing wrong with the P/A. The
station was put back on the air with the repaired Astron supply. Was on the
air for about two weeks, and failed again while I was talking to another ham.
Went back to the tower and found the fuse blown again in the supply. I took
the PA offline and brought it back to Crescend, told them of the issue with the
P/S, and that I needed them to check the PA for problems. Their service tech
called me and said he’d had the PA running on his workbench as we spoke, and
had it transmitting for about 45 minutes with no problems – all operating
within spec (~32A nominal - 38A max draw @ 165W output). After we talked some
more, he said he’d leave it run all night. If it was OK, he’d ship it back. I
got the PA back the following Tuesday. I put the station back on the air…
In the meantime, I spoke with an engineer from Crescend who told me that they
had some experience with RF getting into Astron supplies… so when I took the PA
back to the tower, I put ferrites on the A+ and ground leads to the P/S from
the PA. (There are about 10 wires altogether in the power cable going to the
PA – two bundles of three A+, and four Grounds.) I put three ferrites
altogether on the DC lines, and made three turns through the ferrites with each
bundle. These were installed as close as physically possible to the power
supply. I also put one turn on a ferrite for the entire bundle at the PA end.
(Couldn’t do more than that – was running out of cable length for hook-up.)
I replaced the fuse again, and got the station back on the air. Worked for
about 45 minutes (or long enough for me to be far enough away from the tower
where I couldn’t make a return trip that day) and promptly blew the fuse again.
(Or so I suspect.) I haven’t had a chance to go back to examine the cause of
the failure this time – yet.
Now – here’s the WEIRD part… when I was at the tower with another tech and
replaced the fuse the time before the last failure, we tried to use his DMM to
check the P/S fuse for continuity. His meter acted as if the battery was dead
– but later investigation revealed that the meter was getting hit with RF from
another transmitter at the site. So I’m thinking that the RF problem may or
may not be directly related to MY transmitter. (There is VHF 100W MICOR
transmitter directly next to my equipment rack that is on 161.325, and
transmits 24/7/365.)
OK – here are the questions: 1) Has anyone experienced an issue with RF
getting into Astron power supplies, and how did you remedy the issue? 2) Since
the P/S fails only when my transmitter is on the air, could I be getting a mix
of RF (the VHF and my UHF) that is causing this? To answer a question that may
be posed, I do not have a SpecAn available to me to check for spurs, but I am
contacting the county radio tech (it’s a county-owned tower) to see if he can
assist me with this.
Any ideas, other than purchasing a “hardened” power supply, that could remedy
my situation?
Thanks,
Mark – N9WYS
