Astron RS-35M P/S: Low voltage under load?
I have an Astron RS 35M linear power supply. It's rated for 13.8 VDC ± 0.05 volts at 25A continuous. These are usually noted for their reliablilty and robustness.
However, on mine the output voltage under ~14A load sinks to ~12.4V. The P/S isn't new - at least a few years old - so I'm wondering if there are any internal service items I should look at that might go bad - electrolytics, zeners, etc.
I'd be grateful for ideas.
- Les, W9XC
The first thing to check is the voltage on the collector of the pass transistors. You can measure that without even opening the case. You should have 24+ volts with no load, and better than 20v when loaded. If you are failing that voltage test, the problem may be with the rectifiers or filter capacitor. If that voltage is good, the problem is probably with the regulator, pass transistors, or equalizing resistors.
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Joe, thanks for the quick and very helpful reply. I also found an
annotated schematic and a QST article analyzing the RS-35M:
The actual problem was more mundane and really taught me something:
take Ohm's law seriously.
I had originally been measuring my voltages toward the rig end of the
power distribution chain - which in my case included lengths of 10ga
and 12ga wire, fuses, a RigRunner panel, and multiple Powerpole
connectors. When I went back and measured the no-load and full-load
voltages right at the P.S. they were 13.8v NL, and 13.6V at 20A load. So
the Astron was certainly fine, and the drop of about 1.4V was
occurring in the power cabling.
I shortened and simplified the cabling, which reduced the drop quite a
bit. Even then, when I measured the actual drop in the cable
with just ~5ft of 12ga wire and one set of powerpoles, at 20A load I
had almost .2V drop PER SIDE, or ~.4V overall. Within measurement
error this basically agrees with the standard resistance figures for
12ga copper wire.
Lesson learned: Even small series resistances accumulate in the power
cable chain, and at high current can cause a surprising degree of
voltage drop. Use big short power wires. (And don't blame the P/S).
This is one of the downsides of low-voltage/high-current arrangements
that we have to live with.
- Les, W9XC
I was going to ask where you measured it. I'm relearning this lesson myself.
I wired my mobile rigs up initially by running a lead about 15 feet long directly to the battery. My IC-7000 is somewhat intolerant of low voltage, and I was having some issues, so I tried grounding the negative lead closer to the radio. That made a substantial difference! A few weeks ago, I had an issue that required me to shorten the leads by a couple more feet and eliminate some connectors. This made a further improvement. One of these days, I am going to approach this more scientifically, and I'll probably end up running much heavier cable to the battery.
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Those long power cables can also be little antennas to get RFI from a marginal antenna system, back into the rig ! More problems, cured by short, direct power cables.
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Thanks for the comment.
At a Scout campout a couple of years ago I hooked up an IC-7000 (on a picnic table) to the car battery through a regular 100' construction-grade 3-prong extension cord usually used in a 125V circuit (14ga wire I think). I was all set to demonstrate the delights of Ham radio to the eager Scouts. Receiving worked fine. Then I hit the mic button and everything shut off. I'm thinking I've somehow hosed the shiny new 7000, especially when it kept failing. It was only after I got home after the campout, hooked up the rig as usual in the shack, and it worked flawlessly that I realized what had happened. It was my first non-theoretical introduction to voltage drop in a low-voltage setting.
In the situation below, the real issue was tracking down the cause of low power output with my Flex 3000. It has been much more sensitive to the lower input voltage than my other rigs. Of course the shack cabling is a lot different from the long extension cord experience so I didn't think of ohmic voltage drop at first - until I actually measured it, which was an eye-opener. But at least I now know it's not a higher-cost P/S issue.
- Les, W9XC
I'm not so sure about that. I have an Astron RS-35M, too. I think there is plenty of room for improvements. Take a good look at:
Originally Posted by W9XC
... and you'll see what I mean.
We've all seen that. And if I were using mine in repeater duty, I'd make some modifications.
Since I'm just powering a plain old transceiver, and since the Astron has been chugging along without complaint, I think I'll let well enough alone.
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