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Author Topic: Astron RS-35M P/S: Low voltage under load?  (Read 4663 times)
W9XC
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Posts: 16




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« on: June 11, 2012, 10:51:23 AM »

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
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KA4POL
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Posts: 1917




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« Reply #1 on: June 11, 2012, 11:48:22 AM »

A diode of the rectifier bridge might be bad.
The schematic shows voltages for the LM723 under no load and full load. Check those also.
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AC5UP
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Posts: 3825




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« Reply #2 on: June 11, 2012, 04:39:23 PM »

An LM-723 regulator circuit typically needs 3-4 volts of headroom above the intended output voltage. Meaning......... If you want 14 volts out you'll need 17 plus a scoshe or two extra on the big filter cap.

First thing I'd do is pop the lid and see wassup across the big cap when the output drops to twelve and a half vees. If it's < 17 your problem is in the transformer / bridge rectifier / filter cap section of the stadium. If the unregulated voltage on the big cap holds stout at 19 - 20 vees while the regulator goes south, look for an open pass transistor or other oddity in the regulator bleachers. And don't forget to check the two screws on the big cap to make sure they're still tight and making good contact with the PC board. If your best Columbo-like efforts at troubleshooting come up with absolutely nothing wrong, replace the LM-723.

You can find one on a peg at the local RadioShark if ya' gotta'.
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K8AXW
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Posts: 3672




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« Reply #3 on: June 11, 2012, 06:53:47 PM »

The Astron schematic has a voltage chart at the bottom left.  Follow 5UP's lead with the voltages Astron provides and you'll find your problem pretty quick.
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KA4POL
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Posts: 1917




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« Reply #4 on: June 11, 2012, 09:55:26 PM »

According to the voltage chart that I mentioned in my first reply the cap for the LM723 part of the circuit should have 29V at zero load and drop to 25V at full load. If anything is wrong here the two diodes CR1 nad CR2 could be the problem. However, as I said, the main path with the bridge rectifier could cause the voltage drop.
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KA1MDA
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Posts: 543




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« Reply #5 on: June 12, 2012, 01:29:20 PM »

Set your meter to measure AC volts. Hook it to the power supply output and load down the supply. If you see AC voltage on the meter, you have excessive ripple. My guess would be to check the filter capacitor. If your meter also has a frequency counter setting, measure the ripple frequency of the supply under load. If the ripple frequency is 120 Hz, both rectifier diodes are probably good.

Tom, KA1MDA
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KA4POL
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Posts: 1917




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« Reply #6 on: June 12, 2012, 09:41:22 PM »

If the ripple frequency is 120 Hz, both rectifier diodes are probably good.
The two diodes are only supplying the 723. The main path is through the bridge.
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W9XC
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Posts: 16




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« Reply #7 on: June 13, 2012, 09:30:11 AM »

Thanks to everyone for the quick and very helpful replies. For future reference, I found the annotated schematic and pointer to a QST article analyzing the RS-35M: http://www.al7fs.us/astron-rs35m%20annotated%203.pdf

The actual problem was more mundane and really taught me something: take Ohm's law seriously. 

When I went back to start the diagnositic process, I measured the no-load and full-load voltages right at the P.S. They were 13.8v NL, and 13.6V at 20A load.  Hmmmm...

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. So the Astron was certainly fine, and the full-load voltage drop 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.

Thanks again,

- Les, W9XC
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K1ZJH
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Posts: 901




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« Reply #8 on: June 13, 2012, 06:30:49 PM »

I was going to ask that... the voltage sensing is at the supply terminals. With some regulated supplies the sense
leads can be extended to the load to compensate for R losses in the cable.
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KA4POL
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Posts: 1917




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« Reply #9 on: June 13, 2012, 09:58:13 PM »

A failure diagnosis can only be as good as the failure description. The failure was stated as 'the output voltage under ~14A load sinks to ~12.4V'. So everybody had to assume that the voltage drop was at the output.
Another result of this experience is that a power supply should always have a volt/amp indicator. At least my homebrew projects all have indicators.
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KB3HG
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Posts: 404




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« Reply #10 on: June 21, 2012, 06:01:07 AM »

Les,
Here's an idea for you, there is a variable resistor R5, bump up the voltage a tad. Radios take 13.8 v +/- you could raise the voltage to 14.4v and the radio should be happy.

or

Increase the wire size.

I^2 R  losses decrease with the large wire.

In any case the down stream power contacts should be cleaned it will help.
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