On Thu, 20 Nov 2003 11:36:39 -0500, "Bruce W...1"
put finger to keyboard and composed:

My Astron VS-35M power supply (http://www.astroncorp.com/linear.shtml)
isn't working right. Unable to get support from Astron I'm hoping
someone here can help me fix it. I'd really hate to ship it to
California, this thing must weigh 30 lbs..

Here is a symptom of the problem:
When the power supply is set at 13V and the current knob at zero, ...

When you say "zero", do you mean "minimum"?

... on connecting it to a 3-ohm power resistor the current goes to
2 amps and the voltage drops to 6V.

Sounds like the PSU is working as designed, ie you have set the max
current limit to 2A and the PSU is obliging with foldback current
limiting. You will probably find that a 4 ohm load will produce a Vout
of 8V, a 5 ohm load a Vout of 10V, etc.

When the current knob is then set to full, the
voltage returns to 13V and the current goes to 4 amps (should be 35A).

No, it should be 13V / 3 ohms = 4.3A.

Here's voltages I measured on the LM723 voltage regulator:
[No load voltages]
Pin Voltage
2 13.69
3 13.26
4 6.38
5 6.38

Pins 4 and 5 are the inputs to the error amp. It looks like the PSU is
regulating properly.

6 7.18
7 0
10 14.25
11 29.33
12 29.31
13 15.51

Here's the schematic for the power supply:
http://www.didah.com/astron/circuit.gif
And here's the data sheet on the voltage regulator:
http://www.didah.com/astron/LM723.pdf

I'm no electronics expert (or I probably could have fixed it). But I am
good with a soldering iron and have lots of test equipment. Any
suggestions on what to look at?

Thanks for your help.


- Franc Zabkar
--
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