Larry Gagnon wrote:
Hello. I have a homebrew linear 13.8V 20A power supply I have been using
for some years to power an IC-735, a 2M rig and some accessories. The PS
has these characteristics:

- well filtered (35000uF)22AMP, 21 volt DC output from diode bridge circuit
- LM-317 regulator circuit (as per LM-317 datasheets)
- MJ2955 NPN feeder transistor from regulator driving 4 - 2N3055 PNP
power transistors in parallel
- large heat sinks (I don't think heat is a problem)
- good air circulation with fan

I have the regulator set to 13.7 VDC out. Lately however, I have had
some problems with voltage drops: while drawing 20 amps the voltage
drops to about 11.3 volts and I have had reports of a rough CW signal as
a result.

I have tested the reg circuit and even replaced the LM-317 to no avail.
I cannot see where the problem lies, unless perhaps the MJ2955 feeder
transistor is not working as it should? (I don't yet have a replacement
transistor to test that idea, hence this post).

Any ideas greatly appreciated.

Larry VE7EA

It's probably not dropping to a constant 11.3V -- it's probably dropping
out on the valleys of the rectifier's output. A steady low voltage
would either render your transceiver inoperable or lower your output
power; the "rough CW" is probably modulated at 120Hz.

Most likely that "well filtered (35000uF)" has turned to "lots of hum
(dried up electrolytics)".

I assume you don't have an oscilloscope. If you do, clip it onto the
power supply output and take a look while you run it close to it's
limit. Any filtering problems should become immediately apparent.

If you don't have fancy equipment just measure the output with your
voltmeter switched to "AC" -- most VOMs will filter out the DC and just
let you see the wiggly part. Do this under both heavy load and light --
if there's a honking big AC component as the voltage sags then there
needs to be some new capacitors in your future.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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