Astron supply generating RFI

[Tom W2ILA]

My Astron RS35A 12 volt supply generates hellacious RFI at about 2150KHz.  The products go way up and down the bands.  This is not a switching supply.  It only generates noise when something is pulling a little current.  With no load it is quiet

I called Astron and they recommended removing paint from the AC and DC ground points and putting a .1 mfd cap across the transformer secondary.  No change.  

Everything is well grounded to a common ground bus.

Has anyone else found / fixed this?

Tom W2ILA

[w3jn]

THe regulator is oscillating.  Bypass the input and output of the regulator chip as well as the pass transistor(s) with .1s and 1 ufs.  I've seen this happen on many other power supplies.

73 John

[Tom WA3KLR]

I have an Astron RS20A and a RS35A.  Any of these simple linear supplies can have diode rectifier hash.  It's going to be worse the more current the supply has.  I first discovered the hash on 160 meters years ago when I only owned a RS20A.

These problems can be hard to spot at first since the power supply must be on and other gear in the shack that are turned on may bypass the line due to their line filter caps.

Bypass each side of the power line to ground in the power supply with a proper 0.1 uf cap.  Be sure to put the one on hot side of the line past the fuse.  In one power supply I put the hot line cap. ahead of the a.c. switch but after the fuse.

I can mail you some X-Y film caps. rated for the power line service.

[WA1GFZ]

Also try a .001 across each rectifier diode.  Fast rectifiers will generate wide band noise.  Sometimes the size of the output cap will cause oscillation if long leads are used at the output. I have a large linear 12 volt supply that suffers from that problem. I need to install a larger output cap.

[W2JBL]

that noise gets out the DC output too. over the years Astron has had various different bypass caps on the output of their supplies. most do little or nothing. i use an RS-20 as a small bench supply at work sometimes, and another (NASTY) problem with the design is that you can get RF INTO it. and when you do, sometimes i gets into the regulator and the output goes to about 20 volts- YIKES!!! i smoked a brand new Motorola M1225 that way.  good quality ferrite clamp on cores on the AC cord and DC output cured that, and knocked down the noise it generates too.

[w3jn]

Again, problems wiht RF getting INTO the regulator can be cured with the proper bypass caps on the regulator chip(s) and pass transistor(s).

73 John