HELP with Heathkit DX-20
I am fairly new to Amateur Radio. I got my license almost 2 weeks ago. I wanted to get a license ever since I was 18, that was 50 years ago. Anyways, I have a DX-20 that I have had for a couple years now and I wanted to restore it back into working condition. For background I understand radio circuits having rebuilt several of AM raadios and 4 Hallicrafter S-38's and had a dual trace oscope as well as a ton of other test equipment. I have an annoying buzz coming from the 5 Henry filter choke. The power supply circuit is properly wired. I replaced the 2 electrolytics, the .1 mF 1.2kv cap and had to basically run through the whole construction project because it was miswired by 7 wires. Suprising that it did not blow up when I first pushed up the voltage on the variac. All the tubes tested good on my I-777-B and TC-3 tube testers. Even with the buzz I was able to set the grib at 2 ma as required and set the plate to 120 ma as per recommended settings. With a little tweaking I could get about 20 watts out on my SWR on 80 meters with an end fed 35 foot dipole. I was able to talk to one of my buddies on CW (that is all it does) with no problems and got an RST of 599. Soooo, could anyone shed any light on why I get this buzz in the filter choke???? Almost sounds like 60 cycle buzz to me but I am not sure about that.
Likely the choke has become delaminated. If lucky, there may be four screws at the four corners, try tightening them up and see if the buzz is at least diminished if not eliminated. If no screws there may be holes at the four corners of the choke, you could try using proper sized machine screws and nuts with lockwashers to attempt tightening.
If the delamination is the actual coil itself, the above won't help much, the choke will likely have to be replaced.
The laminations can be loose on older chokes and transformers, creating a frame buzz.
Is that the issue you are experiencing (hearing)??
Nullius in verba
Thanks for the info. Looks like someone tried that fix by putting pieces of rubber band under the filter choke casing. One screw was loose but that is not the problem. Looks like it may be the coil inside because that is where the buzz is coming from. Best info I could find on this part is that it is 5 Henry, 100 ma but I do not know what the voltage or the ohm rating is. I measured 200 ohms across the two wires and I assume that being each plate on the 5U4 is 600volts that it is rated at least that much. How important is the ohm value of this filter choke?? Appreciate any help on replacing this part. Thanks.
Before you replace the choke, as a last resort, try soaking in a sealed can of varnish or one of the newer "clear" type wood finishing products. You want the can sealed because, otherwise, the solution will start to dry in a hurry. After a couple of days, remove the choke and let it "air dry" for a couple more days. For decades, chokes and transformers were "cured" in varnish and that helped to keep laminations "tight". It won't hurt anything and may just "fix" the problem.
I can't remember for sure, and I'm not going to take my DX-20 apart, but the choke may be filled with "tar". If so, you can remove it from the chassis and then put it "top down" on an electric stove burner set on around "medium". Let the choke "cook" until the tar is liquid and then turn off the burner. It will take several hours for the choke to cool but this also "fixes" a lot of problems.
On my original DX-100, the high voltage transformer developed problems including internal arcing. I "cooked" it for about a half-hour and then let it cool. After that, the transformer was still working when I gave the transmitter away to an employee of mine about 15-years later.
Oh, by the way, "cooking" a choke or transformer is best done when your wife, if you have one, is not home! Wives just don't understand what is being done and also, usually, object to the odor!
Gary, WZ1M provides a transformer and choke rewinding service.
I note on his web page, that he is having eye surgery this month,
So he should be contacted before any shipments.
Nullius in verba
I will try the cooking on the stove considering the choke is inside a metal can and it appears to be filled with some sort of
solidified material. I will give that a shot and see what happens. Thanks for some ideas.
IF you only need to raise the temperature to ~ 190 degrees F
I will try cooking this choke on the stove, considering the choke is inside a metal can and it appears to be filled with some sort of coating.
THEN using boiling water in a double boiler config may reduce mess.
I use that trick to remove hardened solder from plastic parts --
the plastic softens in the water bath -- so the solder Pops Off the plastic in one piece.
"Potting" compounds have changed significantly since WW2
Many newer formulations are easier to apply and safer (environmentally friendly).
Last edited by W9GB; 04-13-2012 at 04:03 AM.
Nullius in verba
Thanks to all that answered. I put the choke in a small frying pan filled with water, boiled it for 1 hour or so until the tar softened (not in the kitchen but on a single portable burner I bought when I kitchen was being redone), cooled it down and the buzz is now gone. I can keep my sanity while communicating. Thanks again folks.
Excellent fix! Good old time ham stuff.