HELP with Heathkit DX-20

Discussion in '"Boat Anchor" & Classic Equipment' started by K2DEP, Apr 13, 2012.

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  1. K2DEP

    K2DEP Subscriber QRZ Page

    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.

  2. KE3WD

    KE3WD Ham Member QRZ Page

    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.

  3. W9GB

    W9GB Ham Member QRZ Page

    Steve -

    The laminations can be loose on older chokes and transformers, creating a frame buzz.
    Is that the issue you are experiencing (hearing)??
  4. K2DEP

    K2DEP Subscriber QRZ Page

    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.
  5. K9STH

    K9STH Ham Member QRZ Page

    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!

    Glen, K9STH
  6. W9GB

    W9GB Ham Member QRZ Page

  7. K2DEP

    K2DEP Subscriber QRZ Page

    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.
  8. W9GB

    W9GB Ham Member QRZ Page

    IF you only need to raise the temperature to ~ 190 degrees F
    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: Apr 13, 2012
  9. K2DEP

    K2DEP Subscriber QRZ Page

    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.
  10. WX7G

    WX7G Ham Member QRZ Page

    Excellent fix! Good old time ham stuff.
  11. WA7KKP

    WA7KKP Ham Member QRZ Page

    There is no doubt in my mind that you have loose laminations causing the buzz. Follow the suggestions posted and they should do the job . . .

    As far as voltage rating -- most chokes of that era were good for 450v at least, and 1000v as a top figure. Most MIL surplus components will have all the ratings printed on them. The resistance is only critical if you are pulling a lot of current -- remember Ohm's law here (voltage drop). A 10 H choke wound with #30 wire will have a greater DC resistance (and smaller size) than the more common #20-22 wire. ISTR that the DX-20 only had about 400v plate voltage, and maybe 100 ma plate current, so a replacement choke shouldn't be a hard thing to find, if the home fixes don't cure the buzz.

    I remember as a kid I had a slot-car set that had loose windings in the power transformer. I tried using a vise to re-clamp the frame to the windings, but that never lasted very long, so I just lived with the buzz . . . I guess I should have returned it back to the dealer, but I was only 10 or so at the time, and d didn't know much.

    Still don't know much . . .

    Gary WA7KKP
  12. K2DEP

    K2DEP Subscriber QRZ Page

    Thanks Gary, that is good information to know in case this choke goes by the way. Right nopw the melting down of the tar and cooling eliminated
    the buzz. Hope it lasts. The transmitter seems to be very stable now. Thanks again for the info.
    Steve KB3YLB
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