DX-100 VFO DRIFT

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Shannon M. Latham:
Hi, Elmers!  I am having a lot of fun with a DX-100/R-390A combo on AM.   But I'd like to use the rig on cw too, but the transmitter VFO is just too drify for it.  My cw note is not really chirpy, but it wanders around!  Do you have any tips or advice on how to tame this beast?  It would be appreciated, thanks.

73' Shannon-W3SML  

Allen C. Ward:
You didn't say whether the VFO drift was something new or something the rig has always done.  You also didn't give a drift rate.  All Heath tube VFO's drift some as do most vacuum tube VFO's unless they have been engineered to compensate for drift.  Most of the drift is due to temperature variation usually during warm up which may last for hours.  Some of the things that have been done to minimize drift is to heat the VFO continuously by putting a power resistor on the VFO chassis to dissipate a few watts.  A small incandescant lamp (like 25 watts) can be placed in the rig.  With an older rig like the DX-100 (built from a kit) one should examine the lead dress and soldering in the VFO and make sure the supply voltage is fairly well regulated.  Some capacitors can become microphonic and change capacity with physical movement.  A wooden pencil eraser can be used to tap the components (while monitoring the frequency) to see if any are particularly sensitive.   With the DX-100 you can just bang on the cabinet to see if this is a probable cause of the problem.  If you change any components BE SURE they have the SAME ratings as the original (like NPO, or N750 etc.) or the problem may become worse.
Good luck Allen

Chuck Guenther:
Although I never owned one, I recall that DX100's were pretty drifty when they were brand new.

To minimize drift in the WRL external VFO that I used during the 1960's, I left power on 24 hours a day.  The oscillator tube only lasted a few months under these circumstances, since it was contained in a small shielded assembly.

Gl & 73,

Chuck  NI0C
     

Pat Bailey:
My first General (actually Conditional) transmitter was a DX-100B, which my high school electronics teacher had built a year or so before.  It was a great rig, but one evening the transmitted note started sounding like a drunk nightingale - almost literally.  I sent a quick letter (this was in the very early '60s) to Heath, and they shipped me a whole envelope of parts - quite a few silver mica capacitors and some precision resistors.  They'd had reports of VFO stability concerns and had engineered the fix.  After following the immaculately-detailed instructions, I fired it up and voila!  a stable, "X"-quality signal.

My point is that you might have parts heading south.

Ed Brooks:
The original DX-100 had VFO mechanical stability problems.  I had two of these transmitters (I did not build either one of them) back in the early 1960s. Thumping the front panel, or hitting the table, or even flipping the Plate switch, any vibration, caused a VFO shift, but as for 'drift' it was fairly stable.  At least for back then!  My DX-100B did not have the mechanical problem, as I recall, and drift was minimal.  They were acceptable for use on AM, but rather difficult on CW due to this problem.  I know of rigs, though, that worked very well, so undoubtedly part of it at least was in the construction care taken.  

Some drift with these old rigs is just going to happen.  You can, of course, rebuild the VFO and improve it a lot.  But the big problem I noted in mine, and heard about in quite a few others, was the mechanical problem.  Going in and tightening everything possible, resoldering every solder joint, did make a difference, and I did that on both of mine.  I never completely resolved the problem, though I did manage pretty dramatic improvement.

I would suggest you do that.  "Reconstruct" the VFO mechanically as well as electrically.  It really is worth it.  

ed

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