DX100B Restoration

Garey Barrell k4oah at MINDSPRING.COM
Thu Mar 3 11:26:13 EST 2005

Bry -

The DX-100 VFO is essentially the VF-1.   This VFO as designed is quite 
capable of chirp-free, drift-free (for it's era), operation.   Changing 
the VR tube voltage or regulating the plate voltage have little or no 
effect.  It is an Electron Coupled Oscillator, and the plate voltage has 
no effect on the oscillator, and very little on the loading.

Part of the drift problem is a result of the temperature variations in 
the DX-100, both warm-up (at least an hour because of the huge mass of 
metal), and from transmit to receive.  The latter is especially 
noticeable with AM "old buzzard" transmissions.

Solder, mounting hardware, and especially the bus wiring are all 
contributors (or detractors) of stability.  Make sure the bus wiring 
especially is well soldered and not "under tension" from 
point-to-point.  It also is a good idea to "exercise" the trimmer caps a 
little and verify the ground connection.  DON'T over tighten the 
mounting hardware, they will break!  A little liquid Deoxit on the 
switch contacts and tube pins ONLY.  DON'T spray anything all over 

Replacing the 2.2k 1/2W resistor in the grid circuit with a 1W helped, 
both with drift and minor instability, and probably a carbon film would 
be even better today.

The silver mica caps are reliable, but some have replaced them with 

A GOOD VR tube is essential, just because they glow doesn't mean they 
are stable.  Many "dither" a few volts.

Selection of a GOOD 6AU6A is also necessary.  There is a BIG difference 
from tube to tube, even "new" ones, especially with regard to chirp.

73, Garey - K4OAH

Drake C-Line Service Manual

Brian Carling wrote:

>Say guys,
>I just got this DX100 and it is clean when using crystal control but 
>the VFO drifts and chirps really badly.
>I was thinking of dropping the voltage regulator from 150V to 105V.
>Any thoughts on this?
>73 de AF4K, Bry

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