[Boatanchors] DX-20 Advice Please

[Jim Wiley]

Curt -


A Heath DX-20 was my first transmitter, lots of years ago.  1958,  
actually.  Bought it from my paper-route money, and built it from the 
kit.  It was  my first  Heathkit, in fact my first major electronic 
project of any kind.


You are correct, the RF choke is necessary for the crystal oscillator to 
function.  Bypassing the choke was necessary when driving the 
transmitter from an external VFO.   Some guys installed another  slide 
switch on the panel that allowed the operator to quickly reconfigure the 
set for either crystal of VFO operation.  You _could_ use the set with a 
VFO without bypassing the choke, but operation was marginal at best. 


Most guys used the Heath VF-1 VFO.  It worked OK, but had a few 
problems.  For 80 and 40 meter use, it was pretty good.  The combination 
tended to chirp on 20 meters and higher. The VFO definitely needed its 
own power supply - robbing VFO power form the DX-20 was theoretically 
possible, but very iffy, and led to all sorts of problems.


You are correct in the lack of what we would consider today as 
necessities, such as a power supply fuse.   I would recommend that you 
install a fuse in the AC line. A 1.5 ampere 3AG slow-blow type should be 
suitable.  If the fuse blows too often, try a 2 amp unit,  but don't go 
any larger.   If you don't want to cut a hole in the chassis for a 
post-type holder, you can install an under-chassis unit, but that 
entails having to remove the cover to change fuses. In-line fuses work 
too. 


Heath transmitters of that group (DX-20, DX-35, DX-40) all suffered from 
too-small transformers for the load drawn.  As long as the rigs were run 
on CW  (or controlled-carrier AM) without too much key-down time, they 
would make it, but anyone who used one for any kind of constant-power 
key-down service, such as a driver for a higher powered AM rig, was 
taking a big chance on failure.  My original DX-20 managed to avoid 
this, although I don't know why, as I used it to excite a 500-watt AM 
rig for years. I probably got away with it because I only needed about 
12 watts RF to drive the PP 813's in the "big" rig, and it ran on 75 
meters only, so the DX-20 didn't have to work too hard.   I did manage 
to burn up the transformer in a DX-40 that I converted to external plate 
modulation by being too long winded. .


Adding a power-on lamp is easy enough.  A friend of mine who also had a 
DX-20 used the unused operate-standby switch contacts to operate a small 
relay from the filament circuit that allowed him to have a red "HV ON" 
lamp as well (across the relay coil), and then used the relay contacts 
to supply switching for an external antenna relay and receiver muting 
function.
I used external T/R switching with these rigs.  If I remember correctly, 
the DX-20 does have an extra set of contacts on the operate-standby 
switch that could be used for controlling a TR relay. 


The panel meter is problematical.  Being a moving-vane type, it tends to 
bounce erratically when the transmitter is keyed,  and is really only 
good for steady-state measurements, such as during tune-up. 


But, all this being said, a DX-20 is a really great minimalist rig.   
Sometimes I kind of wish I was 13 again with that brand-new novice 
ticket, a BC-312 receiver and the DX-20.  My current rigs are clearly 
more versatile, but I am not sure they are as much fun.


- Jim, KL7CC




Curt wrote:

> Recently received a DX-20 that won't oscillate.  Filter caps replaced and PS 
> working ok.  Previous owner removed the cathode choke on the oscillator and 
> shorted the cathode to ground.  Don't think it can oscillate with signal 
> grounded that way.  I'm winding a 1mH inductor on an FT37-43 toroid as a 
> replacement choke, hope that will work because don't have any Hammond chokes 
> laying around.
>
> Never owned one of these, but am a little amazed at lack of what seems 
> basic, like a fuse for the power, light to indicate power is on, any way to 
> power an antenna relay from the standby/transmit switch.  What was 
> experience of folks who owned these rigs?  Items mentioned "needless 
> frills"?  At a bare minimum I'm buying a fused line plug.
>
> 73, Curt KB5JO 
>
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