[Boatanchors] Thoughts on Reviving the Heathkit DX-100

[David Stinson]

Morning.  I just finished restoring a Heathkit DX-100
for a good friend.  The information I needed was supplied
by the kindness and generosity of several of our members.
Our people are the best!.

Many of the grounds had gone "Hi-Z." A small "tweak"
of the ground screws usually fixes that. 
 It's a common problem with old radios and the 
first thing I address when one hits my bench.

This transmitter had a bad LV transformer.  It had also
been chopped and lopped to use the SB-10 Sideband 
adaptor.  The previous owner had done other things 
"his way" and it was quite a puzzle to undo all the mayhem.

It's a pretty transmitter, but I'm not a fan of the DX-100.
The DX-100 runs everything on the "hairy edge," putting
max voltages on modulators, PAs and even the LV 
stages.  One of the 5R4 HV rectifiers had a failed and
arcing tube base- not surprising given how hard Heath 
pushes everything in this rig.
http://home.netcom.com/~arc5/5R4arc.jpg
The set was designed when the AC line voltage 
averaged 110-112 VAC.  Today's lines run 
122-125 VAC.  That's better than 10% high and 
runs the HV north of 800 Volts.  Not nice.  
Things start arcing easily at 800 Volts.
No wonder people report 115W out on 75 Mtr AM;
they're kicking the stuffings out of everything in there.
I'd bet breakfast that a bunch of blown-up DX-100s
were fried for just this reason:  High primary voltage.
If it were my rig, I'd buck the line down under 110V,
take my 80W average out on 75 AM and call it "good."

Moreover- the terminal strips used to build the set are 
cheap and prone to breakdown.  I had the 6 VAC buss
from the transformer try to weld it's terminal post to 
the chassis and I'm lucky I didn't lose the replacement
transformer.    I think one should coat the terminal
strips and the chassis around them in Corona Dope,
just for safety's sake.

The 5V4 "Low Voltage" rectifier: I noticed that one of 
the wires that crosses the 5V4 socket had a burned 
spot and the replacement transformer also had a 
burn spot in the same place.  I put shrink tubing over
the burned spot.  Well, I got a new burn spot because
the wire crosses above a filament pin and the insulation
was breaking-down, causing an arc!   
http://home.netcom.com/~arc5/5V4arc.jpg
I double-shrink-tubed the entire length of the wire, 
plus the wires going from the LV filter choke near 
the chassis because I thought I saw an arc to ground 
there as well.
The DX100 is the "arcing-est" transmitter I've ever seen.
When transmitting, one needs a good ground because
RF where it doesn't belong encourages arcs in this rig.

All this leaves me with the impression of the DX-100
as a fragile beast, just itching to let its smoke out 
and roll over on you.  If you run one, I think toning 
the primary voltage down around 110V and accepting
a little less output, applying Corona Dope to all the 
terminal strips in the power supplies and checking all 
you grounds would be excellent investments.

GL OM ES 73 DE Dave AB5S