[Boatanchors] Thoughts on Reviving the Heathkit DX-100

[Glen Zook]

Heath ignored the maximum input power, for the 6146 AM modulated, which is 67.5-watts.  The maximum CW power input is 90-watts.  The AM power input maximum is 75% of the CW input.  As such, the DX-100 should not be run, for AM, at more than 135-watts input.  Johnson, with the Viking II, which used 2-each 6146 tubes, rated the transmitter at a maximum of 135-watts input whereas Heath rated the DX-100 at 160-watts input for AM which, of course, pushes the 6146 tubes beyond specifications.


For best tube life, the power output, on 160-meters through 20-meters, with brand new tubes, should not run more than around 100-watts output for CW.  For 15-meters, around 90-watts output and for 10-meters around 80-watts output.  Frankly, the station, on the "other" end of the QSO, is not going to be able to tell the difference between 100-watts and 120-watts but the final amplifier tubes will definitely know it!


For AM, the carrier output needs to be 25% lower because of the 25% reduction in the specifications of the 6146.


As for the higher line voltage which, for a number of years, major electric companies are now using 125 VAC as the "target" voltage +/- 2-volts.  Especially during the warmer months, because of additional load due to air conditioning, the actual line voltage is 127 VAC.  At least with my Heath DX-100 and my Heath TX-1 Apache, the higher line voltage has not presented a problem and my line voltage has often run at 127 VAC at least since the early 1970s.


The 6146 tube family can easily handle the higher plate voltages caused by the higher line voltage.  However, one definitely has to decrease the plate current (really cathode current as read by the meter in the "plate" current position) to keep the power, of the final amplifier, within the specifications of the 6146.

 Glen, K9STH 

Website: http://k9sth.net
 

      From: David Stinson <arc5 at ix.netcom.com>
 To: boatanchors at mailman.qth.net 
 Sent: Wednesday, January 6, 2016 8:59 AM
 Subject: [Boatanchors] Thoughts on Reviving the Heathkit DX-100
  
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.