1) Replace the125uf 450v high voltage supply
caps with 400uf 450v caps.
2) Replace the 20uf bias supply caps with a
50uf 100v caps.
3) Replace the shorting type switch with a non-shorting
type switch in the meter circuit. (if your unit has a shorting type switch)
4) Add a current limiting resistor (thermister)
in series with the high voltage transformer primary. (5 amp continuous,
25 ohms cold, 0.5 ohms hot)
5) Break the connection between the point where
the 15k high voltage bleeder resistors meet, and connect to the CW-Phone
switch. Add a 100 ohm 1/2w resistor at that point. This is the contact on
the CW-Phone switch that supplies the 1625s with screen voltage in the phone
mode, and removes it in the CW mode.
6) Replace the power cord and plug with a 3-prog
7) There are two fuses shown on the drawing.
Remove the fuse shown on the right, and move the fuse shown on the left,
to between the RFC/.005 cap and the power switch. This fuse holder can
be mounted under the chassis with a single 3AG fuse holder. I used a 5amp
slow blow fuse.
8) Move the6AU6 28uhy plate choke's B+ source
from point "M" to point "L".
9) Replace the 15K 5W resistor on pin 1 of the
OA2 with a 15K 10W resistor.
10) Replace the OA2 with an OB2.
11) Replace the 5V4, 5R4s and 6AL5 with solid-state
General mods #1 and #2
Are simply to reduce power supply ripple.
General mod #3
Have you ever changed your meter switch position while
on the air, and as you monitor your audio with a receiver, you hear annoying
pops in the audio as the switch transitions from one position to another?
Me too. For whatever reason, Heathkit decided to use a shorting type switch
in the meter circuit rather than a non-shorting type, at least in my unit.
What this does is momentarily connect things together that shouldn't be
connected together. If you look at the schematic carefully, you will see
that this can adversely affect important things like the PA bias circuit,
etc. Replacing the meter switch with a non-shorting type will completely
eliminate this undesirable effect. I found a beautiful, unused, Centerlab
2-pole ceramic non-shorting switch at a local electronic surplus store for
$1. Electronic Surplus Inc
General mod #4
Is to reduce the high inrush current that develops when
switching the Plate Switch from off to on. Installing a thermister in series
with the High voltage transformers primary will do the trick without a big
compromise in supply regulation. The current limiting thermister that I
used in my DX-100 is 25 ohms cold, then drops to less than 0.5 ohms within
about 1 second. It's nice to switch the rig to transmit without having a
brownout in the shack. It's easier on the rectifier and filter caps too.
Current limiting resistors are getting more prevalent and less expensive
now that switching power supplies are getting so popular. Most switchers
rectify the line directly, then filter it, without a transformer. This wouldn't
be practical without a current limiting resistor, allowing time for the
filter cap to charge.
General mod #5
Is to allow time for the .1uf screen decoupling cap to
charge. If you switch from CW to Phone while the 800volt supply is on, you
will take a little bite out of your mode switch. The scenario is; the 800
volt filter caps are at full charge, and the .1uf screen decoupling cap
is at zero charge.then you close the switch....BOOM! Adding a 100 ohm _
watt resistor between the filter caps and the switch will allow time for
the .1uf cap to charge without excessive current. The drop across the resistor
with full audio output is only a few millivolts, so the screen voltage it
virtually untouched. It will also protect that beautiful ceramic switch
in the event the 1625s develop a screen grid short, or the .1uf decoupling
cap shorts. You will loose a $.05 resistor not a $25.00 switch.
General mods #6 and #7
Are to reduce a shock hazard that exists in the original
design. It's never a good idea to fuse the neutral line, and a 3-prong plug
ensures that the chassis is sitting at zero volts with respect to earth
ground. I drilled a #6 clearance hole in the dividing plate between the
low voltage ac input section, and the PA section, and used a 6-32 screw
and nut to mount a Littlefuse 3AG fuse holder and a 3AG 5amp slow blow fuse.
This way, there are no extra holes drilled in the rear apron to accommodate
the new fuse. It does however require you to open up the unit if the fuse
blows, but there is probably a good reason the fuse blew, and you will have
to open the rig up anyway.
General mods #8 thru #10
Are to reduce the plate and screen voltage on the 6AU6
in the VFO. This will reduce the plate dissipation, which will reduce the
temperature rise, which will reduce the drift, and improve tube life. The
6AU6 is rated for 330V max plate voltage. With 400 volts on the plate, the
plate current can be as high as 14ma depending on the band selected. This
is excessively high for the 6AU6. The 6AU6 is only rated for 3.5 watts of
plate dissipation. The VFO will deliver more than enough output with the
plate and screen voltage as low as 80 volts. That was the reason for selecting
the OB2 over the OA2. The 15K resistor will keep the OB2 at around 20ma
with the 6AU6 off, which is in the middle of its range. This will however
subject the 15K resistor to about 6 watts, which is the reason for replacing
it with a 10-watt device. The plate/screen voltage on the 6AU6 will stay
at constant 106 volts ±2 volts with this configuration. This mod
will nearly eliminate any heat related VFO drift.
General mod #11
Reduces the overall heat developed by the DX-100 and improves
1) In noise sensitive areas of the audio amp,
metalized polyester film capacitors were chosen to keep the transmit audio
as clean as possible. Metalized polyester film capacitors have very low
dielectric noise, even with a large DC component present, and they're rather
2) The pi network filter in the negative feedback
circuit (Figure 1) using the two 1500pf capacitors
and the 2.5mHy choke is in place to ensure RF doesn't get into the audio.
3) I used three 200K 2W resistors in series
for the feedback circuit voltage divider (figure 1). This keeps the voltage
seen by each resistor under 300 volts. If you use other resistor combinations,
try to stay within the voltage rating of the resistors you choose.
4) The solid-state rectifiers that I chose were
mixed. The 5V4 and 5R4's were replaced with off the shelf, 8-pin tube replacement
plugs. For the 6AL5, I used two 1N1566A's. These are radial lead TO-5 type
diodes that also happen to have .040" leads, which allowed me to plug
them in directly into the existing socket. (the pins on a 6AL5 are .040"
5) Before operating the rig: If you feel comfortable
working with the unit open and the 800 volts on, temporarily connect the
string of 200K resistors to the mod iron's 500 ohm tap through a resistance
substitution box (starting at several meg). Apply a small amount of audio,
then slowly drop the value in your substitution box. If the modulation level
drops with a drop in resistance, your polarity is correct. If the amplitude
increases, turn off the plate switch, and swap the plate caps on the 1625's.
If you're squeamish about 800 volts, with the unit warmed
up, as briefly as you can, turn on the plate switch. If you hear it oscillating,
immediately turn off the plate switch and swap the plate caps on the 1625's.
No audio input is needed for this test. It will scream if the polarity is
wrong. On my rig, the black lead on the mod transformer primary is on the
plate of the 1625 closest to the front panel, and the white lead is on the
rear 1625. There are a few places were the polarity can be switched in the
audio chain. Any one of them will make your new negative feedback circuit
look like a positive feedback circuit. (not good.) If the mod or driver
transformers were not wound the same from unit to unit, or if the units
were not wired the same by the original builders, the polarity cannot be
6) Have fun and be safe. Use the "volts"
position on your panel meter switch. It tells you when it's ok to touch