DX-100 Modifications

by Ed - AA8TV


Audio Mods

1)  Replace cathode caps from pin 8 of the 12AX7 to gnd with a 220uf 35v electrolytic and 1uf 35v tantalum.

2)  Replace 510pf coupling cap between pin 6 of the 12AX7 and the volume control with a .05uf 600v metalized polyester film.

3)  Remove the 2uf cap from pin 3 of the 12AX7 to gnd. Pin 3 will be the point where negative feedback is injected. See Figure 1

4)  Replace the .1uf supply decoupling caps with 10uf 450v electrolytic caps. (one is on the 47K dropping resistor and the other is on the 100K)

5)  Replace the 510pf coupling cap between pin 1 of the 12AX7 and pin 2 of the 12BY7 with a .05uf 600v metalized polyester film.

6)  Replace the 2uf cathode cap on pin 1of the 12BY7 to gnd. with two caps. One 100uf 35v electrolytic and one 1uf 35v tantalum.

7)  On the power supply side of the driver transformer primary, (labeled "M") add two .05uf 500v ceramic disc caps to gnd.

8)  On the secondary center-tap of the driver transformer, add a 1000uf 100v electrolytic cap to gnd. (+ side to gnd.)

9)  On the mod transformer where the primary center-tap and secondary hot side pick up +800v, add a .01uf 3kv ceramic disc cap to gnd.

10)  On the mod transformer, replace the .02uf 1.5kv cap across the secondary with a .01uf 3kv ceramic disc.

11)  Replace the .1uf decoupling cap on pin 3 of the 1625s to gnd with a .1uf 600v metalized polyester film.

12)  From the 500ohm tap on the mod transformer to pin 3 of the 12AX7, build the circuit shown in figure-1 to apply the negative feedback.



General Mods

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 plug.

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 devices.


Mod explanations:

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 supply regulation.


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 cost effective.

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" dia.)

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 guaranteed.

6)  Have fun and be safe. Use the "volts" position on your panel meter switch. It tells you when it's ok to touch things.


Back to the Modifications Page



17 December 2000