This all began when a couple of us decided to take a step back in time and to try AM after a forty year lapse from this mode. We didn't want to get into real "heavy metal" like some of the big guns on AM, but we did have certain criteria. It had to have tubes that glow in the dark and get hot to the touch. We started looking for receivers and rigs that would allow us to continue our usual afternoon chats using AM instead of the normal SSB.
I found a reasonably priced Heathkit DX-40 on an auction site. I'll tell you about my modifications of it. It nice enough to try an older mode, but hat doesn't mean that you can't improve on the original so I did quire a few modifications. One mod was - in the days of separate transmitters and receivers a "change over" system was necessary. I decided I could build in a relay to switch to the receiver without resorting to an external system.
Looks pretty good for a little over $100 shipped. Cosmetics are fine. the cheesy slide-switch for the meter has been changed to a bat handle switch. This is a common mod. The Heath slide-switch was the cheapest component in the DX-40 and did not last long. After removing the components from th top, use a lot of 409 and Contact Cleaner and "elbow grease" to clean off the dirt and grime. After that, replace the old and unsafe two-wire plug with a modern plug with a ground wire, which is simply soldered to a convenient point on the chassis. The fifth picture shows the old fashioned mic connector which was common in "the good old days. Next you can see how it was directly connected to the input. The builder left out a resistor so the seventh pix shows how I replaced it with a volume control. I'm not sure this was necessary, as I will explain later. One final note on cosmetics - some of the old gear was built on a steel chassis. If you buy from eBay be sure there is no rust on the gear before you put your money down.
Sometimes, if an old piece of gear has capacitors that don't show visible leakage, they MAY be restored by using an auto-transformer to bring up the power slowly slowly over a day or two. BUT if thee is any doubt, replace the caps with new ones. You'll find modern caps are also much smaller than those old paper electrolytics. The power caps are in the left and right-hand upper corner of the under chassis picture. I replaced those because one showed leakage. The DX-40 provides 120VAC on two pins of the accessory plug when the MODE switch is in the PHONE or CW position. This will key the relay and switch the antenna between transmitter and receiver. The ASTRON cap and two-watt resistor RC circuit had to be reoriented 90 degrees to make room for the relay. Note that the original transmitter output has been replaced with a very short length of RG-58 to one of the relay terminals. At this point I anticipated using the additional relay terminals for switching purposes. This turned out to be unnecessary and a SPDT relay would have been smaller and would work fine. Too late now! Pix 10 shows an additional PL-259 for the antenna input and pix 11 shows it installed along with a new microphone connector. In pix 12 is the original section for VFO input which was an RCA jack. Not good. Also, for an unknown reason, the builder installed red and black banana jacks but they were not connected to anything. ??? The next pix shows a PL259 for a new VFO input.
Here is one more shot of the change over relay and next a pix of the rear panel with all the mods indicated. I did not include a schematic, but there is on in the PDF manual listed below. The mods are simple to do, all it takes is some time and some chassis work. The DX-40 uses a 6146 final tube capable of 75 watts input (which was max for Novice licensees). To avoid a heavy modulation transformer and extra tubes and circuitry, the DX-40 uses screen-grid modulation that, by its nature, restricts modulation to about 80%. This was good because it kept Novices from causing distortion. It is also an excuse for me to have left out the volume control, but one never knows if the mic used is going to have higher gain which could itself cause distortion. I used an old Shure444 with low gain so the volume control stayed wide open all the time.
The DX-40 is a good little AM transmitter. If you can team it up with a decent receiver you can hook this one (with the change over mods) to it and an antenna and be in business on AM. In its day, amplitude modulation was KING and there is no question that the audio was superb, which was its downfall. The wide bandwidth taken up by AM was just too great. SSSC (Single-sideband suppressed carrier) came along and took up half the room and had none of the disadvantages. If you'd like to revisit AM days, there are AM nets on the air that would be glad to have you join. I hesitate to add this, but I had a "newcomer" email me and ask where the "speaker" output wason a DX-40. At first I thought it was a prank. When I realized he was serious I had to advise him that the Heath DX-40 was a TRANSMITTER, not a transceiver!
How does it work? It works just like Heath intended it to work. Even the big boys will let me play AM radio now! Here is a manual for the DX-40 in PDF form which you can download for yourself. DX-40 Manual