Keywords: on dx40
Description: This all began when AI8H, W4MLC and I (W4BTX) 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
This all began when AI8H, W4MLC and I (W4BTX) 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 on 3.702 Mhz using AM instead of the normal SSB.
I found a reasonably priced Heathkit DX-40 on an auction site, and W4MLC found a real beauty of the same type. AI8H got a DX-60 which is essentially the same as a DX-40 except in a more modern, for the era, style. I'll tell you about my modifications first.
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.
Not bad on the inside also. Dusty, as you might imagine, but no rust or corrosion. For once, I might have lucked out on an auction buy. I've really been stung by some of my "buys" but this one looks good. Let's hope it works without much trouble.
A good point to make here is this: Old equipment is usually dusty, maybe dirty. The people who sell these things seldom clean them, they just want to make a quick sale. Don't be put off by that. Ignore dust - but look carefully for rust or corrosion. If necessary, ask the seller for more pictures. Barring rust or corrosion, old gear can usually be easily cleaned.
Also clean beneath the chassis. The original builder did a nice neat job. Following Heathkit's instructions he carefully penciled in numbers on everything, including the terminal strips. This is a good place to give some advice. It is first impulse to fire the new/old rig right up. DON'T DO IT! If you want to try to bring back those old high-power capacitors hook your new purchase up to a Variac and run the gear in "warm up" or "standby" conditions at about 60 to 75% of full AC power (in this case, 70 or 90 VAC) for several hours, preferably over night. Sometimes this will restore those old, now hard to get and expensive, electrolytics. BUT check them and if they show "bulges" or leakage, replace them before they blow up and destroy a lot of good circuitry.
Now we're looking better. Some 409 cleaner, and a liberal spraying of contact cleaner are making my DX-40 look nicer all the time. Actually, the 40 year old paint was in remarkable condition, and as you can see the aluminum looks like new. Elbow grease works wonders. Note the sectioned band coil behind the front panel. That original meter slide switch was between the coil and the panel. I'm sticking with the previous owner's bat switch. Unless you are a stickler for exact "restoration" don't make extra work for yourself.
Another view of the cleaned chassis. Removing the tubes makes the job easier, and while you are at it, shoot some good contact cleaner into the tube sockets, and examine the tube pins to make sure they haven't corroded.
Now I've move the cleaned transmitter to the work bench. It's time to decide what mods I am going to make. Note that the 5U4 rectifier tube has been reinstalled. Previously I installed all the tubes and found to my joy that the old DX-40 still worked, putting out about 50 watts on CW. More details on the DX-40's modulation system later.
First thing is to replace the AC cord. All these old radios and transmitters had two-wire cords on them. Today with 120VAC as standard, and a third wire for "safety" ground, it is not only commanded by the Electrical Code, but a good idea for your own safety to install a three prong plug and cord. (Yes, I know it looks like it is giving you the finger, but it's just an illusion caused by late nights working DX.) Incidentally, the DX-40 does NOT have a fuse in it, so now is a good time to wire one in.
Here we have the typical mic input for old transmitters without PTT. A shield for ground and one center connector for the "hot" side of the microphone. These were never good. The center solder blob would flatten over time making the connection intermittent. And the more you tightened the connector the more flattened it became. This will be replaced. There is also room between the mic connector and the antenna input for another SO-239, because I am going to put an antenna change-over relay inside the chassis.
The mic cord (in the upper part of the picture) is gummy and sticky but it is going to be replaced. I need room for the antenna change-over relay and there is a nice open spot just below the mic cable. Unfortunately, the reason it is open is that there is a heavy iron choke on top of the chassis at that spot. That's not a bad thing. In fact, a point in favor of the DX-40 is that Heath used heavy-duty choke input power supplies in many of their early transmitters. Choke input was common when using tube rectifiers and provided a strong, steady power supply. With solid state rectifiers, capacitor input eliminated the iron choke.
The relay is in place. Some double-sided "sticky" tape and the wire leads will keep it planted. 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 and two-watt resistor RC circuit had to be oriented 90 degrees to make room for everything. 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 worked fine. Too late now!
Both antenna cables are now connected to the change-over relay. Probably not necessary, but I was able to keep short pieces of shielding on both. Note that the former transmitter antenna cable has now been moved to the open pin of the TPDT relay.
Both SO-239s are in place (white insert goes to receiver, yellow insert is antenna input). The mic connector has been moved to the original ground lug spot and changed to a simple open-circuit jack.
Moving right along - the original vfo input was a chintzy RCA phono plug. That must go. Also, for reasons unknown, the previous owner installed two banana jacks (black and red). But they are not connected to anything. I think I have a plan.
I think I can use the former phono plug bolt holes, and with careful use of the nibbler tool in the area of the banana jacks. I can.
A quick check of the audio finds that my trusty old Shure 444 microphone has too much "punch" for the DX-40. So off we go to AI8H's workshop. As you can see, Jeff likes to get right into his work. And what he found was that the builder had left a 1 meg resistor from the mic input to ground, out of the circuit. Why? Who knows? Maybe he had a microphone that was low on output. If these old transmitters could talk, we could give up SSB and spend hours listening to tales of the olden days.
Well, we are almost through now. We went from a dusty old transmitter to an up to date version and here are the modifications we made.