On the surface, this transmitter seems to be a Hallicrafters knock off on the Heathkit DX-60. Moreover, you would expect better design and quality from Hallicrafters than Heath. From reading the eham reviews of the HT-40, you would expect a great trip down nostalgia lane.

Surprise. Ain't so. There is no way I can give a pass on this one, its gonna be harsh. This is not an unsubstantiated bash job on a good radio. Read it to the end, please. I sold this rig after working on it for a while, so I have no photos preserved from that time. But I feel so strongly about this rig that I have to give you a heads up. Hallicrafters made some fine equipment, but this one did not live up to that reputation.

First, the DX-60 uses a commonly available 6146. The HT-40 uses a black and white TV sweep tube with an octal base, the 6DQ5. This could get scarce. Could you change it over? Maybe. Would it be worth it? Be aware that the 6146 could be a better choice for 6 meters, but the HT-40 doubles in the final for 6 meters. Power levels are not that different for the tube choices. But the screen modulation characteristics of the 6146 and the articles on DX-60 have a good track record for excellent performance. See the second point below for AC0OB's solution for the modulation issues.

Second, the controlled carrier modulation does not get above 80% in stock form. AC0OB has a nice write up on fixes for that, some power supply problems, and metering issues. So it is fixable. We hams love to tinker with audio upgrades. I admire his persistence, and he seems to have gotten it right. http://amfone.net/Amforum/index.php?topic=34555.0

Third, the metering does not include plate current. It uses maximum output. You can fix that by using the cathode resistor for a sampling point for current, as pointed out above. Maybe just personal preference, but it bugs me.

Fourth, the HT-40 suffers from the rotary function switch problems that plagued the DX-60, the Johnson Ranger, and the Eico 720. Use the push to talk mods to reduce the stress on the switch. Its a common problem of the rigs of the period.

Fifth, the PA may or may not be neutralized, depending on the date of production. It greatly stabilizes grid drive and loading on the upper bands. You may be able to fix this. See the AC0OB mods.

Finally, the worst flaw this rig has is the potential for spurious output on sub-harmonics if you try to use any of the commonly available affordable VFOs of the era. This would be the Heathkit VF-1 or the E F Johnson 122. Subharmonic is probably a term that you have not heard, if you are a newcomer to the hobby. How the heck does a transmitter divide the frequency by 2 or 3? It doesn't. The sub-harmonic term is a misnomer. The VFO signal at 1/2 or 1/3 the intended frequency passes through along with the desired signal. Usually the exciter section doubles or triples the VFO signal to get to the desired band. The HT-40 resorts to a Pi network inter stage coupling network to the final's grid circuit. This is not that rare, but you have to recognize it as a low pass filter. That means the desired signal and all frequencies below it pass through to the output. The final coupling network is also a low pass Pi network. The DX-100 and Apache use the technique. But they do take steps to avoid the problems inherent in the HT-40. When the subharmonic problem manifests, the rig appears to tune up OK. The only indication of problems is high SWR if you are using a mono band antenna. The SWR is higher than anticipated because the unwanted spurious output does not match well into the antenna tuned to the desired frequency. If you check the output RF with a monitor scope as you tune the PLATE and LOADING controls, you will clearly see the lower frequency RF. This will happen, for instance, if you use a VF-1 output, which is 160 meters, to transmit on the 80 meter band. It will also happen if you try to use 160 meter crystals on 80, or use 80 or 40 meter crystals on 20 and up. I tried to attenuate the low frequency energy by decreasing the RF choke in the exciter stage to no avail. The AC0OB mods page note there is another version with 50 pF coupling capacitors in the exciter section, which may have helped some with the problem. There is no good solution to this problem other than using fundamental crystals or a much more expensive VFO that has cleaner output.

Hallicrafters solved this with their HA-5 VFO. It is a beautiful heterodyne VFO with very clean output on the exact band selected. If you are lucky enough to find one, it will cost you. This will work nicely on any legacy rig. See one at: http://www.universal-radio.com/used/sold513.html

If you bought the HT-40 as a novice operator back in the 60s, you would likely operate it on crystal control and trade up to a better rig, and never need a modulator, unless you obtained a tech license and used it on 6 meters. Even in kit form, you did not get a good deal for your money on the HT-40.

The other VFO fix is to use a modern synthesized VFO with a driver. Using a synthesized VFO with output on the same band on which you wish to transmit will work. This one is specific to the HT-40, so it will work right out of the box. http://www.wa1ffl.com/VFO_driver_amp.pdf

Failure to implement either of these measures will probably score you an OO report.

BOTTOM LINE: Avoid this rig unless you are a real Hallicrafters fan or can obtain one with a working HA-5 VFO. Instead, get a DX-60 or Ranger, or even the Johnson Adventurer or Challenger. Consider a Globe Scout 680. All of these rigs use a parallel tuned exciter which does not have the sub-harmonic problem. You will have lots of fun with vintage radio, AM, and have a clean signal without expensive extreme measures to get it.

Again, this is not a slam on the otherwise sterling transmitters and receivers that Hallicrafters produced. The HT-40 just did not live up to the legacy.

UPDATE 8/8/2015

AC0OB has written a comprehensive article on repairing and improving the HT-40 for Electric Radio Magazine, the July 2015 issue #314. It explains an unusual and dangerous quirk in the HV supply switching. It shows how to fix the irritating output meter to change it into a plate current meter. It also has a valuable discussion of the overall circuit, and shows significant improvements to the AM modulation circuitry. The original design produced excessive sidebands, which, when run through a linear amp, could cause co channel interference. A discussion of the neutralization problem shows how to implement it. Most importantly, it addresses the spectral purity issue, changing L1 to 1 mH in the 6CX8 circuitry. I attempted something similar, and still was not able to get acceptable performance with a Heath VF-1; the Hallicrafters HA-5 VFO works but is expensive (worth it) and hard to find. He changes some other capacitor values, which may help with some issues. I encourage you to read the article in reprint, available from: http://www.ermag.com/.

Note: The important spectral purity information in the first issue is partially incorrect, so you have to get Electric Radio August 2015 #315 and see page 29 to get the full information.