High Frequency AM

I picked up a Heathkit DX60B transmitter with matching HG-10B VFO at a local flea market. The DX60 in my opinion was fairly well designed for a novice transmitter (except for a troublesome Function switch) and I found out when in stock form it did produce noticeable key clicks. The transmitter includes a low pass filter, RF cage and buffer/multiplier, things that were missing from many other novice transmitters. The final amplifier tube is a 6146. Physically the transmitter and VFO were in reasonable shape but they suffered from a multitude of problems and took a couple of weeks to sort out working in my spare time. This included modifications to fix the key clicks.

The DX60 turned out to be a fairly good AM transmitter although low power. It uses controlled carrier modulation and apparently the DX60 has become a somewhat desirable transmitter when used to drive an amplifier. This one is used without an amplifier. Audio reports generally have been very good. The desk microphone I use was bought at the above mention flea market. The microphone (UD-844) appears virtually identical to the Yaesu YD-844 available as an option with some early Yaesu equipment. This one doesn't have the Yaesu name on it.

On AM the transmitter runs about 65 Watts output on modulation peaks on 20 meters. I used my Bird 43 Wattmeter and an oscilloscope with an isolated TEE to measure the power output at modulation peaks. Despite the low power I have managed to work some DX on 20 meters. The antenna is a Mosley TA33jr beam at about 50 feet. The popular 20 meter AM frequency is 14.286 MHz and boat anchors are mainly used there. Most of my operation is on this frequency. On 40 meters the popular AM frequencies are 7.295, 7.290 and 7.285 MHz but I have also heard Amateur AM between these frequencies.

Power for the HG-10B VFO is normally taken from the transmitter. In order to improve the drift characteristics of the VFO I am now powering the VFO from an external source and applying B+ continuously. Some drift is accepted on AM with these boat anchors and the switched voltage originally from the transmitter did not impose a real problem but I wanted to improve it. Drift is very noticeable on 10 meters with many of the boat anchors used there. One concern I had leaving the VFO run while receiving is RF leakage from the VFO getting into the receiver. The VFO operates either on the receiver frequency or the VFO frequency is multiplied to the receiver frequency assuming you are not operating split which seems to happen unintentionally on occasion especially on 10 meters J. The leakage turned out to be just a minor inconvenience at least on 20 and 10 meters. I have not tried running the VFO continuously on CW and I donít know if there would be a backwave issue.

This DX60 is a "B" model but the circuitry appears to be from an earlier version. I assume it may be just an early "B" model. The power supply has been rewired using "B" model circuitry as it needed to be repaired and I did not have the required part for the earlier version.

There is a known problem with the Function switch used in these transmitters. When transmitting the high voltage is essentially switched on using contacts on this switch. It is a ceramic wafer switch and a contact used to switch the high voltage tends to burn. I repaired the switch by drilling out the rivet holding the contact and replacing it with another contact using a #1 machine screw in place of the rivet. There is a mod for this problem that uses a relay to switch the high voltage and using the Function switch contacts to operate the relay coil instead of switching the high voltage directly. I plan on doing this mod when I locate a suitable relay.

The AM set-up here consists of the DX60B, HG10B (or more recently Heathkit VF-1) VFO and a Drake R4B receiver. A Dowkey relay is used for TX/RX switching and receiver muting. See the Photos section of this website for pictures of the equipment.

UPDATE: I recently acquired an EICO 720 transmitter with the outboard 730 plate modulator and now have them operational. I plan on using them in place of the DX60B for a while to see how well they work out.

ANOTHER UPDATE: I finally got around to doing the DX60B Function switch high voltage mod as mentioned above. I used a relay with a 120 VAC coil. It seems to work fine.

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