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Thread: Heath DX-40

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  1. #1

    Default Heath DX-40

    Can anybody tell me if the HG-10 VFO is compatible with the DX-40. I know orginally the VF-1 was used with it,but they seem to drift so bad and I have the HG-10.

    Thx
    Walt/K6EGE

  2. #2

    Default

    The HG-10 will work fine with the DX-40. Just do NOT rob power from the transmitter to power the VFO.

    As for the VF-1: with 2, non invasive, "modifications", the VF-1 VFOs are very stable. The first is to replace the 6AU6 oscillator tube with a 6AH6. No wiring changes have to be made, just unplug the 6AU6 and plug in a 6AH6. The calibration may shift a few kHz due to the fact that their is an inter-electrode capacitance difference between the tube types. All that is required is to "touch up" the variable capacitors used for calibration on the bands. It is not necessary to do any tuning on the inductors. Next, replace the 0A2 screen voltage regulator with an 0B2. This drops the screen voltage from 150 volts to 108 volts. Finally, do NOT power the VF-1 from the DX-40. The power transformer in the DX-40 is already running on the "ragged edge" and the high voltage varies all over the place when transmitting. Use the 0A2 removed from the VF-1 to regulate an external power supply to 150 volts and then use that to power the VF-1.

    Although Heath designed the HG-10 series VFOs so that they "might" be powered from the DX-60 series transmitters (which have a transformer that is not being overloaded), there is a definite recommendation that the HG-10 series be run from a separate power supply, again with 150 volts regulated. Running the VFO from the DX-40 power supply is a sure way to hasten the failure of the power transformer in the DX-40. There are a LOT more DX-40s "out there" with blown power transformers than those that are still working. The DX-35 power transformer is even worse than that in the DX-40. Only in the DX-60 series did Heath actually install a power transformer that could actually "handle" the requirements with even a little reserve.

    The VF-1 VFOs usually have more output than the HG-10 series and therefore the VF-1 will give more drive. However, the output of the HG-10 series is plenty to drive the DX-40.

    No matter which VFO is used with the DX-40, it is definitely prudent to allow the VFO to run constantly when the transmitter is in the transmit position. If you try to key the VFO along with the transmitter, in the vast majority of cases the VFO will "chirp". Allowing the VFO to run almost always keeps it from chirping.

    I have 2 VF-1 VFOs, one early model and one of the much more common models. The basic design is exactly the same, there are just 2 widely different front panel designs. I occasionally use those VFOs with either my Hallicrafters HT-20 transmitter or with my Heath DX-35 transmitters. Both have the tube changes and are run from a separate power supply. Both, after about a 15 to 20 minute "warm up", are very stable.

    Glen, K9STH

  3. #3

    Default

    Thanks for the info Glen. Looks like I'm going to have to build a small power supply if I use the VFO.

  4. #4

    Default

    A simple way to get a suitable power supply built is to use 2 filament transformers wired "back to back". That is, the low voltage windings wired to each other. The primary of the 2nd transformer, when rectified with either a half-wave or a full-wave bridge, will produce a voltage that is high enough to be regulated to 150 volts. If 6.3 VAC filament transformers are used, ground one side of the 6.3 VAC windings and then feed the heaters in the VFO from the un-grounded side. If you don't have suitable transformers, Radio Shack has inexpensive 12.6 VAC transformers with a center tap giving 6.3 VAC. Ground one side of the 12.6 VAC windings, connect the other side of the 12.6 windings together, and then use the center tap for the 6.3 VAC heater requirements of the VFO.

    I use that configuration for building power supplies for VFOs, tube type receiving converters, and even for transmitting bias supplies when the power transformer doesn't have a suitable winding.

    Glen, K9STH

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