Wednesday, February 15, 2017

Working on HW-100 and V-7, build of Digital Direct Synthesizer oscillator


I bought a kit to build replacement probes for the VTVM, which arrived yesterday and is assembled and ready for use. Heath used a phone jack for the main DC probe, plus banana jacks for ground and AC/Ohms probes. 

New probes for the VTVM
 I reinstalled the potentiometers that are now turning freely. With them soldered in place, I just need for my replacement capacitors to arrive from Digikey before I can wrap up the restoration and put this meter into service. They are projected to arrive on the 16th.


My remaining tube and the power supply are still drifting through the USPS system. The tube was due yesterday but is now showing "delayed delivery" without an updated date. The power supply is supposed to arrive tomorrow. 

I bought a direct digital synthesizer (DDS) to replace the VFO in the transceiver, giving me higher accuracy and stability for both transmit and receive. It was a kit that I quickly assembled and checked out today. It appears to work well, passing self test and displaying appropriate messages on the LCD panel.

The unit will mount atop the HW-100 cabinet to avoid having to hack the front panel apart to mount controls. I will need to manually synchronize the band settings on the transceiver with the appropriate DDS bands in order to have the LCD display the true frequency. 

Otherwise, the HW-100 would still work properly, tuning an intermediate frequency over a 500KHz range, which is added to the crystal selected frequency to yield the desired band.  That is, the DDS always produces signals over a 500KHz span, but it might display this relative to the wrong band's starting frequency. 

If on 80 meters, the unit is tuning to 3.500 to 4.00 MHz, while on band 2 (40 meters) it is tuning from 7.000 to 7.500 MHz. The VFO produces the same IF value in either case, but displays based on the DDS logical band I have selected. If I put the HW-100 in band 1, tune to 3.720 Mhz, but tell the DDS I am on band 2, the DDS LCD display will have 7.220 Mhz showing while I am truly tuned to 3.720M.

The HW-100 first produces a 3.395MHz IF, modified slightly depending on whether SSB or CW is chosen. The VFO then produces a signal that ranges from 5.000 to 5.500 MHz as the second IF. This is mixed with the first IF to create a third IF at 8.395 to 8.895 Mhz. The crystal oscillators and bandpass filters adjust this IF to the final RF frequency of 3.5-4, 7-7.5, etc all the way up to 10 meters at 29.5-30 MHz. 

You may wonder how it produces a final frequency of 3.5 Mhz with an IF that ranges from 8.395 to 8.895. The crystal frequencies are mixed and produce but the sum and difference. The bandpass chooses only the difference signal. Thus, the crystal at 12,395 MHz is mixed with 8.395 to give 21.79 sum and 4.0 difference. As the VFO increases from 5.000 to 5.500, the second IF goes up to 8.895 giving 21.29 sum and 3.5 difference. 

From this, you can see that the HW-100 VFO actually starts at 5.500 MHz when the dial is at .000 and decreases to 5.000 while the dial states .500, added to the band start. By this, I mean that to tune 3.510 the VFO has to be at 5.490 to give 2nd IF 8.405 and difference 3.510. 

Fortunately, the DDS can be set up with a negative or positive IF offset, which means I can have it move downward in frequency between 5.500 and 5.000 as I tune upwards through each band. It just requires some setup parameters to define my eight HW-100 bands and the appropriate offset. The goal is to have the DDS always generate between 5 and 5.5 MHz. 

By the end of the evening I had set up the first 5 bands and the following morning I wrapped it up. Putting the DDS on the frequency counter, I verified that the output frequency is always 5 to 5.5 Mhz on all bands, while the LCD display is showing the band. However, I still have a problem.

The dial direction is still inverted. That is, on the 3.5MHz band, with the dial set to 3.500 MHz output, I should be seeing 5.500MHz and as the dial gets up to 4.000 MHz the true output should have dropped to 5.000 MHz. Instead, this begins with 5.000 MHz at a dial indication of 3.5 and goes upward as the indication moves to 4.

I have figured out that there is a way to reverse dial indication, but it means a new round of configuration of the DDS. I will take care of this later today.


We may get a test shot late Friday, but several people have doctors appointments in the morning that restrict our time together. I would check the ability of the tool to write a bootable image and to read an existing cartridge correctly, assuming I get the chance. 

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