Thursday, February 23, 2017

C-3 and HW-100 work, construction of DDS box


While removing the old capacitors that will be replaced, I found a funky modification made to the underside of the chassis. Someone had soldered 20 ohms of high wattage resistors across a new tie strip that was added. It is a poor quality change, just as I found quite a few solder joints which were amateurish at best.

Resistors inserted in series with magic eye tube filament
Upon investigation, I found these resistors were added in series with the filament on the magic eye tube. Looking at the tube, I discovered it was a 6E5 magic eye, not the 1629 tube supplied with the kit. Aha - the filament on the 1629 is 12V but the 6E5 requires a lower 6V supply. The series resistors were intended to drop the voltage.

Until I replace the capacitors and power this up, I really don't know if the 6E5 tube is working. If it works well, I will dress up and leave this little mod in place but if I need a new tube, I will just go back to a 1629 and drop the extraneous resistors. The wiring of the socket for the 1629 is different from that of the 6E5, thus I have to rewire that also if I switch back.


I am working on the layout for the external box that will hold my direct digital synthesizer that replaces the VFO inside the HW-100. I need to lock in the orientation I want and the placement of the LCD, rotary encoder and three pushbuttons. Further, I need to decide on the power supply and RF output connector types and placement.

Aluminum box for DDS VFO

DDS and LCD to mount inside
Rotary encoder and RF output jack

Buttons and knob to complete the front panel
Perhaps I should create a small board to deliver 12V from inside the HW-100 for use in the DDS VFO. I would likely take the filament power, rectify it and regulate it, putting some small board inside the HW-100 to accomplish all of this.

Meanwhile, the microphone arrived - a historically accurate choice, an Astatic D-1604, and I hooked it up to the transceiver. Now, I could perform the transmitter alignment steps from the Heathkit manual. That is, I could if I hadn't disassembled the DDS VFO. First, I have to complete the construction of the DDS in its box.

I punched the holes for the three pushbuttons and mounted them. The hole for the LCD is cut out, although a bit uneven because of the way I had to make the rectangular slot. My hole for the rotary encoder is too small, thus I have to step up one size and punch again.

I will need to carefully measure, mark and drill the small holes for the mounting hardware that will hold the LCD in place behind the panel. At the same time, I should create the holes on the bottom to mount the main DDS PCB.

I still have not finalized the RF output and power input jacks, thus can't punch the holes in the rear yet. I have a number of heathkit style rubber feet and will place four of them on the bottom of this enclosure to allow it to stand a bit above the HW-100 cabinet top.

I chose to run over to Anchor Electronics to pick up the RF and power jacks, which would let me complete the project tonight. Sadly when I mounted the LCD panel, the backlight broke from flexing of the board. It works but need external light.

After dinner, I had both boards mounted and was wiring up the controls. I finished at night but wasn't ready to test it out until the morning.


  1. Wait, isn't that a 0-F Hex rotary knob from an IBM 360 era front panel? (Or a... gasp! 1130 panel?!?!)

  2. Yes, indeed, it is from a 360 era panel - in this case I think it was a 3803 panel or similar.