Something I had been wanting to look for this hamfest season was an old VFO. They are typically more rare than transmitters from that era, because most transmitters in those days were crystal controlled, or had built-in VFOs.
My old novice transmitter is crystal controlled, and my old xtals are in the wrong part of the band now, and xtals are hard to find. They are no longer being made for the ham market. Ones that you can find are not cheap anymore.
So for a bit more convenience, I had looked at various options. On eBay, old VFOs are expensive. $100 – 240, often the seller had no idea if they work. A modern design digital VFO “kit” is also not cheap, and may take some effort to get to work with my transmitter.
With not many sellers at the AWA Swapmeet, and already spending $80 of my $105 dollars in my pocket, I saw this Eico 722 VFO that a latecomer put out. I don’t think the seller was a ham. He didn’t know anything about the VFO. It was missing one of the two control knobs. The cabinet looked well used, but the front was good. I asked him what he wanted for it, he said he really didn’t know, and finally said $25. I was willing to take a chance.
When I got it home, I looked it over. Someone had wired in three 20 mF electrolytic capacitors to replace the 3 section one on top of the chassis. However, they just wired them in parallel with the old one, which can be a problem if they are electrically leaking. It made the wiring easier, but I did not want to leave it that way. So I unsoldered the caps and clipped the old terminals off the original cap. Then I put in a 3 position terminal strip to remake the connections. I didn’t see any other issues that were obvious. I did also replace the old crusty 2 wire power cord with a new 3 wire one.
I found a manual online that I downloaded. I had an external HG-10 VFO around 1972-1973 for my Heathkit DX-60B, but I don’t remember much about it, I just connected it up per the manuals. The Eico manual was a bit sparse. Besides the output phono jack, there was a key jack, and two screw terminals on the back that the manual never mentions but to call them for “transmitter keying”.
The manual talked about three ways to use the VFO. Essentially, it seemed like I should use the one that described the VFO being always on and just key the transmitter.
I powered it up, to see if any major issues. None seen. Then tried to see if it was outputting anything. Nothing seen on an oscilloscope, so spent some time re-reading the manual and looking at the schematic. I measured some voltages, all looked OK. I wondered what the manual meant by always on. Did it mean it stays in the transmit position? Did it need to be keyed? I put a key in the key jack in the back of the VFO and tried again. It worked, there was my 3.600 mHz signal. It seemed to be functional. Now I needed to interface it to my homebrew transmitter.
I had watched some videos online about VFOs, and it seemed like the best bet was to just feed the VFO output into the crystal socket. My transmitter used an 8 pin octal socket. Pins 1 and 3 are a perfect fit for a standard crystal. I thought about sacrificing an old crystal, but I took one apart and it didn’t seem that feasible. Then I remembered I had an old 8 pin male octal socket from way back that I had used as part of a T/R switching setup in my station. It was in a box in the basement, and surprisingly I found it within 5 minutes. I usually look for something like that for days, and realize it probably was thrown out.
I wired up pins 1 and 3 with a piece of RG-174 coax, and a phono plug at the other end. Plugged it into the VFO and transmitter and fired everything up. No luck. I could key up either the VFO or the transmitter, but not both. So I put a shorted key into the VFO key jack (VFO on all the time, as per manual), and key the transmitter. It works. Progress being made. The problem is the VFO tone is constantly heard in the receiver between clicks. I can switch it off when listening, but annoying while sending. The manual mentions keying both with a double-pole relay, but why not key both directly? I made another cable, so my key fed the VFO and transmitter. Maybe a bit risky, but both are on the cathodes of the oscillator tubes. What the heck, I’ll try it. It worked, no annoying tone on all the time. It actually seemed to lessen the slight chirp that transmitter usually has. I probably should look closer at this though. A relay is probably the way to do it.
I still need to figure out my receiver muting. All these Hammarlund receivers have a manual Send position that mutes it, but would like to have it automatic. The HQ-100 that AB2RA modified, has a unique muting scheme. For some reason I have been unsuccessful getting it to work as I believe it should. So that is my next task. But the VFO seems to be working great. Here is a pic of VFO and transmitter.