Here are two useful mods: 1) replacing the "Mode" switch with a 3 pole 3 throw shorting rotary switch, available from Mouser Electronics, their stock number 105-SR2511F-33S at $2.77 (my thanks to Hank W0TNM for this info), or a similar switch, and 2) adding blocked-grid keying capability so you can use the VF-1 with blocked-grid keyed rigs like the DX-60 or HW-16 .
By the way, by "shorting" above is meant the type of switch which keeps the first set of contacts connected until it reaches the second set of contacts, when it then releases the first set. This prevents the filaments from momentarily turning off between switch positions, causing a small amount of drift. This is NOT the same as a "progressive-shorting" switch, which keeps the first set of contacts connected, then keeps connecting subsequent contacts to those until it reaches the last contacts where ALL contacts are then connected in parallel.
The "Mode" switch modification changes the switch functions from "Off-On-Operate" to "Off-On-Spot". Before this change, the "Operate" position also keyed any cathode-keyed transmitter connected to the VF-1, making "Spotting" nearly impossible when using cathode keyed transmitters with the VF-1.
After this change, the "Spot" position keys JUST the VF-1 on, yet leaves all other functions as they were before. "Spotting" is therefore easily done. The Mouser switch was chosen because the knob-positions correspond exactly with those of the original switch.
All that is required for this mod is the switch listed above and some bits of wire.
The second modification, adding grid-block keying capability, requires a DPDT toggle-switch, and a disk-ceramic bypass capacitor of .001 to .1 mfd, 250 VDC or better. The only external change required is the removal of the rear-panel key-jack and the substitution of the DPDT toggle switch in its place. This modification was suggested by Tom N0JMY of www.hayseedhamfest.com, and this supercedes my earlier modification. This is much simpler than mine, and makes switching from cathode-keying to grid-block keying infinitely easier. Just flip the switch instead of removing the VFO from its case, and un-soldering two jumpers and re-soldering them.
The first schematic diagram shows the VF-1 in its original condition with all un-important (to our purposes) wiring left off. The second schematic diagram below shows the modifications.
They are really completely self-explanatory. We hope you all find these useful.
There is further useful information under the schematics.
These schematic diagrams can be printed directly from this web page.
The cable that connects the VF-1 to the transmitter accessory socket should be the 100% foil-with-drain wire, shielded type. Also, all power and keying leads brought into the VF-1 case should be bypassed with .01 mFd disk ceramic caps as close to where they enter as possible. Otherwise, feedback from the transmitter can cause chirp.
The VF-1 makes a pretty good flea power VFO transmitter when connected directly to a good antenna coupler. Output will be over 1 watt on 160 and 40 meters.
Although I have found the VF-1, when properly constructed, or when properly restored to be at least as stable as other VFOs, and that it will key very well if necessary, there are two other extremely simple changes one can make to improve it even more. These are 1) replace the original 6AU6 with a 6AH6 or the military equivalent, and 2) replace the original 0A2 150 volt regulator tube with an 0B2 108 volt regulator tube. The higher voltage is not really needed, and the 6AH6 is often much less prone to both drift and chirp than the 6AU6. The 6AH6 has exactly the same base connections as the 6AU6, and requires identical filament voltage and current.
In fact, I prefer the VF-1 to the later HG-10(*) VFO, both for its appearance (it looks so "retro" and I really like the funky green dial) and for its electrical characteristics. I have had two HG-10s and several VF-1s and the VF-1s were always both more stable and keyed with much less chirp than even the best HG-10. Furthermore, I have found that one must carefully hand-select a good 6CH8 for the HG-10 in order to find one that will not chirp when keyed, while the VF-1 will key cleanly with just about any 6AU6, and has even fewer problems when the 6AH6 is used.
Lastly, I must reiterate two points: 1) if you are having problems with chirp when using the VF-1 with any transmitter, the most likely cause is RF from the transmitter getting back into the VF-1 by improperly shielded leads into or out of it, or the contact between the chassis and front panel of the VF-1 is not good, and 2) the octal plug connections to power the external VFO in the DX-35, 40, 60, and HW-16 all have identical pin-outs, as mentioned on the bottom of the schematic above.
The VF-1 can be used with any transmitter using cathode-keying, and if the modification shown above is installed, also with any transmitter using blocked-grid keying. In the Heathkit line up, such transmitters include the AT-1, DX-20, DX-35, DX-40, DX-60, and HW-16. However, blocked-grid keying must be used with the DX-60 and HW-16, while cathode-keying must be used for any of the others. The VF-1 also works extremely well with the Johnson Viking II, Harvey-Wells TBS-50, and practically any other transmitter requiring an external VFO.
The only other improvements one could make to the VF-1 would be to 1) install an additional vernier of some type on the front panel, in series with the original tuning system, as the tuning rate is rather fast for us old-timers, and 2) install a Class A buffer stage, similar to that in the WRL-755A VFO, between the oscillator and the output.
We must thank Glen Zook, K9STH, for the info concerning the 6AU6 to 6AH6 sub, and the VR tube sub.
Ken Gordon W7EKB