Price below design commentary: Heathkit Model VF-1 VFO Design Commentary Everything You Wanted To Know About The VF-1 But Were Afraid To Ask Just a few design notes in no order of importance: The Heathkit Model VF-1 VFO, announced in the Summer of 1954, covers 7 bands, 160M - 11M, in 3 switch positions (160/80/40, 40/20/15/10, and 11). Measuring 7”H x 6-1/2”W x 7”D, it weighs 4 lbs. Its tube compliment is a single 6AU6, used as a Clapp oscillator, and a 0A2, 150V voltage regulator. It sold for $19.50. Mechanically, and electrically, the VF-1 is relatively simple. A separate LC network is used for 160M and another is used for 40M. For 11M, a variable trim capacitor is added to the 40M LC network. Assembly and wiring is very straight-forward, and can easily be accomplished in two evenings work. When announced in the Summer of 1954, the VF-1 had a copper-plated chassis. Sometime around mid-1958, Heathkit changed the VF-1 chassis to non-copper-plated until it was discontinued in 1961, replaced by the HG-10. The VF-1’s dial is very easy to read and has a green glow due to the green filter behind the dial, illuminated by a #47 bulb. The green filter is split in the middle creating a narrow, white, fiducial for setting the desired output frequency. Installing a 10 ohm 1/2W resistor is series with the #47 bulb will lengthen its life and reduce the dial illumination a bit. The main tuning capacitor is an E.F. Johnson P/N 169-36. It is a special build for the VF-1. It is a dual-variable with a specified maximum capacity of 35uufd (160M) and 11uufd (40M), comprised of 5 and 2 stator plates respectively. It is NOT an off-the-shelf “catalog P/N”. The Summer of 1954 flyer announcing the VF-1 has a close-up photo of the main tuning capacitor. It is NOT the same one used in later production VF-1s. The one in the photos is bolted together and the production version is soldered. It’s also clear from the schematic that accompanies the flyer that the temperature compensating capacitors and their value had been determined with the bolted version. The temperature coefficient of that variable depends on the linear temperature coefficient of the expansion of the metal used - brass being the most common. However, the capacitor’s structural rigidity is also important, and in this regard, there are clear differences between the two variables. The VF-1’s friction dial-drive is a bit clever. The front panel is slotted for the installation of the ¼” drive-bushing and friction drive shaft which engages the circular cutout in the plastic dial disk. The plastic dial disk is 0.030” thick. Some experimentation is required to get the engagement “just right”. Too loose and it will slip. Too tight and it’s difficult to turn. That’s why the hole in the front panel is slotted. The procedure I’ve used over the years is to press down on the top of the bushing with a short length of 0.062” PCB material, applying even pressure while the bushing’s 3/8” nut is tightened using a 3/8” nut-driver. However, here’s the problem with the VF-1’s dial drive: to cover the entire 160M/80M, it takes less than two revolutions of the main tuning knob - about 1.8 to be precise. And about 1.3 to cover the 40M band. There’s just not enough resolution to precisely set any given frequency. Compare this to the Heathkit HG-10’s relatively complicated 28:1 gear drive vernier tuning system. Hint: during the rebuild of VF-1 #3, the dial assembly’s 4-40 hardware was rusted. I replaced it w/new 4-40 pan-head screws and 4-40 nuts. However, after the final assembly, the dial wouldn’t turn more than about 90°!! Turns out that the 0.200” diameter of the new 4-40 screws caused an interference-fit with the friction drive!! I kissed each screw with a bench grinder and then positioned each screw on the dial assembly to avoid the interference-fit. Hint: the easiest way to install the front panel and the friction drive components is to mount the friction drive and ¼” bushing to the dial assembly at the bottom, engaging the dial, and then just lay the front panel over the two switches and the bushing.. The 160M coil is divided into two sections - 82T and 4T separated by 0.3” on a 0.87 dia. Ceramic coil form. I think the intent of the design is that adjustable core interacts more with the 4T coil than the larger 82T coil. The VF-1's schematic says the coil is 114.5 uHy. In the 40M position, with one end of the 160M coil not connected to anything other than the open switch contact, the 160M inductor measured 109.6 uHy. The calculated inductance of the coil, assuming 86T and a coil length of 1.06”, and no core, is 96.4 uHy. The coil, without its core measures 102.2 uHy. It should be noted that I have several archival photos that shows one continuous winding of the 160M coil - no separation - on both copper-plated and non-copper-plated chassis.. The 40M coil is also divided into two sections - 17T and 5T separated by 1/8" on a 0.87 dia. Ceramic coil form. I think the intent of the design is that adjustable core interacts more with the 5T coil than the larger 17T coil. The VF-1's schematic says the coil is 9.3uHy. In the 160M position, with one end of the 40M coil not connected to anything other than the open switch contact, the 40M inductor measured only 7.6 uHy. The calculated inductance of the coil, assuming 22T and a coil length of 0.64”, and no core, is 8.88 uHy. The coil, without its core, measures 7.8 uHy. Per Heathkit, the 160M and 40M coils are wound w/Litz or double cellulose wire coated w/Polystyrene cement and baked for humidity protection. The iron cores are identical for both coils and measure approximately 0.73”L x 0.42” Dia. They are mounted on a length of 6-32 brass stock approximately 1.25”L and slotted at the end for adjustment. However, the length of their 6-32 bushings plus their 6-32 “jam-nuts”, limits the maximum adjustment range to approximately 0.8”. The primary frequency determining components for the 160M band are: the 35uufd variable, a 4.5-25uufd NPO ceramic variable trim capacitor, a 47uufd silver mica capacitor, and a 10uufd N750 ceramic capacitor. The primary frequency determining components for the 40M band are: the 11uufd variable, a 4.5-25uufd NPO ceramic variable trim capacitor, a 22uufd silver mica capacitor, and a 4.7uufd N750 ceramic capacitor. The 11M band uses an additional 4.5-25uufd NPO ceramic variable trim capacitor to obtain 6.740 MHz - 6.808 MHz when switched into the 40M LC network. There is a dual-section coil assembly in the plate of the 6AU6. It is comprised of two coils in series. The top coil is for 160M and is specified at 50-150 uHy. The bottom coil is for 40M and is specified at 5-15 uHy. The top coil is shorted out in the 40M and 11M positions. In my rebuilt unit, those coils measured 70.6 uHy (pins 1 and 2) and 5.4 uHy (pins 2 and 3) respectively. Both are slug-tuned and adjustable. The bandswitch is essentially a 3-position 3-pole switch on a single ceramic wafer. Like many Heathkit parts, this is also not an off-the-shelf item, although a standard 3-pole 3-position ceramic switch could be made to work. The ceramic wafer is at the end of 1-3/4” 4-40 spacers, and I have found that there is a certain degree of flexing as the switch is turned, causing some mis-alignment of the contacts. When I rebuilt my VF-1, I shortened these spacers to ½”, and cut down the switch accordingly. The OFF/STANDBY/ON switch is a 1-pole 3-position progressively shorting phenolic switch. The VF-1 manual, P/N 595-91, dated 1954, specifies the power requirements: 6.3 Vac @ 0.45A, and 250-350 Vdc @ 15-20 mA. Experience has shown that the B+ power requirements can easily exceed 25 mA in 40M. However, the VF-1 was designed as a companion to the Heathkit Model AT-1 4-band 25W Amateur CW Transmitter which was announced in the Spring of 1953. The AT-1’s B+ can easily exceed 500 Vdc key-up (see below), and, the VF-1’s power cable is terminated for use with the AT-1. Using my rebuilt VF-1 as a “benchmark”, the power supply current at 160M and 40M at a supply voltage of 300 Vdc is14.83 mA and 21.36 mA respectively. This translates into a power consumption in the box of 7.3W and 9.2W respectively. On Sunday 10Feb19 and Monday 11Feb19, the measured temperature-rise at 160M and 40M after 60 minutes of operation was 8.3°C and 10.1°C respectively. At a supply voltage of 300 Vdc, the 15K 5% 10W dropping resistor for the 150 Vdc 0A2 regulator tube is only dissipating 1.5W. However, the VF-1 is designed to be used with the Heathkit AT-1 transmitter. At key-up, the AT-1’s B+ measured 523 Vdc @ 115 Vac line voltage. Under those conditions, that 15K is now dissipating 9.28W!!! That resistor, for use with the AT-1, should be at least a 15W-20W wirewound resistor!!! 9.28W is a recipe for early failure, especially with the typical construction of a 10W cement-block resistor!!! Well, the situation gets much worse. The VF-1 is also widely used with the Heathkit DX-20 CW Transmitter. At key-up, the DX-20’s B+ was 599.4 Vdc at 115 Vac line voltage!! Under those conditions, that 15K is now dissipating 13.46W!!! It should be noted that there is NO accessory socket on the DX-20 for connecting the VF-1, and the DX-20 manual clearly indicates using a separate power supply for the VF-1. However…………... It should also be noted that the original P/N 595-91 manual indicates that the series-dropping resistor for the 0A2 is a 15K 5W. It was a Sprague Koolohm P/N 5KT 15K 5W ceramic power resistor. I have several archival photos still showing the original Sprague resistor in place, although these components have no date-code. The Sprague Koolohm was eventually replaced by an IRC P/N 3-8J 15K 5% 10W “cement-block” power resistor. In the beginning, the 3-conductor power cable, P/N 347-4, was relatively large and black (braid=GND, grn=key, red=B=, and yellow=6.3 Vac). Sometime later, but during the use of the copper-plated chassis, a smaller, gray, 3-conductor cable was used (braid=GND, white=key, red=B+, and black=6.3 Vac). I suspect this transition was around the time that the Sprague Koolohm was replaced by the IRC 10W cement-block resistor. The VF-1’s output voltage is specified as “10 Volts”. Is that P-P or rms?? My “benchmark” VF-1 has been rebuilt with a BNC connector instead of the permanently attached RF cable. The RF output is very sensitive to the cable’s cap load, so I tested the output voltage with a 3’ length of 3C-2V 75 ohm coaxial cable (measured 93uuf) attached (B+ = 300 Vdc, 0A2 installed). The output voltage is both band and frequency dependent. On 160M, the RF output voltage varied from 5.48 Vrms to 8.31 Vrms from 1750 KHz to 2000 KHz. On 40M, the RF output voltage varied from 6.49 Vrms to 4.95 Vrms from 7.00 MHz to 7.3 MHz. It should be noted that the Heathkit HG-10 VFO’s output is specified at 5 Vrms, open-circuit, with an RCA phono connector, and a similar 100pf coupling-capacitor. The dial calibration is tube-specific. If you replace the 6AU6, you will need to re-calibrate the VF-1. In testing eight tubes, there was a 10 KHz spread at 7.150 KHz. I didn’t test the end-points. In addition, the warm-up frequency drift of the VF-1 can also be tube-specific. If you have the tubes, it’s a very good idea to select one for the best warm-up drift. Speaking of tubes, conventional-wisdom on the internet suggests replacing the 6AU6 with a 6AH6. I have tested three VF-1s with three different 6AH6WA tubes from two different manufacturers and found that, in all three cases, they worsen the VF-1’s warm-up drift. They also draw more power and increase the VF-1’s internal temperature-rise. In my opinion, it is better to test and select 6AU6s for performance than to switch to a 6AH6. In any event, taking the time to select either the 6AU6 or 6AH6, for best warm-up drift, can result in some spectacular before vs after warm-up performance. My “keeper” VF-1, that I have had since 1991, had an original 6AU6. After testing eight 6AU6, 6AU6A, and 6AH6WA tubes, the original tube came very close, but I replaced it with an HP-branded 6AU6A made by G.E. Notes and References: 1. “Heath of the Month #79 - VF-1 VFO”, by Bob Eckweiler, AF6C, 2017. 2. “Heathkit - A Guide to the Amateur Radio Products”, Second Edition, By Chuck Penson, WA7ZZE, 2003, page 268. 3. “Assembling And Using Your….Heathkit Variable Frequency Oscillator Model VF-1”, Heath Company, P/N 595-91, Copyright 1954. 20 pages not including the cover. A small 7”W x 9”L manual.. 4. During the use of the copper-plated chassis, Heathkit purchased a run of the P/N 40-56 6AU6 plate-coils and these were date-coded 632 - the 32nd week of 1956. Sometime around mid-1958, Heathkit changed the VF-1 chassis to non-copper-plated. It is interesting that a VF-1 with a non-copper-plated chassis with a 15K 5% 10W resistor date-coded 214842, the 42nd week of 1958, still had a P/N 40-56 date-coded 632!! That’s over 2 years from the same run!! All three of my non-copper-plated VF-1s have had P/N 40-56 coils date-coded 632!! And, the cans are not stamped in the same place or in the same orientation!!! 5. Judging by their routine appearance on eBay, the Heathkit VF-1 is widely available. However, like most kits from this era, their condition varies from very good, almost pristine, to downright horrible!! As I write this in March of 2019, many VF-1s w/copper-plated chassis have been in the field for 65 years!!!! Written By David W. Ishmael - WA6VVL - 28Feb19 Latest Revision 27Mar19 $300 plus shipping.