Crystal calibrators

Crystal calibrators from Bud and Heathkit

Vacuum tube communication radios typically do not have precise tuning indicators such as a digital radio with phase lock loop and frequency synthesizer would have. Crystal calibrators were invented to help solve the problem of precision in tuning an analog receiver.

Bud FCC-90A crystal calibrator
Bud FCC-90A crystal calibrator

A crystal calibrator can provide a precise crystal-controlled signal of 100 KHz. That signal and its harmonics can be adjusted to zero beat with WWV, typically at 10 MHz. The calibrator signals would then appear accurately at every interval of 100 KHz allowing relatively easy marking of the band edges in amateur radio and in determining more accurately the frequency to which an analog receiver was tuned. Some of the more expensive radios had built-in calibrators. Those radios sometimes also allowed the tuning scale to be physically moved a bit to aid in accuracy of tuning in band segments between calibrator harmonics. Other radios offered calibrators as accessory add-on devices. For those radios for which a calibrator was not available, companies such as Heathkit, Bud, Lafayette and others offered self-powered calibrators as accessories.

The Bud FCC-90A calibrator uses a 35W4 rectifier and a 50C5 as oscillator tube. The chassis-mounted removable 100 KHz crystal looks more like an aluminum electrolytic capacitor than a quartz crystal.

The Bud FCC-90 was missing a rectifier tube as purchased. I replaced it and the electrolytic inside the case. The two toggle switches for power and Standby were both open. Deoxit by way of the bat handles solved that. I noticed that one side of the power line was switched to B-. The B- was not connected to the chassis but was accessible on the frame of the fine adjustment variable capacitor and could thus be touched on top of the chassis. I polarized the line cord and then rewired the cord inside so that the neutral wide blade would be permanently connected to B- to minimize the danger and the possibility of contacting line voltage. The calibrator did not work. I checked the crystal by placing it in series between a signal generator and oscilloscope. It had a pronounced resonant peak at 100 KHz indicating it was fine. I noticed that a contact on the crystal socket had broken. I replaced it. On plugging the crystal back in, it worked but was intermittent. The capacitors are all ceramic with one mica. I finally checked all the resistors and found a one megohm resistor that had drifted upwards to 2 megohms. Replacing it solved the problem.

The 100 KHz signal was easily heard at 10 MHz for zero beating with WWV. The scope showed the actual signal as a saw-tooth.

Heathkit HD-20 crystal calibrator
Heathkit HD10 crystal calibrator

The Heathkit HD-20 uses a single transistor. A former owner had replaced its battery terminals to accommodate a new single-ended rectangular 9 volt battery. However, in the process the on-off switch had been bypassed so that the unit was always on. The binding post output terminal on top was frozen, probably because it had not been used for some time. Turning it caused the entire terminal assembly to turn. I took the HD-20 apart, resoldered the battery wiring and used a bit of deoxit to loosen the output terminal. The original battery hold-down clamp was designed for a round battery but readily accepted a more modern rectangular battery. Testing the unit showed a classic sine wave on the oscilloscope. The slug-tuned tank coil was adjusted for maximum signal on the scope. The fine frequency adjustment trimmer capacitor was set to within 1 Hertz using a frequency counter. Afterwards, the counter was tested and accurate at zero beating WWV at 10 MHz.

The calibrators are very useful with both tube and transistor analog tuned radios. Here is a picture of one of my favorite casual listening solid state portables, a luggage-sized Lloyds Electronics model 9N24B-37A picked up at a yard sale many years ago. I had to replace an RF transistor for the FM side at the time. A handy shop radio, the set is quite sensitive with four shortwave bands, broadcast, and long wave as well as three VHF bands and FM. It also has a handy audio input jack for connecting an MP3 player and coax inputs for AM and FM antennas. I think of it as a 1970s version of a Zenith Transoceanic clone. A calibrator is a handy addition for such a set.

Using the Heathkit HD-20 to zero-beat 10 MHz WWV on a Lloyds model 9N24B-37A
Lloyds 10 band radio

Calibrators on the scope
Scope display from Bud FCC-90A (left) and Heathkit HD-20(right)
Crystal calibrator on scope Crystal calibrator on scope

The Heathkit HD-20 manual can be found at Tubular Electronics

The Bud FCC-90B schematic is at The FCC-90B combines the On-Off switch and the Calibrator-Standby switch in a single three position toggle switch but is otherwise the same as the 90A.


The Gonset "Super-Six" (model 3030) Converter was the previous item on the bench.

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