- 2025
- Jan
- 2
The Heatkit AF-1 Analog Frequency Meter, Part 1.
Here’s a relatively unusual piece of equipment up next on the bench, an analog frequency meter (not counter!) made by Heathkit - the Model AF-1. This is similar to pieces made by HP before they moved to digital displays in the 60s, and this was completely obsolete by the time the 1970s rolled around. This one appears to be one of the older models, using the red and beige face along with chicken-head knobs. It’s in ok shape for the age.
It’s not really a counter, per se, because it doesn’t count. If I understand the theory correctly, it works akin to a light dimmer model. In a light dimmer, you cut part of the sinewave off - the more sinewave remaining, the brighter the lamp. With this unit, it shapes the incoming signal down to a spiked pulse - the more pulses, the higher the average voltage presented to the meter, and the more deflection you get. It’s as accurate as the meter makes it, and is basically a frequency to voltage converter. It uses 7 ranges, individually adjustable, and covers 0-100kHz. Front panel is sparse, with input, range, power, pilot, and meters.
This is kind of an oddball piece of equipment. It was made from 1951 to 1959, and by the time digital technologies rolled around in the 60s, this piece of equipment was obsolete - it’s trivially easy to count frequency with digital devices, so these probably went in the bin. Other devices - voltmeters, signal generators, even scopes are still useful today and can provide viable data, but this is just a relic of yesterday.
When I received the unit, it had 7 holes drilled into the top. I wasn’t sure why, but found out later when I opened it.
Inside, we see the tubes, transformer, and 7 calibration pots. That’s what the holes are for.
Tube compliment:
6X5 rectifier for a full wave power supply.
VR150 / OD3 150V voltage regulator to provide a stable voltage to the measurement circuit
6SJ7 as an amplifier and pulse shaper
6V6 as an amplifier and pulse shaper
6H6 as a rectifier for the incoming signal
Since this device needs to convert to a known voltage in order to do it’s work, it has a relatively unusual VR150 tube for the supplies. This tube glows a lovely purple when energized, and operates in a manner similar to a neon bulb.
This set of tubes has some hours on it, as evidenced by the tungsten burn-off on the rectifier:
The bottom is what you’d expect for a device of this era:
Nothing unusual here. The random array of drifted resistors and bad wax paper firecrackers adorns it. Of note are the adjustment pots - someone has replaced a handful of the 200Ω rheostats with 5k potentiometers. They’re of varying styles, so this was a junk bin build (or rebuild.)
I did some checks on the unit and didn’t see anything shorted, so into the isolation transformer it goes. It comes up ok, but is wildly out of cal - here it is measuring the 984Hz output from my IG72 signal generator:
In addition to being out of cal, it probably has leaky and dead capacitors everywhere. The ripple on the power supply certainly shows the filters are bad:
Yes, that’s 11.6Vp/p of ripple.
However, it’s operational. That’s what I’m here for. Next is to put some filters on it and see what happens.
Next part of this series: https://wereboar.com … eter-part-2-testing/