RETROVOLTAGE

I just had an unexpected run-in with Murphy’s Law and found myself needing a modulated signal generator in short order. Bad news when my modulated signal generators decided to die right when I went to perform an alignment! My Leader Standard Signal Generator has developed a fault in the power supply somewhere which trips the crowbar, and my EICO seems to not want to produce any output.

Lacking the time to troubleshoot either, I purchased a “new” signal generator: the LogiMetrics 921A RF Signal Generator, refurbished by KISS-Electronics in Cornelius, OR via their eBay Store.  I generally take care of my own test equipment, but for something like this where I needed it done quickly and done right, it was worth it to spend a little extra for a professionally refurbished one.  The owner is a fellow amateur radio enthusiast as well which was encouraging. KISS-Electronics specializes in vacuum tube and transistor test equipment from the ’60s and ’70s, which definitely describes the 921A:

You might also notice the upgraded digital frequency read-out. More on that in a bit. This is a very cool generator with big analog panel meters, showing the calibrated output voltage (1 uV ~ 3V in 1-3-10 steps) and the modulation percentage.

This generator will produce an unmodulated sine, or a 400 Hz or 1 kHz tone from an internal oscillator; there’s also a panel jack for an external modulation input. There’s also the frequency analog output, which produces a voltage proportional to the frequency of the output, and the Freq Shift input which can be used as a remote frequency vernier or to produce an FSK/NBFM signal. It’s “ham radio FM”, not broadcast FM, but it might come in handy if I need to align a communications receiver at some point.

The output ranges from 50 kHz to 80 MHz in 7 bands. The replacement counter is bright and easy to read, and there’s dual-speed tuning plus a fine tuning adjustment near the frequency read-out. It’s pretty stable once warmed up, but it does take about an hour to stabilize.

Inside, it’s a well built machine. There are shop notes written on the cover to the left: it looks like this one was reconditioned on 1/5/2014 and sat around for a few months before coming to live with me. The tuning mechanism is under the shield, with the tuning knob and band switch passing through the plate.

One cool upgrade is the N3ZI Compact Counter, which replaced the Nixie Tube frequency counter this was originally fitted with. The original counter was good enough for what it was, but nixie tubes require a high voltage power supply along with a variety of high voltage logic chips that haven’t been made for decades and really never appeared in any other kind of equipment; they also tend to go bad over time and go bad due to other failures in the high voltage circuitry. All of that means that it’s generally not worth the trouble to fix up a nixie tube display. Swapping the nixie tube counter out for a modern counter saves time, money, power and weight. The shop calibrated the counter against their GPSDO for accuracy and I checked it against my bench counter and it’s spot on.

Underneath the cover, there’s a board, and the power supply for the N3ZI compact counter. Underneath on the bottom of the unit, there’s a few more pieces of gear. The HV power supply components have been removed, since there’s no need for an HV supply with the digital counter.

Quality work. A bunch of discrete transistors, and a few early op-amp ICs in Ceramic DIP packages, along with plenty of adjustments. It looks like it was aligned perfectly as the internal counter and my external counter perfectly tracked each other through the whole tuning range. Hopefully these close-up board photos of the LogiMetrics 921A circuit boards will be helpful to someone – according to various forum posts, the service manual is a bit hard to locate.

You can never have too many oscilloscopes; this GoldStar OS-904RD 40MHz dual-trace analog oscilloscope is a recent gift. There’ll be a post about it separately at some point, but I used it to check out the output waveforms on the LogiMetrics. It produces beautiful, clean, accurate RF envelopes. Here’s 430.2 KHz modulated by 1 KHz at 50% modulation:

It is a bit large, so I’ve had to rearrange my bench to make room, but it’s going to be great. It certainly will beat the EICO in any task, hands down, and has a greater frequency range than my Sencore AM/FM Stereo Analyzer which only works on the U.S. AM and FM broadcast bands. I’m going to get a lot of use out of this while aligning shortwave and amateur receivers.

I hear these are likely to drift a bit, and mine does seem to go about 200 Hz low as it warms up. The counter in the photos is showing 430.2 KHz, when the generator was set to 430 KHz and re-tuned over about an hour, so it’s definitely shifted some, but like any piece of precision equipment some time to warm up and stabilize is key. The seller already has another one for sale, and it’s the slightly enhanced model which I believe has an external 10 MHz timebase input on the back – great if you have a precision external timebase lying around.

KISS-Electronics eBay Store