Last year I bought an incomplete GenRad octave analyzer, as part of my overall move back to analog stuff. This machine is especially rare and useful, so it’s worth fixing. The microphone is missing, and I’ve been watching Ebay since then, hoping to grab up the mic or another ‘parts car’.

So far no mic, but another rare SLM appeared this month: a 1565A, the square predecessor to the 1565B. Since I had already done a tribute to the square, I had to buy the real thing.

Here’s the square SLM with its square leather case:

This unit was owned by NASA Langley. The meter and the leather case are both heavy and tough, and show considerable use. There’s no obvious opening or door on the meter to replace the batteries. The manual says to unscrew the knurled nut. There’s only one knurled nut, encircling the mic. Unscrewing it lifts the top cone.

Ingenious structure. The single C cell is clamped inside the nose cone.

How is a single C-cell enough to power a transistor circuit? Clever answer:

The first transistor is a ‘tickler-coil’ oscillator, humming at 130 cps and powering a secondary that gives 22V after rectification and filtering. This type of elegant analog design has disappeared.

There was a folded paper inside the leather case, documenting the last calibration of this meter in 2001. Note that the calibration was ‘initiated’ on 9/11/2001.

The lively 1970 handbook gives the original price as $365, plus leather case $15. Total $380, times inflation of 7.3, gives $2774 as the modern equivalent. The actual modern price for the newer round version is $3500. So the lawyer premium was pretty much the same in 1970, which surprised me. I had assumed the overgrowth of litigation started during the ’70s.

The UI and UX on these two devices are opposite.

The round has much better UX. Buttons let you choose fast or slow response separately from A/B/C weighting. The square has a single dial to select A slow, A fast, B slow, B fast, C slow, C fast.

The square has much better UI in terms of visibility. The meter face is easy to see from any angle in any light. The meter of the round is hidden behind part of the rounded body, and it’s hard to see even in good light.

The round is easier to hold but harder to set down vertically. It wants to tip over. The square is harder to hold but nice and solid in a vertical position.

Even the output plug is affected by round/square. The jack on the round is a submini, and it’s submerged inside the round case through a strange D-shaped channel. I had to file down a submini plug to reach it, and even then the plug didn’t make firm contact. I have to wiggle it to get non-scratchy audio. The output on the square is a big full-sized earphone jack, and it makes firm contact. So the square turns out to serve my purposes better, though I wouldn’t have bought it solely for this reason.