Tektronix 492 Spectrum Analyzer Backplane Pin Spacing

Tek 492 Memory Board
Tek 492 Memory Board

My buddy Eks asked me to help fix his new-to-him and guaranteed broken Tek 492 spectrum analyzer, which turned into a tour-de-force effort. One sub-project involved sucking the bits out of an existing “known-good” Tek memory card, which meant building a backplane connector and a circuit that behaved like a 6800 microcontroller… fortunately, it could be a lot slower.

[Update: It seems searches involving “Tektronix 492” produce this page. You may also be interested in these posts…

If those aren’t what you’re looking for, note that the correct spelling is “Tektronix“.

Good luck fixing that gadget: it’s a great instrument when it works!]

You can tell just by looking that this board was designed back in the day when PCB layout involved flexible adhesive tape traces and little sticky donut pads. Ground plane? We don’t need no stinkin’ ground plane!

Actually, it’s a four-layer board done with the usual Tek attention to detail. They didn’t need a ground plane because they knew what they were doing. Remember, this is in a spectrum analyzer with an 18-GHz bandwidth and 80 dB dynamic range; a little digital hum and buzz just wouldn’t go unnoticed.

Tek 492 Backplane Geometry
Tek 492 Backplane Geometry

Anyhow, the backplane pins are on a 0.150-inch grid within each block. The center block (pins 13-36) is 0.200 inches from the left block (pins 1-12) and 0.250 from the right block (pins 37-60).

That means the left and right blocks are neatly aligned on the same 0.150-inch grid, with the middle block offset by 50 mils. You can’t plug the board in backwards unless you really work at it.

Of course, Eks had some genuine gold-plated Tek pins in his stash: 24 mils square and 32 mils across the diagonal. They have 1/4″ clear above the crimped area that anchors them to the black plastic spacer and are 1/2″ tall overall. They’re not standard header pins, but I suspect you could use some newfangled pins in a pinch.

Here’s what the reader board finally looked like, hacked traces and all, with the board connector to the rear. The memory board didn’t use all the backplane pins, so I only populated the ones that did something useful. The power-and-ground pins (left side of right pin block) stand separately from the other because I had to solder them to both the top and the bottom of the board: no plated-through holes!

Tek 492 Memory Board Reader
Tek 492 Memory Board Reader

I cannot imagine this being useful to anybody else, but I defined an Eagle part for the connector so I could CNC-drill the board. Drop me a note and I’ll send it to you.

[Update: this turned into a Circuit Cellar column, so you can fetch a ZIP file from their FTP site that has all manner of useful stuff.]

Memo to Self: The drill size follows the pin’s diagonal measurement… not the side! Duh.