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.

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  1. #1 by John Miles on 3-December-2009 - 16:18

    Hi, Ed –

    Congratulations on a heroic troubleshooting effort. :) The memory-board reader is a great hack, and actually, that EAGLE part definition might come in handy if/when someone needs to create service board extenders for the 49x line. (They were available for a time from Norway Labs in Beaverton but aren’t currently being produced.)

    As I mentioned offline, the firmware for many Tek analyzers, scopes, and other instruments can be found in the Manuals / ROM Images section at http://www.ko4bb.com . The 1.6 firmware for the 492 was posted fairly recently, probably too late to save you the R&D effort in this case but it might be helpful to other users. Luis Cupido, who contributed the firmware, also documents a process for burning it to a larger 27xx-series EPROM that may be easier to find.

    Having repaired a few of them for local hams, I’ve put together an ad-hoc collection of service notes and FAQs for these analyzers at http://www.ke5fx.com/49x_notes.pdf . Every RF hobbyist should own one, if only for the troubleshooting experience!

    • #2 by Ed on 3-December-2009 - 16:27

      Excellent!

      I should burn a 27256 with that image and give it to Eks, just for completeness… it certainly beats those weird-ass not-quite-EPROMs we used!

  2. #3 by Martin on 30-August-2010 - 18:04

    Hiya, I am having to build some extender cards for my 495/P. I would be very pleased to have the Eagle connector part you defined. It will save me a lot of work, and it will help me source some sockets to pick up the pins on the motherboard…
    Best regards, Martin

    • #4 by Ed on 30-August-2010 - 21:17

      It’s on its way directly to you… let me know if it doesn’t match reality!

  3. #5 by Clay on 2-September-2011 - 05:57

    Ed,

    Could you also email me the Eagle connector part, I am also ‘attempting’ to make a card extender for the 492?

    Regards

    [Ed: Done!]

  4. #6 by Andreas Wieck on 27-January-2012 - 09:24

    Hey guys,

    I got the Tek492 (opt,1,2&3) and it is really a great instrument! Even today, it is hard for the competitors to do it better. Tek itself discontinued this frequency range, sad! Unfortunately, I do not have the -P version, so nor IEEE-bus. May I upgrade it or at least may I read the data by the analog outputs (X,Y, pen up/down) at the rear? Comes the parameter readout along with the trace? Does anybody of you do something similar? My email is andreas.wieck(at)rub.de

    Great to speak with specialists,

    yours Andreas

    • #7 by Ed on 27-January-2012 - 09:48

      read the data by the analog outputs (X,Y, pen up/down) at the rear?

      Probably the best place to get an answer will be the Yahoo TekScopes group at .

      I don’t have one, so I have no idea what’s available on the rear panel!

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