I’ve been looking into old Tektronix osciloscopes and related gear lately, and I thought I should write-up some of what I learned.

Last year, I posted a few installments in my saga of figuring out what to buy for my first osciloscope. I ended up with a Rigol DS1074Z, and while I haven’t gotten a lot of use out of it, yet, when I have used it, its saved me a lot of troubleshooting time.

Recently though, I’ve been looking for ways to address some of the limitations of my scope. In particular, I’d like to be able to do low-noise differential measurements on one or more channels. In part, this allows more flexibility in using all my scope channels to look at power supply circuits. It can also be useful for looking at power supply output noise and ripple.

One approach is to use the math function of the oscilloscope to calculate a differential between two of the input channels. This has its uses, but suffers from slow-update speeds and the fact that some of the signals I’m looking for are already at the limit of the DS1074z’s resolution.

Another approach is external differential probes. Unfortunately, these are expensive. New they start at $300 or so. Used are a little better, starting at $100, but most seem targeted at high-voltage rather than high-sensitivity use.

This brings me to the Tektronix gear. I’m less interested in the 7000 and 5000 series scopes themselves, than in all the various modular “plug-ins” (particularly high-sensitivity differential amplifiers) Tektronix developed for them. Tektronix also sold a line of stand-alone chassis called the TM500 and TM5000 series, and an accompanying line of plug-in modules.

Now, the first thing you need to know is something I was lucky to figure out before buying anything on ebay, which is that, while the plug-ins for the 7000 series, the 5000 series, and the TM500 and TM5000 series all appear to have superficially similar form-factors, they are incompatible. You can’t use a module intended for a scope in the stand-alone TM500 or TM5000, or vice-versa. Nor can you use a module for a 5000 series scope in a 7000 scope, or vice-versa. There are other important distinctions too.

Within the 5000-series of scopes and modules, there is a distinction between “slow” (~2MHz bandwidth) and “fast” (50MHz bandwidth). You can use slow modules in fast scopes, but you can’t use fast modules in slow scopes.

Within the 7000-series, which cover an even wider range of bandwidths from 25MHz all the way up to 1GHz, most scopes are compatible with most plug-ins, according to Tektronix.

For the stand-alone mainframes, modules for the TM500 will work in the TM5000, but the reverse isn’t always (usually?) true.

My inclination is to get a 4-slot stand-alone chassis like the TM504 to save space and minimize shipping costs. Unfortunately, it seems that the AM502 differential amplifier module is rather rare and relatively expensive. There is just one on eBay at the moment and only a few in the available history of past sales, and the prices seem to start at $100.

Meanwhile, there are multiple examples of the equivalent 7A22 or 5A22N modules for the 7000 and 5000 series scopes, with prices starting below $50. The necessary scope and chassis can be had for as little as $100 or so more, about 2x what a TM500 chassis might go for, with the downside of added shipping costs and the (possible) upside of a second scope. Moreover, there are apparently pass-thru outputs so would still have the option of using any modules I acquire with my existing digital scope. I’m also interested in other modules, like function generators.

The smartest thing, at this point, would be to put this project on hold and finish up the half-dozen Keithley 197A multimeters I’m in the process of restoring and repairing, or the Power Designs TP340A I’m in the middle of fixing (destroying?). If wisdom prevails, I’ll have this post to remind me of what I’ve learned, should I ever come back to the idea of buying some old Tek modules.

To that end, here are some of the resources I found useful in researching this:

• Christian Weigel’s TM500 module guide and 5000-series scope guide
• TekWiki
• KB7RGG’s list of 7000-series Plug-Ins
• BarryTech’s Compilation of Tek Information