Built up a little something...this is a test bench build for Eurorack.
ModularGrid Rack
This uses a powered 4ms Pod 48X cab. From left to right:

A CG Products PCV3 and a Mazzatron Testy allows you to test ANY synth patchcable...1/4", banana, and 3.5mm.
The Animodule TrAniModule gives you a 1 watt output amplifier and speaker to check audio outs.
The Joranalogue TEST 3 lets you check individual module current draws and other important DC level checks.
A 2hp DC gives you three possible fixed voltages...very useful for checking CV scaling and tracking.
The big draw here, of course, is Jones's O'Tool's latest revision, offering frequency measurement, an o-scope, voltmeter, and a pile of other test functions.
And last, a Metro Modular Test Eagle Option 10 gives you four dual bananas for connecting external test gear, with appropriate impedance and level settings for typical devices of that sort.

This isn't a synthesizer, of course...but if you have a sizable Eurorack (or other) modular rig, a little skiff like this with these sorts of modules definitely gives you 95% of the key test functions you need to keep that synth operating to spec.

I've been super impressed by my O'Tool Plus so far, I'll have to check these other modules out. Thanks @Lugia.

Great idea. I've recently found myself in need of several of these functions. Will put the O'Tool and Test 3 on the list.

Hi Lugia,

That's a very interesting idea indeed. Thank you very much for sharing this with us :-) Kind regards, Garfield.

For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads

Yeah...I don't recommend putting diagnostic gear into a functional synth build...but if you DO have a large setup, having a little "test pod" like this is damned useful. For one thing, it's small enough that you can move it around to where it's needed. And the O'Tool's functions are also pretty useful for checking calibration, etc on most ANY musical instrument, so this is set up so that it'll take any typical synth patchcable and you can still set up the 3.5mms on the pod to do any sort of test you'd normally do with a NON-modular synth. Plus, if you want to "relic" some calibration settings on a synth so that you've got a bit more of an "aged component" sound, you WILL need several instruments such as a frequency counter, voltmeter, o-scope and so on...or, just this pod.

Pretty cool potential addition to your test bench:

Nice find @farkas