💾 Archived View for soviet.circumlunar.space › insom › 2021-01-24-Oscilloscope-Things.gmi captured on 2023-05-24 at 18:18:47. Gemini links have been rewritten to link to archived content
⬅️ Previous capture (2021-12-03)
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I just picked up an analogue 30MHz scope from a colleague who is moving. It's in perfect working order and I'm very pleased with it.
It's great timing, because I intend to get back into amateur radio. I have the Advanced class license (the top one, here in Canada) which lets me built my own gear. I have some kits coming from QRP Labs, all the way from Turkey -- a Tayloe mixer style software defined radio which connects to the PC soundcard for final processing and a power amplifier I can feed some WSPR signals into from a software and hardware stack I already have (but which is quiet).
A recent impulse purchase of a magnetic-loop antenna has kicked this off again for me, after I gave a lot of my ham stuff away. It just seemed like a hobby I was past -- I don't actually enjoy talking to other operators and racking up QSOs, I like building radios -- but I suck at it. However enough time has passed and I've gotten better at building _audio_ frequency analogue circuitry, I think I am ready to give RF another try.
Of course, I actually already had an analogue scope. Two. I won the cheapest CRT scope I could find on eBay back in 2019 and they sent me the wrong one. I contacted them about the mistake and they sent me the right one, and let me keep the one they already sent. (CRT scopes are heavy, didn't make sense to courier it back to Washington state).
They are in the Tektronix 5110 range -- 2MHz entry level mainframe scopes. Mainframe means that it's a little like a modular synth: the scope is actually reasonably dumb, mainly consisting of the power supply, CRT and deflection amplifiers. The real smarts are provided by up to three plugins you slide into the front of the chassis.
Tektronix 5110 TekWiki Page on the Big Web
The plugins in slots 1 and 2 are designed to amplify or attenuate signals for the Y axis. The plugin in slot 3 is designed to provide a time-base (basically, to move the beam from left to right a certain number of times per second). You can use one of the vertical plugins in slot 3 to make an X-Y display, which is actually the main reason I wanted the scope in the first place.
I have various plugins that came with the two scopes, but they range from "suspect" to "definitely faulty". At their very core, though, the vertical amplifier is basically an op-amp with a +/- 30V range, and the time base is a variable rate sawtooth wave generator (with a comparator for handling the trigger function). X and Y position controls are literally just adding or removing DC bias.
So, I've decided it'd be fun to build my own plugins, based off of modern ICs. I've spent the evening going over the manuals and tracing the function of every pin on the connectors. (Fun surprise: pin A1 is 200V! This is just used for lighting up an optional neon light, as this scope comes from the era before LEDs)
Before going and fully designing a board, I wanted to be able to trace one of the amplifiers that I have while it's in use. I needed an extention board. Needless to say, Tek ones are unobtanium on a 40-something-year old device that wasn't even _that_ popular to begin with.
I measured the parts I have an searched on Digikey for 56 connector 0.156" pitch edge card sockets. There was exactly one match. The minimum order was 480 (~$8000) and the lead time was 13 weeks. Ouch.
As a last ditch, I tried putting 56 connector 0.156 into an eBay search and discovered that these sockets are the same ones used by the popular JAMMA arcade machines. There's an active hobbyist scene around these and connectors can be had for $4. I have some coming from Markham, Ontario, they should arrive next week.
This project is going to take a little time, and in the end, it'll be pointless, but actually I think I am okay with that. I've already had a really enjoyable evening trying to puzzle out the meaning of all of the pins on the cards. Hopefully the future evenings I spend on this will be as fun.