A couple of months ago, I bought an adorable little 1989 Tektronix 222 miniature CRT portable oscilloscope locally on Facebook marketplace – I love old electronics test gear, and the 222 is particularly desirable among collectors due to its cuteness.
The Tektronix 222 is a portable battery-powered 10 MHz dual-channel CRT oscilloscope with digital storage (8-bit resolution, 512 samples) and independently isolated inputs.
The 222, as well as similar oscilloscopes such as the 224, featured an 8V 2100mAh sealed lead-acid (SLA) battery pack – these are unfortunately long since dead, so it’s common for 222 owners to replace this pack with an alternative.
This scope was no exception: the previous owner had built a replacement pack using seven 1.2V AA NiCd cells, with a nominal voltage of 8.4Vdc – these were just float charged directly from the 9.5V SLA charge voltage that the scope puts onto the battery pack when it is powered from a 12Vdc centre-positive PSU via a 2.5mm DC jack on the back of the unit.
This had worked well for them, but the NiCd cells were getting old and didn’t hold much charge. I could have swapped them directly for a set of more modern NiMH cells, which would have better energy density, but I decided to do something a bit fancier.
I’d read about a very nice replacement pack designed by Kitsune-Denshi, which is described in this video – this uses three 18650 lithium-ion cells in parallel, each with its own charge control and protection circuits to charge the cell from the 9.5Vdc SLA charge supply when a PSU is connected, and a boost converter to boost the cell output to 8Vdc to power the scope when a PSU is not connected.
A schematic, bill of materials, and PCB files are available on their website – I decided to buy a pre-made PCB on eBay rather than get my own PCBs manufactured as I only needed one, and I ordered the other parts including three Samsung 3.7V 2600mAh 18650 button-top cells and all other components, which I’ve got a Digikey bill of materials for – I’d recommend ordering spares for the smaller passive components, as they’re very easy to lose (I know from personal experience, a couple got sucked into my fume extractor).
If you’re looking to build one of these up yourself, be warned: it’s quite tricky and requires fine soldering skills and decent equipment. There’s a lot of small and fine pitch components, and you’ve got to be extra careful to ensure that these are placed in the correct location with the correct orientation, and without any shorts.
In total, it took me a couple of hours to do from start to finish – I started with U1 as this has a ground pad underneath, which I placed with a hot air station, then I worked my way through all lowest-profile components up to the highest-profile components, finishing with the battery holder. I then trimmed down and attached the original power cable.
Once I’d built the pack up, I tested it out with the scope – I charged the batteries off the DC PSU for a little while then disconnected the PSU and tried to power up from the new battery pack, and the scope seemed to work OK.
The scope also worked on the PSU while the batteries were charging, but I noticed quite a lot of noise on the inputs (probably interference from the SMPS in the PSU).
Now that the pack was built and tested, I wanted to put it in a case – I 3D printed a nice enclosure for it and built some light pipes for the status LEDs out of some cut-down hot-glue sticks wrapped in heatshrink, which works quite well.
I tested the status LEDs, and they seemed to look pretty good in the case.
With the pack complete, the oscilloscope is ready to go whenever now and I don’t have to worry about runtime – I keep it on display in my home office. It even has an RS-232 output for data logging and control, and there are serial libraries available to use this functionality on a modern PC, including drawing graphs!
Does this scope have X/Y mode?