Another visit to the little Tektronix 212

[Suchatavi]

I've been looking at posts on the old Tek 212. My intention was to remove the long dead battery packs to guard against leaking but now it looks like I may have a repair to do. First, I need to determine if the dead batteries or removing the batteries could result in the attached wonky waveform (no input)? Are the batteries performing a filtering function under normal circumstances? Could they be replaced with some electrolytics if I don't intend to use it battery powered? If not the batteries, do you agree that a problem exists in both the horizontal and vertical circuits or before? Traces don't normally fold back on themselves! I did get my hands on the service manual, and I do intend to DEox all of the contacts. I'm looking for a knowledgeable hint from you guys for the best way to start. Perhaps someone has seen this situation before? TIA Rick

[edavid]

It looks like the CRT was damaged when the scope was dropped 

[Suchatavi]

Oh snap that's a shame. Internally it's like new.

[inse]

There is no filter cap in the snippet you provided.
Probably the batteries serve as filter caps.
Supply the scope from a lab psu at the battery connector.

[Andy Watson]

I cannot see any significant mains filtering apart from the batteries - I would find a suitable DC power supply to check it with.

Also, note that the input, including the battery, is not isolated from the mains.

[inse]

On second thought, this might not be the entire power supply of the scope.
The battery is charged via a capacitive dropper which hardly can supply the current required to run the scope.

[oz2cpu]

wow you got the 212, those tiny ultra rare tek scopes are fantastic, i feel you pain mate,
I got a 213 with a blown input dual fet, no suggestions yet about a replacement..
maybe i can chear you up with a little video of mine :
https://youtu.be/l8WaflwgxMs
it is ofcourse click bait, so only click responsible.

[wasedadoc]

It looks like the CRT was damaged when the scope was dropped 

Doesn't look like a damaged CRT to me. My first suspect would be power rail(s).

[Andy Watson]

Here's a little bit more of the power supply circuit. The only significant capacitance I can see is the 100uF on the centre-tap of the transformer. I would definitely find another source of power, or batteries before condeming it.

[inse]

I am still puzzeled by the power supply.
The two capacitive droppers C210 and C212 in parallel deliver 225mA at 115V/60Hz.
This may be sufficient to charge the batteries, but not to operate a scope with 2.7W of available power?!?

[med6753]

The batteries need to be installed and functional in order for the PSU to operate properly. Chances are the original NiCd cells look like this:



I decided to replace them with 6V NiMH cells and surprisingly they work fine without any other circuit modifications. Check connector P3 for +69V, +5.6V, and -5.6V. If OK then your issue lies elsewhere. And unfortunately to me at first glance it appears that the CRT might indeed be damaged. Analysis of the waveforms shown in the manual should prove/disprove that once you insure the PSU is OK.


 

And yes, the primary side of the PSU is not isolated from the mains so be cautious of that. Re-assembly of one of these mini scopes can be tricky and frustrating. I've talked with former Tek service techs and they hated working on them.