Need help at troubleshooting my Tektronix 222 A - Page 1
Forum section: electronics / repair

author: 128er
author_rank: Regular Contributor
timestamp: December 21, 2014, 08:21:00 pm
content: Hi!I recently bought a broken Tektronix 222A mini scope. You turn it on and it beeps. But the CRT stays dark. At a cold start it draws about 583 mA, within 30 seconds it settles down to 552 mA at 12V. No battery is installed. It's powerd via the external power connector. I use the Tek 222 service manual, couldn't find anything useful for the 222 A model.Link to the manual:http://www.ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_222The battery charger SMPS seems to work fine (sits on the main board). It delivers the 9,8 V, as mentioned in the manual. The CPU board seems to work, too. I can measure the X/Y signals, also the Z signal is there.The fault at my unit, I think, is very obvious at the A4-Power Supply Board.The pages 26 and 70 in the manual are quite useful. Page 26 shows a block diagram of the power supply section and a little bit about the theory of operation. On page 70 you can find the interconnect diagram of all boards.On the A4 PSU/CRT board is a TL594CD PWM controller. It gets the DISPLYE signal that is required for the IC to start working. Could not figure out if the feedback circuit is working correctly (But I assume this is a very important thing) . The FET's Q401 and Q402 are switching, but they are really hot (~90°C).There is nearly nothing on the output rails Vcc, Vee and AVcc.Here are the measured values - with respect to GND - on the CRT connector (with CRT removed):1 - Grid: -269V2 - Filament: -233V3 - Cathode/Filament: -246V4 - Focus: -162VMeasured resistance at the CRT filament pins: 6,2 OhmsAnyone here who had a similiar problem, or can give me a good hint?

author: tautech
author_rank: Super Contributor
timestamp: December 21, 2014, 08:57:56 pm
content: There was a very recent thread on the 222:https://www.eevblog.com/forum/testgear/tek-222-portable-scope-schematics/msg527059/#msg527059It was for display problems too.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 22, 2014, 01:55:54 pm
content: The only source I could find for manuals with full schematics for the 222A was Dave at artek:http://artekmanuals.com/manuals/tektronix-manuals/The voltages on the cathode and grid are too low - they should around -1600V.

author: c4757p
author_rank: Super Contributor
timestamp: December 22, 2014, 02:52:11 pm
content: Quote from: Andy Watson on December 22, 2014, 01:55:54 pmAre you sure? That's close to what they should be for a full-size scope!

author: Andy Watson
author_rank: Super Contributor
timestamp: December 22, 2014, 03:10:30 pm
content: Quote from: c4757p on December 22, 2014, 02:52:11 pmYep! Beyond the range of all my DVMs - I had to resort to the trusty AVO 8 to measure it.The manual says:Quote

author: 128er
author_rank: Regular Contributor
timestamp: December 22, 2014, 03:20:39 pm
content: Andy was faster . . .Yes, about -1550V. Even my differential probes are not capable of that voltage. My P5205 is only capable of 1300V.But I think my first step will be, to replace the TL594. It's reference output is only about 4,09V. Should be stable 5V with 1% tolerance.I don't like these "one part orders" . . . .