Tektronix 422 display issue - Page 1
Forum section: electronics / testgear

author: mstck
author_rank: Contributor
timestamp: September 03, 2014, 12:43:17 pm
content: Hi there, I have seen a few threads on the Tek 422 in this forum a determined that there is some expertise here that might be able to assist me. I recently acquired a 422 unit which I am restoring. As stated in earlier threads, this is definitely not "state of the art", but I find it "cute" and would like to fully restore it. I have the basic unit working, and I can see a stable trace but it is very dim. I am forced to turn the intensity knob fully clockwise to see it. I do not have the means to measure the very high voltages on the CRT and was wondering if anyone here has had a similar problem and how they solved it. If not, then are they any suggestions as to where I might concentrate my diagnostic efforts to identify and hopefully resolve this problem.

author: tautech
author_rank: Super Contributor
timestamp: September 03, 2014, 12:51:23 pm
content: Grab the first link in the Repair board written by Tek for troubleshooting CRO's.A PSU spec check is sort of mandatory with old CRO's.Links to manuals in the Repair board too.

author: PaulAm
author_rank: Frequent Contributor
timestamp: September 04, 2014, 12:01:11 am
content: Most Tek scopes have a CRT bias adjustment. Unfortunately the 422 doesn't.On page 6-9 of the service manual is a procedure for setting the unblanking level. You can try that.Also, measure all of the voltages. The CRT voltage is -1385 +/- 69.5V You'll need a HV probe to read that one.Check the values of the resistors and pots in the intensity/focus string. They can drift out of tolerance and not allow the intensity to be turned up.It's possible the accelerating HV supply is not putting out +4900V. That's a tough one to measure.If you don't have it, be sure to download a manual. Free ones are available on the net with readable schematics and parts lists.It's also possible the CRT can be at end of life. If it's not too beat up, it's probably one of the other items though. CRTs show up once in a while, but they're pretty scarce.Edit: I should have added this is a very nice basic 2 channel scope that's relatively simple since it lacks delayed sweep (not a big deal though). Tek is very conservative with their bandwidth ratings. The 422 is rated to 15 MHz, but it's quite usable to 25. The front end nuvistors, if used in your model, can be replaced with MPF102 FETs with no circuit changes. There's a post somewhere on the eevblog that details it. I kept the nuvistors in mine, they're pretty neat old tech.The available manual has the AC/DC power supply. If you need the AC only power supply schematic, let me know and I'll post it.

author: xwarp
author_rank: Frequent Contributor
timestamp: September 04, 2014, 12:59:25 am
content: I have 2 of these 422's and they are very convenient for basic stuff.Let me know if there is anything I can possibly help with.

author: mstck
author_rank: Contributor
timestamp: September 04, 2014, 04:30:09 pm
content: Thanks very much for the responses. I have the service manual which i downloaded from the web. I also found the manual for the AC supply. Thanks for the offer. I do not have a high voltage probe. I will make some checks on the components surrounding the intensity control etc. as suggested. I have checked the other voltages on the test points and everything is within spec.The unit does function and the trigger is quite stable. I can display the calibrator signal on both channels.I just hope that it is not an exhausted crt. I know a replacement would be almost impossible to find.Once I have done the additional checks I will post the results. Thanks again.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: September 04, 2014, 07:30:21 pm
content: Why not make one?http://www.instructables.com/id/DIY-High-Voltage-Probe-for-digital-multimeters/?ALLSTEPS

author: Macbeth
author_rank: Super Contributor
timestamp: September 04, 2014, 08:28:07 pm
content: Back in the old days of TV repair we could use something nicknamed a "bopper" or "reactivator" to rejuvenate a few more years out of a fading and worn out CRT. I never actually got to use one, the shop I worked at would just buy new CRTs as they had massively fell in price by the early 90's.But I can't see why the same principle wouldn't work on an Oscilloscope CRT. Of course make sure the oscilloscope PSU is providing the correct heater and EHT voltages before you go ahead "bopping" the tube! Commercial EHT probes can be expensive, but you can make a cheap one using lots of high megaohm resistors. Just be careful and cover in insulation!

author: mstck
author_rank: Contributor
timestamp: September 05, 2014, 01:25:38 am
content: Hi all. Great info. Adding a HV probe to the workshop is not a bad idea. I will likely have to source the resistors from one of the electronic component suppliers since the source cited is not readily available here. Thanks, I appreciate all the help so far.

author: xwarp
author_rank: Frequent Contributor
timestamp: September 05, 2014, 02:45:56 am
content: Is this scope the only one you have?

author: MarkL
author_rank: Unknown
timestamp: September 05, 2014, 02:11:14 pm
content: Quote from: SoundTech-LG on September 04, 2014, 07:30:21 pmI wouldn't mess around trying to make your own high voltage probe. Your equipment and possibly yourself are at risk.And in particular, the referenced design appears to have a high voltage resistor chain folded up inside the probe handle. This puts resistors with a high voltage delta in close proximity to each other and provides an opportunity for arcing through any weak spots in the heat shrink tubing. And once you get an arc going around some of the resistors, you may have well exceeded the voltage limits for the remaining resistors and they go too.The claim is that this is good to 20kV. Where did that come from?Really, buy a proper probe if you're going to measure high voltages.

author: Macbeth
author_rank: Super Contributor
timestamp: September 05, 2014, 02:20:50 pm
content: QuoteThe DIY probe I remember back in the early 90's was for measuring 40kV (Colour CRTs would go up to around 35kV). It had big beefy hi meg hi voltage resistors (much like the ones in Daves ECG teardown) all in a single long chain. I agree that folding over thing is cause for much concern.However, I really wouldn't worry about my old probe for EHT stuff. I mean I've been zapped directly from EHT CRT's to know what it feels like anyway. I've survived - even the incident where I had to hold onto a heavy CRT I took out of a TV for dear life while getting zapped in order to not drop it on the floor and let it implode instead.

author: MarkL
author_rank: Unknown
timestamp: September 05, 2014, 02:36:24 pm
content: Quote from: Macbeth on September 04, 2014, 08:28:07 pmHere's a useful page that describes the rejuvenation process:http://www.ke5fx.com/crt.htmlIt provides specific current levels and timing. I was able to restore a very dim Tektronix 2465 CRT using this method.But only consider rejuvenation as a last resort once you're sure it's the CRT by checking all other voltage levels and signals supplying the CRT as others have described. It's possible to permanently damage it.

author: tautech
author_rank: Super Contributor
timestamp: September 06, 2014, 02:48:52 am
content: Quote from: PaulAm on September 04, 2014, 12:01:11 amHave a hunt for an old AVO meter, many do 2.5-3KV and can be cheap.Quote+1QuoteQuadruplers normally work or they don't. If other parts of the circuit are to spec and you still have a problem, then investigate. Easier to work around it.Quote from: MarkL on September 05, 2014, 02:36:24 pm+1

author: mstck
author_rank: Contributor
timestamp: September 07, 2014, 12:59:22 am
content: Hi All. I decided to order a HV probe for my Fluke. Awaiting delivery. I too was a bit uncomfortable with the DIY design after I read the article a second time.xwarpQuoteI also recently acquired a Rigol DS 1102E. I have a particular fondness for the older analogue scopes and would like to "collect" a few more .

author: xwarp
author_rank: Frequent Contributor
timestamp: September 07, 2014, 01:44:35 pm
content: Quote from: mstck on September 07, 2014, 12:59:22 amAre you sure about that?.........LOL