Tektronix 576 CRT Question - Page 1
Forum section: electronics / repair

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 06, 2023, 09:31:45 pm
content: I recently bought another 576 this time with blurry CRT dot and trace.Wondering if any of you CRT experts could help me with a question?Checked all voltages as per manual and all are within spec.My problem is the trace... I have to turn the Intensity knob 3/4 way up to get a dot and then I can focus but it only lasts for a few moments and goes out of focus again.I have worked the Invert, Zero and Cal buttons a number of times and I do get a more focused trace for a minute or so and then back to being unfocused.What I am wondering, is this a sign of dying CRT or are there resistors and Capacitors that need to be checked.As I said, I have 4000 volts on CRT and all other voltages are within spec. I have the manual and am looking through for ideas.I am even thinking of pulling out the switches and cleaning each one individually as these have been culprits in other units I have had.

author: Gyro
author_rank: Super Contributor
timestamp: March 06, 2023, 10:13:30 pm
content: From what you say, I think it's unlikely to be a CRT problem (yet). Note that I am not a 500 series expert though.If you have to turn the intensity way up to get a dot, it sounds as if the CRT grid bias is set wrong. It should be set so that, with the intensity control fully anticlockwise, the dot is just on the threshold of visibility (under dim room lighting) - that's on a scope anyway, it's presumably the same for a single dot on a curve tracer.You say that you have checked all voltages. I suspect that these don't include all of the taps on the high voltage resistor chain, including the intensity, focus and astig controls. This is a high resistance chain that would be affected by the loading of the meter. The resistors in these chains are notorious for drifting, potentially taking the focus control out of correction range (and maybe causing the intensity problem too). It would be worth checking all of the values. I don't know if the thermal drift is fast enough to cause what you are seeing (again, it doesn't sound like a CRT problem though).What you say about the Invert, zero and cal buttons introduces another possibility - noise pickup, but doesn't really tie in with the focus control observations.Hopefully a couple of possibilities anyway.

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 06, 2023, 10:58:05 pm
content: Thank you GyroI do know that there is a known problem with these switches that make the trace go wonky.I am on my 5th Tek 576 but usually can get through the trace problems by spraying the 3 switches with a little Deoxit D5.But so far no luck on this one.Think like you said, I will start checking HV board resistors.

author: james_s
author_rank: Super Contributor
timestamp: March 06, 2023, 11:59:04 pm
content: I've yet to see a CRT problem where it can be brought into focus and then drifts out. When CRTs get weak the most common symptom is a dim picture that gets blurry when the brightness is increased. This sounds more like a problem with the focus voltage or one of the grid voltages.

author: jonpaul
author_rank: Super Contributor
timestamp: March 07, 2023, 02:19:39 am
content: high value resistance in CRT, focus, ckt, are known as drift up, over the decades.Second suspect is HV transformers or multiplier.Occasionally the CRT socket / neck to pins connections may be oxidizedFinally the 576 CRT same,as 7603 and other Tektronix large display CRTs.Bon SoiréeJon

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 08, 2023, 03:03:50 pm
content: Looks like the problem is Intensity and Focus pots.I compared ohms reading with my working unit and the readings are way off.Looking for replacements now.var. potentiometers are Tektronix part # 311-0254-00 and 311-0397-01

author: james_s
author_rank: Super Contributor
timestamp: March 08, 2023, 05:40:45 pm
content: Are they constructed such that you can clean them out with some control cleaner?

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 08, 2023, 08:06:38 pm
content: I haven't pulled them out yet.Measured the 2M ohm pot with wires removed, I believe the pots are sealed, these are H.V. units with insulated shafts.I found new replacements, after removing wires and turning the 2M Ohm pot through its range, it read from 15 Ohm - 38M Ohm - 2.54 M Ohm.The good pot read from 1.0 Ohm smoothly to 1.82 M Ohm.

author: james_s
author_rank: Super Contributor
timestamp: March 08, 2023, 08:13:59 pm
content: It might just be dirty. Try squirting some control cleaner down the shaft, enough might get inside. Turn the pot back and forth a couple dozen times and then measure it again.

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 08, 2023, 09:40:23 pm
content: Well, I took the control out and took it apart. It was not sealed and it had cleaner in the pot.After drying it out and putting it back together it was the same jumped from 15 Ohm all the way to 38M Ohm and higher before reaching out of range at the end.The track inside was completely smooth.I think it best to replace.

author: james_s
author_rank: Super Contributor
timestamp: March 08, 2023, 10:07:23 pm
content: That's weird, pots in that sort of application don't usually wear out.

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 09, 2023, 12:38:59 pm
content: Someone tried to repair this unit before my purchase. ie, screws missing, high voltage cover missing.The switch I took apart was filled with liquid, I don't know what it was. Although it was extremely slippery and would not dry out.The main part of track was completely smooth with grainy track only on two ends.I may take the other one apart also and see.

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 09, 2023, 05:23:29 pm
content: OK James,I thought about what you said, So I took both switches back apart and soaked them in alcohol to remove whatever residue was on the tracks and now I have a focused trace again. The numbers are not perfect but it definitely made for a temporary workable solution.I will still replace those switches.Now to find any more problems.I set up my work bench again and in the past few days have started fixing some of my equipment.Thank for your help. I haven't been doing much repair for a couple of years now so it is always nice to present the problem.

author: james_s
author_rank: Super Contributor
timestamp: March 09, 2023, 06:24:23 pm
content: Sounds like maybe somebody used the wrong type of product in attempt to clean them.Does the image look good now with the pots cleaned?

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 11, 2023, 12:39:15 pm
content: Posting pic, after cleaning switches.

author: jonpaul
author_rank: Super Contributor
timestamp: March 11, 2023, 01:11:29 pm
content: very good, but seems like the intensity is rather highwe minimize the intensity as,needed.Burning is,easy on curve tracer!Jon

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 11, 2023, 03:57:38 pm
content: Yes, I ordered new switches, trace drops out while bringing intensity down.The resistance of these switches is not linear. Need to be replaced.

author: james_s
author_rank: Super Contributor
timestamp: March 11, 2023, 10:00:10 pm
content: It's remarkable that there does not appear to be any burn on that tube already, most of these curve tracers I've seen have a dark spot in the corner.

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 11, 2023, 10:36:24 pm
content: Yes and it is a very early unit.All IC's are 1968 or before.

author: jonpaul
author_rank: Super Contributor
timestamp: March 12, 2023, 03:18:06 am
content: Johnny10, congrats, 576, is a,fine,machineI.suspect that theliquid filled pot was,lubed with WD40,by a,novice.Check,all other posts and trimmers,for similar abuse...Fail points on 576 are,HV transformers, mult, switches and fiber optic readout lamps.Look for the rare front porches on,ebay or,ham radio Fleas être are several besides the stock100A pulsedprogrammableSCR currentTektronix numbers like172, 176The service manual is also very useful.Jon

author: Johnny10
author_rank: Frequent Contributor
timestamp: March 12, 2023, 12:20:55 pm
content: That's a good thought jonpaul.If wrong cleaner or lube in those switches possibly others also.

author: jonpaul
author_rank: Super Contributor
timestamp: March 12, 2023, 02:12:17 pm
content: Rebonjour Cher monsieur JohnnieAs power electronics designer, consultant and manufacturing in 1970s..1990s, we used both 577 and 576 Tektronix extensively. Tested 20,000 150V HV Zener diodes for production avionics power supply.Very lucky to find these curve tracers in 1980s, in excellent condition with test fixtures and front porch 100A pulsed, programmable and regular. At Silicon Valley ham radio Fleas and auctions.1. use 99% or 91 % isopropyl alcohol to clean test equipment knobs, panel's, PCBs, etc. Very low risk of damage2.please DO NOT use WD40, petroleum distillates, or powerful organic solvent like acetone, trichlorethene, etc severe damage to plastic, components etc.3. Many forum vintage equipment folk are afficanados of Craig Deoxit products for controls and pots.4. We have managed for decades to use ordinary TV tuner cleaner, control lube etc. from Miller Stephenson, GC, etc. Some tektronix equipment have sealed square blue Bourne pots, more difficult to clean.5. Additional common failures of 576 to beware of are the deteriorated plastic on certain knobs and controls, e.g. the small top row focus, intensity controls.Also, especially delicate are the main large control knobs for mode, R/ power/Voltage range, etc.Very easy to split knob at sets screw and nearly impossible to repair or to find replacement knobs. ROTATE LARGE CONTROLS SLOWLY, and CAREFULLY!6. Besides eevblog, best advice, parts, accessories on all Tektronix scopes and curve tracer at tekscopes2 and tekscopes @ groups.io, some veteran Tektronix engineering and technicians...Best information, manuals at W140 and Tekwiki.7. Test fixtures are rare and costly, but DIY versions and PCBs are on both eevblog and tekscopes2Our most used fixtures are TO3, TO247, TO220, diode 4 terminal, and TO92.8. The high voltage safety cover is useless and easy to break. Most simply defeat the interlock switch at lower left of front porch.Hope you enjoy my notes and tips...Just the ramblings of an old retired EE (1968!)Jon