Tektronix 455 bad crt ? Update: Video uploaded. - Page 1
Forum section: electronics / repair

author: lame_turtle
author_rank: Contributor
timestamp: October 24, 2020, 10:18:49 am
content: Hello,I turned on my lovely tek today only to find a fat trace, I tried to readjust focus and intensity but when turning the intensity the screen goes from dim to bright then dim again then flickering bright. Is this a bad crt ?This unit has been fully recapped and been given alot of love. Checked all the voltage rails the are spot on and no ripple .Please any info on this problem is greately appreciated.I know this is hard to explain so I took a video.https://youtu.be/0vVEwHMaEGsKind regardsLM

author: wn1fju
author_rank: Frequent Contributor
timestamp: October 24, 2020, 11:08:56 am
content: Dirty intensity pot? Spray some contact cleaner into it. Also check high meg resistors in focus circuit.

author: lame_turtle
author_rank: Contributor
timestamp: October 24, 2020, 11:13:29 am
content: I did spray contact cleaner in there and measured the pot. Measuring fine. There is a 1200pf 15KV (manual states 7KV) that I haven't replaced in the focus circuit but it measures good no leakage . I'll be looking for resistors.

author: lame_turtle
author_rank: Contributor
timestamp: October 24, 2020, 12:35:25 pm
content: Ok I can find only one resistor that doesn't measure good . It's r563d should be 20Mohms and measures 16Mohms. It is a resistor I haven't seen before, it is a blue bar that has 5 pins. So a resistor with four sections ? I have no idea with what I could replace it with.Could this be it ?I'm also looking in Q544 but I'll have to disassemble a bit more to measure it.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 24, 2020, 12:53:57 pm
content: I suppose you have already tried the focus adjustment.Here is a quote from another group that may be pertinent, even though it is about another Tek Scope:I'd run the scope by an ex-Tek CRT inspector, and she thought the symptomslooked like "low cathode". I interpreted this to mean that, although emissionis still good, it is coming predominantly from parts of the cathode structurethat are not optimum for a nicely focused beam. Another CRT relatedpossibility mentioned at the time was some kind of mechanical damage that affected theinternal alignment of the elements. Whatever the root cause, it turned out theproblem was indeed with the CRT itself, and not the electronics. A new CRT(actually for a 465M) from ebay, a new blue filter from Walter at SphereResearch, and a recal, and it seems to be as good as rain again. With the new tube,the traces are nicely crisp. Not as good as I remember a 485 mind you, butquite nice, and certainly not worse than any other common Tek scope of the 80'sand 90's that I'm familiar with.

author: lame_turtle
author_rank: Contributor
timestamp: October 24, 2020, 01:01:41 pm
content: Yes I did adjust the focus.. even recalibrated all the adjustments that have to do with crt and focus. I'm in great fear it is just the crt.

author: mzacharias
author_rank: Frequent Contributor
timestamp: October 24, 2020, 03:19:10 pm
content: I don't have a great deal of experience with CRT related problems but this sounds like a bad (high value) resistor to me, in the focus circuit itself.

author: David Hess
author_rank: Super Contributor
timestamp: October 25, 2020, 11:01:08 pm
content: Failure of the focus resistor chain is a definite possibility which should be checked.

author: lame_turtle
author_rank: Contributor
timestamp: October 26, 2020, 01:52:53 am
content: What would be an adequate replacement for the focus resistor chain ?Edit: sorry for the silly question. Searched a bit for it and it seems that I can replace it with resistors.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: October 26, 2020, 03:02:39 am
content: HiHigh value resistors go high in value with time and applied voltage, standard problem with the focus and brilliance circuitry on a scope. You cannot use standard resistors as they do not like much more than 250VDC across them.Vishay make suitable resistors in their VR25 range, other do also but I find Vishay easier to purchase.George G6HIG

author: George Edmonds
author_rank: Frequent Contributor
timestamp: October 26, 2020, 03:40:30 am
content: HiDid you measure R563D (20.5M) in circuit, if you did it will be lower resistance than expected.A good test, with the correct instrument, would be to check the 2KV supply on the test point. You cannot use a DMM for this test as its 10M input R is too low.George G6HIG

author: George Edmonds
author_rank: Frequent Contributor
timestamp: October 26, 2020, 03:51:05 am
content: HiForgot to say.Qservice in your country is a major supplier of Tektronix spares, they may have both a spare intensity pot and a thick film resistorGeorge G6HIG

author: lame_turtle
author_rank: Contributor
timestamp: October 26, 2020, 02:27:15 pm
content: Hi,Thank you so much for the answers. Unfortunately I do not have the equipment to measure the 2KV supply. Well actually if a multimeter with an HV probe wouldn't do it, I'm not even sure what test gear I would need. Yes I did measure r563d out of circuit and resistance is lower than expected. Is this a precision resistor or could it be that this was the value from the beginning ?I did find the added the vishay resistors to mouser but I can't find an adequate replacement for the 1200pf 15KV capacitor (7.5 KV would as per the service manual) .I checked the intensity pot on the scope and it is great. ( I should have done this before spraying deoxit since now the great feel of the knob is gone. I may just open it and add grease if I fix the scope)I also found out today that I can replicate the flickering even by just turning the focus pot.Kind Regards

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 26, 2020, 02:44:50 pm
content: I hesitated to answer to your topic because it is difficult to give a clear and precise answer.I give you my personal opinion on your problem, with all reservations.1) I don't think this is a CRT problem2) the suggestion to check and replace the high voltage resistors instead is good3) Personally, I would also check the blanking circuit with another oscilloscope to see if there are any variations in amplitudes which correspond to the brightness fault

author: David Hess
author_rank: Super Contributor
timestamp: October 26, 2020, 11:55:41 pm
content: Quote from: lame_turtle on October 26, 2020, 01:52:53 amTektronix replaced the carbon composition resistors used in the high voltage focus resistor chain with high voltage metal film resistors like the Vishay VR25 and VR37 series.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: October 27, 2020, 10:43:51 am
content: HiI tried to find out what Tektronix used to measure the 2KV supply, this information is not given in the manual, at least not as I could find.If you have access to a moving coil meter that has a 2.5 to 3KV range such as some AVO 8's that should be adequate.All the schematics that I can find list R563D as 20.5M not 20M.If R563D is down to 16M you have your problem, however, you may have a measurement error as most hand held DMM's are getting inaccurate by this resistance.I can see no reason for the 1200pf capacitor needing to be rated at much over 2.5KV.George G6HIG

author: lame_turtle
author_rank: Contributor
timestamp: October 27, 2020, 06:49:50 pm
content: Thank you,I found a honor te-1210 analog meter locally for a few bucks and I will be getting it tomorrow. There is no documentation or actually anything online for this meter but says 3000VDC on it so it should be good.I'm really sorry about the 20.5mohm resistor it was me recalling it incorrectly from memory. I checked my schematics and it is indeed 20.5mohm . And you are of course correct . I checked the resistor with a DE-5000 I just bought and it is spot on correct. I should have this lcr meter earlier .Since the capacitor is fine at 2.5KV (I checked again the schematics no apparent reason why not as you say) I have a good vishay one to put there. I'll do it right now and report back.

author: lame_turtle
author_rank: Contributor
timestamp: October 27, 2020, 07:53:58 pm
content: Focus capacitor replaced and made no difference.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 27, 2020, 08:16:26 pm
content: Checkout the unblanking pulses....

author: lame_turtle
author_rank: Contributor
timestamp: October 27, 2020, 09:16:32 pm
content: Ok, either I'm measuring unblanking pulses completely wrong or my unblanking pulses are just a sinewave ??I'm measuring at r506 as per the service manual.Update: was measuring wrong. Unblanking pulses look fine as of amplitude but terribly noisy for some reason.Btw, now I'm measuring 150mV of ripple on the 32V line, is this a problem ?

author: David Hess
author_rank: Super Contributor
timestamp: October 29, 2020, 02:56:51 am
content: Quote from: George Edmonds on October 27, 2020, 10:43:51 amThe cathode voltage is tightly regulated because it directly affects deflection sensitivity so there are no special requirements for measuring it as its impedance is low. I use a Fluke 80K-6 probe which has a 75 megohm input resistance.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: October 29, 2020, 06:14:09 am
content: HiThere is a fundamental point that is being missed here. The OP does not have vast amounts of test kit available and purchasing something like the Fluke 80K-6 just to make one measurement is not a viable option.A cheap 20K Ohm/Volt moving coil meter has on its 3KV range an input resistance of 60 Meg Ohms, more than adequate to check the stability of the 2KV supply, please note in this case the stability of the supply when the CRT trace intensity is increased that is important, not its absolute voltage.A simple check on the CRT blanking circuits is to display a low frequency sine wave and then look for the flyback trace at normal screen intensity, if no flyback trace is seen the CRT blanking is working correctly.If the trace intensity is increased the flyback trace may become visible, but at a lower intensity than the main trace.The KISS principle should apply at all times.George G6HIG

author: lame_turtle
author_rank: Contributor
timestamp: October 29, 2020, 12:54:46 pm
content: George, I have to say a huge thank you for the vast amount of info you've provided. I have the analog meter rated at 3KV now, bought only for a few bucks and is working good. I will test the 2KV when I'm back home and report back.I have limited test equipment but my rigol 1054z has been proven really useful (although terribly slow) and I try to make do with what I have .

author: lame_turtle
author_rank: Contributor
timestamp: October 30, 2020, 01:48:50 pm
content: I did the measurement. The actual number shown on the meter is around 2300V but I guess that is not to be trusted. The important part is that the voltage is stable enough when altering the intensity, moreover if I find a spot on intensity where the flicker begins, no fluctuations are observed on the voltage measurement.On the other hand day by day the problem is actually getting really worse. Now only two thirds of the trace are illuminated and the rest is in half brightness.I think I have to investigate unblanking more but I think that the unblanking problem wouldn't explain the bad focus problem.I remembered that this problem started while I was modulating the z axis (well within limits of course). On the schematics I can see this is where internal z axis connects too, so the problem might be there ?I have to reassemble the scope and see if the flicker stops when the tracefinder is engaged, since I see a whole lot of this circuitry is bypassed with this function.