Tek 'scope CRTs - how do they work? - Page 1
Forum section: electronics / repair

author: tggzzz
author_rank: Super Contributor
timestamp: February 13, 2016, 05:07:34 pm
content: I've found a Tek 465 scope which seems to be working well except for one feature: the intensity control is ineffective and the trace is so bright you could use the CRT as a torch.My understanding, such as it is, is that the fault is likely to be in either HV DC restorer circuit or inside the CRT itself. My feeling, based on somewhat indirect measurements of 2.5kV circuits, is that the DC restorer is working OK and that the fault is with the CRT's grid - but my lack of experience with CRTs makes me hesitate to jump to that conclusion.Am I right in thinking:the beam's intensity is controlled by the cathode-grid voltage?the more negative the grid relative to the cathode, the less intense the beam? I.e. the negative grid voltage repels the electrons and prevents them being accelerated by the anode?there is a "cutoff" voltage which will prevent any electrons hitting the phosphor?If so,what is a likely value for that cutoff voltage?Not having a HV differential probe, I can't measure the grid-cathode voltage (Vgk) directly, but have to resort to indirect means. Measured values:cathode voltage -2450V, which is correct, and the focus voltage is 20% too lowwith the CRT disconnected, the voltages on the focus plate is as expected and the cathode voltage is slightly higher. My guess is that difference is due to a high electron beamthe low-voltage blanking signal looks sensible: 0Vppwith intensity control on minimum intensity, and up to 60Vppas the intensity (control) increasesVgk= -123Vmeanwith minimum intensity.Is that more than the expected cutoff voltage?Vgk= -68Vmeanwith maximum intensityGiven the duty cycle of the low voltage blanking waveform, -68Vmeanis what I would expect if the DC restorer capacitors and resistors are working correctly.So, is it likely there is a fault inside the CRT?What other tests could I make?Might there be any way of rectifying a fault in the CRT?Thanks for your thoughts and advice.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 13, 2016, 05:38:53 pm
content: http://www.ko4bb.com/manuals/download.php?file=Tektronix/Tektronix_-_465_Oscilloscope/Tektronix-465_Service_Manual/Tek_465_Service_Section-4.pdf

author: tggzzz
author_rank: Super Contributor
timestamp: February 13, 2016, 05:46:02 pm
content: Quote from: PA0PBZ on February 13, 2016, 05:38:53 pmThank you, but I read that and it doesn't contain the answers to my questions.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: February 13, 2016, 07:03:26 pm
content: The following book has the answers you seek.....http://sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

author: tggzzz
author_rank: Super Contributor
timestamp: February 13, 2016, 07:11:30 pm
content: Quote from: Addicted2AnalogTek on February 13, 2016, 07:03:26 pmThanks, yes.IfI understand it correctly, "most" Tek scopes will cutoff with Vkgmore negative than -65V. I see -123V and it is far from cutoff! That's one of the things that leads me to believe there is a tube problem - but I know that I know very little in this area and need a sanity check!

author: tggzzz
author_rank: Super Contributor
timestamp: February 13, 2016, 07:25:55 pm
content: Quote from: Addicted2AnalogTek on February 13, 2016, 07:07:47 pmInexperienced like me!The focus voltage is low (i.e. more positive), but the focus control still has an effect. If it did fully focus the beam, the phosphor wouldn't last long! The focus is a separate part of the circuit, and any problems might simply be a consequence of the beam intensity. I'll look at that after sorting out the intensity.More importantly, the cathode and grid voltages appear correct - as best as I can judge. That's against my understanding of the "weak diode" phenomena. I've also tested the diodes in the DC restorer with a DMM, and they are fine as far as that limited test goes. I've replaced them with BAV21s, with the results given previously. (I wish I'd measured the voltages before replacement, but I was hoping for a quick fix).I haven't replaced the HV ceramic capacitors because they are more difficult to obtainandthere doesn't seem to be a problem with the voltages - but I'd like that sanity checked by some one with more experience.Hence my questions about voltages!

author: Andy Watson
author_rank: Super Contributor
timestamp: February 13, 2016, 08:05:41 pm
content: Have/can you checked the voltages on the pins of the tube?

author: tggzzz
author_rank: Super Contributor
timestamp: February 13, 2016, 08:36:47 pm
content: Quote from: Andy Watson on February 13, 2016, 08:05:41 pmAs best I can using a 100Mohm 100:1 potential divider (3*33M+100k)plus a DMM!I haven't measured the anode, but many electrons are hitting the screen so I doubt that's a problem.The cathode is adjusted to: Vk= -2450VThe grid is a bit lower: Vgk= -42Vmeanto -147Vmeandepending on the position of the CRT bias trimpot and the front-panel intensity control. (i.e. Vgis around -2550V)My understanding is that the grid is physically between the cathode and everything else, and is responsible for controlling the beam current. Or not in this case!The focus is -1060V to -1670V depending on the focus front-panel control. The manual indicates -1700V. If the CRT is disconnected, the focus is -1600V to -1900V. With neither understanding nor evidence I presuming that is a secondary problem that might be caused by the excessive beam current.The other voltages are all reasonable, but I'm guessing they are also unimportant at this stage.So, what would you infer from those voltages?

author: rf+tech
author_rank: Frequent Contributor
timestamp: February 13, 2016, 10:36:08 pm
content: As a vote of confidence in your understanding, with the cathode at -2450 Volts and the grid *more* negative (-2492 to -2597 Volts), the beam should completely cut off within the range of -2515 Volts to -2597 Volts.It does appear that the control grid is open inside the tube, unfortunately.One last thing to try is carefully removing the CRT socket and visually inspecting pins in both. Look closely inside the neck of the CRT for visual clues.RF+ Tech

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: February 14, 2016, 06:14:33 am
content: It would help if I didn't attempt to do anything when I'm half asleep (or more...) including reading and replying on posts. I don't remember actually reading half of your initial post!!Being rather inexperienced, following repair posts helps me learn, though. It sucks that you may have a bad tube!!I have a 465 open on my bench if you need any measurements to compare against.

author: tggzzz
author_rank: Super Contributor
timestamp: February 14, 2016, 10:29:59 am
content: Quote from: Addicted2AnalogTek on February 14, 2016, 06:14:33 amYesand yesThe TekScopes Yahoo! group contained useful information.QuoteI think the only relevant measurement is to stick a DMM across the CRT's grid and cathode, to see what the range of Vkgis. Well, not directly, in case there is a gross fault involving high voltages. I used a homebrew 1000:1 100Mohm divider chain (3 * 33Mohm + 100kohm).With the intensity control set to minimum, there was no LT blanking waveform at TP1486. Hence Vkgshould be largest, in my case ~-100V to -147V depending on the position of the CRT bias trimpot. If my understanding is correct, that should be more than adequate to blank the beam.To find the smallest Vkg, adjust the intensity control for the maximum peak-peak voltage at TP1486 (65V in my case), and select a timebase to maximise the duty cycle to somewhere like 90% high 10% low. Then Vkgshould rise by ~60V to somewhere between -40V and -87V. If my understanding is correct, that the beam should be visible for some of that range.Looks like I need to decide whether to break it up for spares or find a new tube.Apart from that, it is a damn sight easier to work on and repair my HP1740A: no wires soldered to boards, no pairs of components (e.g. resistor+diode) with joints in mid-air for no good reason other than they discovered a need for one of them after the board had been laid out. Yes, it is an early 465, SN106389

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: February 14, 2016, 01:35:27 pm
content: I have a 465 that was damaged in shipping - SN B308858 - which has flying lead connections as well, so I don't think they ever bothered to change the boards to accommodate. That one, being an Option 5, is even worse with the flying leads than my standard 465 B254554.The reason I mentioned "damaged in shipping" is, even though I got it electrically working (for the most part) that I am on the fence about parting it out. In the event I do, I would sell it's known-good CRT for $50 plus shipping. However, if you wait, you could just get a whole parts scope for near or less than twice that.

author: tggzzz
author_rank: Super Contributor
timestamp: February 14, 2016, 08:49:56 pm
content: I might conceivably be interested in that, but I'd like to know the post and packing costs first!A suggestion on the TekScopes group is that I disconnect the CRT, apply a normal heater current, and set Vgkto a few 10s of volts with a current limiting resistor. If they are both connected they should act as a diode and conduct a few microamps. I think I'll try that tomorrow.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: February 14, 2016, 09:32:42 pm
content: It seems I must join the Yahoo Tekscope forum asap... there must be at least a few old Tek techs over there!That reminds me.. I need to get myself a good analog microammeter. I bought bunches of scopes, cal gear, and signal gens and didn't bother buying a single quality bench meter. I'm going to have to try and score one or a few soon.Thanks for taking the time to put much of your repair and troubleshooting info into the thread, though. This type of stuff is an especially helpful learning tool in addition to the books I've been reading!As for the CRT, there would be no packing costs as I'm fully stocked on boxes and packing materials from last years purchases. In case you want to find out, it would be approx 3lbs or a little under, and in a 20" x 9" x 9" box. (50cm x 23 x 23) and would ship out of WV USA 26501.

author: tggzzz
author_rank: Super Contributor
timestamp: February 14, 2016, 10:32:57 pm
content: Quote from: Addicted2AnalogTek on February 14, 2016, 09:32:42 pmIt is an active group and the information can be very precise and detailed.QuoteIf you're not careful you'll turn into a VoltNut. Dont forget that someone with two watches (voltmeters) doesn't know what the time (voltage) is.QuoteAh, you lucky person. You have yet to be delighted by appreciating the much larger amount of effort visible in the TekScope group!My small contribution has already been rewarded by the responses of others.

author: tggzzz
author_rank: Super Contributor
timestamp: March 31, 2016, 01:42:49 pm
content: In my case the 465 tube was terminally damaged: the grid was internally disconnected. Rather than throw it out, I decided to chop it up and put the pretty bits in my display cabinet.The results, showing the disconnected grid and the hidden metal mesh lens, can be seen athttps://entertaininghacks.wordpress.com/2016/03/09/rescuing-a-broken-tektronix-465-crt/Those interested in the Burr Brown equivalent of the Pirelli calendar can look at pretty scans of thick-film hybrids athttps://entertaininghacks.wordpress.com/2015/07/07/images-of-late-70s-burr-brown-thick-film-hybrid-ics/(Yes, other people have described them as "geek porn")