CRT circuit Tektronix 465 - Page 1
Forum section: electronics / repair

author: vangheli
author_rank: Contributor
timestamp: May 27, 2021, 08:13:22 pm
content: Hi everybody. This is a request for help.My tektronix 465 oscilloscope stopped showing the beam : also the beam finder was not helping. So I started to check first all the voltages (excluded the -2450 volt due to lack of a high voltage probe): every voltage was within specifications and the ripples were below two millivolts for all. So I continued checking the components around the HV oscillator : Q1418 tested ok, C1419 tested in circuit 54 uF , fuse F1419 was ok and so also Q1413 and Q 1414 . I also checked with another oscilloscope TP1486 for the unblanking signals : the signal was there nice and clean and the level varied according to the position of the intensity knob. The troubleshouting chart say at this point to check "CRT circuit". what I think to do is to check the signal of the oscillator on the collector of Q1418 , but even if it will test ok ,at the moment I have no way to test the presence of the -2450 volts.Is there a way to indirectly test the presence of this voltage? Can you please give me indications on how to disassemble the board containing the HV section ? Thank you !

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 27, 2021, 08:38:07 pm
content: If you have a multimeter, determine its internal resistance. Probably 10 MOhm.Put 9x 10MOhm resistor in series, cover in several layers of shrink tube and use that as a probe for a makeshift 1:10 divider.Not very accurate, but you can test it against a known DC voltage and calculate the correction factor.That should be good for up to about 5kV.Of course, an insulation transformer is your best friend when doing this. And check if your benchtop DDM is ground referenced. Better yet, use a handheld DMM.If you just want o know if its there at all, just touch the CRT with the back of your hand. You will feel the static electricity lift the hairs on your hand and usually hear some discharge noise if you listen carefully.

author: David Hess
author_rank: Super Contributor
timestamp: May 28, 2021, 12:06:40 am
content: If the cathode voltage and unblanking input are present, then there is a good chance that the z-axis DC restorer has failed. Unfortunately testing it is difficult, and testing the pulled parts not much easier, so the usual recommendation is just to replace the parts in the z-axis DC restorer with new ones.

author: vangheli
author_rank: Contributor
timestamp: May 28, 2021, 09:55:12 am
content: Hi David,thank you for your advice. At the moment I cannot check the cathode voltage ,I have to find a high voltage probe. What I presume I can do now with the tools I have ,is to check the presence of the oscillations on the collector of Q1418 ( the manual reports a frequency of about 50 khz , I suppose the maximum voltage on the collector is no more than 100 volt peak). About the difficulty of testing the components of Z axis dc restorer : do you mean that an high voltage equipment is necessary to test diodes and capacitors? I do not have instruction on how to disassemble the related boards : perhaps you have a cue to the instructions?At the moment , I hope the high voltage multiplier U1432 is still alive , and my hypothesis about the faulty component is that diode Cr1421 is leaking or faulty. Is there a simple way to access this diode? Thank you and have a nice day

author: David Hess
author_rank: Super Contributor
timestamp: May 28, 2021, 11:47:53 am
content: Quote from: vangheli on May 28, 2021, 09:55:12 amI thought you had already verified the presence of the high CRT voltages.Besides the collector of Q1418, the DC voltages around Q1404 will reveal if the output is in regulation.QuoteThe problem is that the parts may only fail at high voltage so testing them properly requires high voltage testing.QuoteI do not, and I have not had my 465 apart that far.QuoteThe multiplier could have failed causing the inverter to stop ... inverting. I think fuse F1419 would blow in this case though.

author: vangheli
author_rank: Contributor
timestamp: May 28, 2021, 12:39:56 pm
content: David , thank you for your help.I will check the voltages around Q1404 : these depend on the feedback from the line of the -2450v ,through R1431D. Perhaps you know what are the base voltages with -2450 present or not?If I correctly understand your thinking ,you say if the multiplier failed with a short to ground also the secondary on T1420 was shorted ,generating an excessive load on the collector side of Q1418 : luckily the fuse F1419 is ok.I will now do all the measurements , and report later.

author: vangheli
author_rank: Contributor
timestamp: May 28, 2021, 05:10:17 pm
content: Hi David. I did the measurementsbase voltage of Q1404=3.85voltcollector voltage of Q1404=3.23 volt with a drop of 1.77volt on the 68k resistor : Vb>Vc so Q1404 is saturated with the final result that also Q1416 is driven into saturation through Q1404 and Q1408, Confirmation :collector voltage of Q1416 0,495 volt , emitter o.469volt Vce=0.26volt .This way Q1418 cannot oscillate and the -2450 volt are not there .I checked with another oscilloscope and what I see is a 31khz signal almost sinusoidal 1volt peak to peak , superimposed to a triangular signal of about 2 volt peak to peak and frequency 200Hz (4 times the local mains). Unable to explain .I tried to evaluate the resistance of the base coil and the collector coil (T1420) obtaining strange and probably wrong results:base coil 0.85ohm ,collector coil 0.45ohm.Conclusion (wrong?) : the high voltage section of the crt circuit is inactive.Reasons one or more of the following : failure of the rectifier diode cr1421 , failure of the transformer , failure of the voltage multiplier causing the inability of the oscillator to create the signal , other I don't know. Thank you to anybody who can give advice.

author: josh132
author_rank: Contributor
timestamp: May 28, 2021, 08:38:57 pm
content: Discharge the CRT to the chassis by disconnecting the anode plug. If there is a 1 CM arc then its likely the HV supply is fine. If it doesn't arc the CRT is dead or the HV supply is dead.

author: josh132
author_rank: Contributor
timestamp: May 28, 2021, 08:39:55 pm
content: or just arc the anode to the chassis while its on. Might damage the HV part so not recommended.

author: vangheli
author_rank: Contributor
timestamp: May 29, 2021, 09:51:09 pm
content: I wish to thank all the persons who answered to my help request about the high voltage circuits failure on my Tektronix 465.One more question (perhaps somebody had this type of issue): if the high voltage multiplier fails, can it possibly damage the high voltage transformer T1420 without melting fuse F1419 , ?Any other suggestion is welcome . Thank you

author: Smoky
author_rank: Frequent Contributor
timestamp: May 30, 2021, 01:07:08 am
content: I found a 465 service manual dated December 1972 and scanned part of the CRT circuit which shows some voltages:

author: floobydust
author_rank: Super Contributor
timestamp: May 30, 2021, 03:45:00 am
content: Quote from: josh132 on May 28, 2021, 08:39:55 pmNo, don't draw an arc from the HV, it kills the HV diodes in the multiplier.

author: andy3055
author_rank: Super Contributor
timestamp: May 30, 2021, 05:25:32 am
content: Quote from: vangheli on May 29, 2021, 09:51:09 pmVery unlikely that would happen. Check if you can see the CRT filament working. Try to remove the CRT base and check with a meter for continuity. If that is OK, see if it glows. Sometimes, in a darkened room, you can see it from the back of the base. Also, check the diode CR1421. If the multiplier goes bad and-if it is shorting, the fuse should blow (in case it is open that will not be the case). There are several threads on the 465 in this forum.

author: vangheli
author_rank: Contributor
timestamp: May 30, 2021, 07:52:03 am
content: Hi Smoky, thank you . This diagram is very useful for me : you spared me a lot of calculations to find the voltages.More : these are precise figures and not mere hypothesis!It would be nice if you can provide me a link to this 1972 manual . The ones I have do not reports the blue voltages . In case this is not possible ,because you scanned from your hard copy, I will highly appreciate if you can post a scan of the second half of the same page , containing the details of the dc restorer and the sketch of the crt itself. Thank you again.

author: vangheli
author_rank: Contributor
timestamp: May 30, 2021, 08:04:20 am
content: Quote from: andy3055 on May 30, 2021, 05:25:32 amI am triing to understand how to separate the boards to check the CR1421 : this diode is located under.perhaps you can suggest me how to access the HV multiplier?I am checking any possible reference to my issue in the Forum .Thank you for your help

author: andy3055
author_rank: Super Contributor
timestamp: May 30, 2021, 04:33:05 pm
content: Check this site for the manual:https://bama.edebris.com/manuals/tek/465/https://bama.edebris.com/download/tek/465/465Service.pdfI don't have a 465 as I gave mine away to a friend. But I believe you can locate it from the above manual and access it from the bottom of the main board.

author: Smoky
author_rank: Frequent Contributor
timestamp: May 31, 2021, 01:06:29 am
content: Here's the other half of the CRT circuit

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2021, 05:26:30 am
content: Quote from: vangheli on May 28, 2021, 05:10:17 pmThe most common failure is the high voltage multiplier. If this is the case, disconnecting its input will allow the inverter to operate and the CRT will display an image although it will be dim, poorly focused, and have lower deflection, but that is enough to verify operation.If C1419 fails, then the inverter will also not oscillate.

author: floobydust
author_rank: Super Contributor
timestamp: May 31, 2021, 06:21:58 pm
content: One test for a shorted module is to disconnect the HV multiplier module U1432GROUNDwire.I would never disconnect the HV multiplier module INPUT from the transformer and then run the scope. Unloaded, there is a danger of an arc which destroys the transformer.More in this thread:https://www.eevblog.com/forum/repair/tektronix-465/msg652402/#msg652402/

author: vangheli
author_rank: Contributor
timestamp: May 31, 2021, 09:37:27 pm
content: Thank you Smoky , these pages are an invaluable resource!

author: vangheli
author_rank: Contributor
timestamp: May 31, 2021, 09:38:59 pm
content: Thank you very much , I am trying to profit of your experience!

author: vangheli
author_rank: Contributor
timestamp: May 31, 2021, 10:19:32 pm
content: Hi David , thank you for your help!I disconnected the high voltage multiplier from ground ,lifting a jumper located on the upper side of the board (A5 coordinates on the layout).Well this is what I observed :(note: I had another oscilloscope connected on the collector of Q1418)pressing beam finder I got the trace ; I was able to catch a sinusoid of 25 volt peak to peak for 5 second , then nothing .The trace disappeared but a sinusoid of 50 khz and amplitude 1 volt only peak to peak was visible on the other oscilloscope.I started poking around and I noticed the blown fuse. I suspected C1419 (I had previously tested it ok) .Short circuit but no smoke , no popping out ,silent failure.After inserting another capacitor , everything seemed to function again , but it lasted only 5 second.Tonight ,in the dark ,I was looking for some luminescence from the crt , to reassure myself that it was still ok , when suddenly the trace came on.Note : the high voltage multiplier was still disconnected from ground . This time the trace was larger than before , occupying about 6 division.I was able to change intensity , focus, move left and right, I also connected the calibrator , everything seemed to functioning perfect (apart for the postacceleration) . At this point I switched off the oscilloscope , to connect the other one: after switching on again no trace at all.Due to my inexperience I am not able to relate these symptoms to a precise cause : are they showing a bad high voltage capacitor or a weak diode in the dc restorer section , or a weak transformer ? I am thinking that perhaps the high voltage multiplier is still good (or this is only a wishful thinking to avoid the inconvenience of find and installing a new one?).greetings and thanks

author: josh132
author_rank: Contributor
timestamp: June 01, 2021, 05:52:36 am
content: Well if it seems as if nothing is working and it seems unfixable, the choice of buying a replacement HV CRT board for the tek 465 is always an option where to get one you say? The holy and the best friend to us, EBAY.

author: james_s
author_rank: Super Contributor
timestamp: June 01, 2021, 05:56:43 am
content: Seems a bit extreme to try to buy a replacement board, these things don't grow on trees even on ebay. It shouldn't be too hard to fix the original.

author: David Hess
author_rank: Super Contributor
timestamp: June 03, 2021, 09:31:30 pm
content: The high voltage multiplier is not difficult to make from parts.

author: Grapsus
author_rank: Regular Contributor
timestamp: June 03, 2021, 11:23:15 pm
content: Hello !I think your HV multiplier box might be good. You are looking for some kind of intermittent issue around the tube. It might be a failing capacitor or even a cracked solder joint that changes with temperature.I would wait for the scope to break again then carefully and systematically inspect everything around the tube:- filament current (pins 1 and 14)- isn't the Wehnelt disabling the beam (pin 2)- do we have acceleration voltage (mesh, pin 12)- is T1420 oscillating ?- if not measure everything on the primary until you debug itI'm not a specialist on CRT but I think having current in the filament and -1700V on the mesh is enough to have a dot on the screen. If anything is getting in the way and deviating it, you can just disconnect it until you get the dot.Good luck !

author: vangheli
author_rank: Contributor
timestamp: June 04, 2021, 10:13:14 pm
content: Quote from: David Hess on June 03, 2021, 09:31:30 pmThank you. I have read the method used by GRAPSUS in an old post .I appreciate more advice ,I am inexperienced on HV.Today I was able to access the multiplier , the transformer and the diode CR1421 .Is it possible that an high voltage ceramic capacitor is bad despite an almost perfect measurement of capacity?Thanks

author: vangheli
author_rank: Contributor
timestamp: June 04, 2021, 10:43:25 pm
content: Hallo Grapsus ,thank you for your help . I spent the day to disassemble the vertical board from the interface board :it was really a pain ,even following step by step the manual. I think that you already targeted the problem.At the moment I do not have an high voltage probe to check the voltages you said. But I can do the low voltages measurements. In my opinion the oscillator is working: the frequency is 50 khz , but I checked with an other oscilloscope a sinusoid of only 2 volt peak to peak on the collector of Q1418. Perhaps this is caused by a failing capacitor (C1421,C1422,C1423) on the side of the secondary of T1420 ,but their value measured with my DMM is perfect.The diode perhaps is the culprit , but at the moment I do not have the equipment to really test it. By the way my diode is different from the one I saw on your pictures . Mine is anonymous , no marks for the cathode , no part number : perhaps somebody changed it.Do you know what should be the correct value of the resistance of the filament of the crt ?I need time to do all the tests do debug the oscilloscope . I hope to profit of your knowledge.Merci beaucoup!

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2021, 02:49:44 am
content: Quote from: vangheli on June 04, 2021, 10:13:14 pmIt is very possible. High voltage capacitors need to be tested at high voltage to reveal if they have failed.

author: floobydust
author_rank: Super Contributor
timestamp: June 05, 2021, 03:28:40 am
content: Another way to test the HV multiplier is using a signal generator and inject a 50kHz sine/square wave of at least 10-20Vpp (because multiplier HV diodes have high Vf). Discharge the CRT (using a resistor, a few in series to make say 1MEG) and then disconnect the HV wire from the CRT. You could leave it connected, the CRT anode then just acts as a capacitor.Just to measure the multiplier's output, it should be around 6x and DC only coming out. I'm not sure- but the multiplier failures are likely a shorted diode inside.Ceramic disk capacitors - if the dielectric is failing at voltage, there's corona hissing and a loud arc you will hear repeating over and over, but not in the potted module.The other mystery HV diode CR1421 is "VG-12X; SILICON 12,000V 5mA" 250nsec 35mA and specs are Vf 22V to 30V, so it would read open circuit with a multimeter unless it shorted.

author: vangheli
author_rank: Contributor
timestamp: November 10, 2021, 05:12:53 pm
content: Hi everybody.This is a very late post about the repair of my scope Tektronix 465 , due to the late finding of the required part.Well , the failed part was the high voltage multiplier. I changed it , and now the scope is working perfectly.I arrived late to the correct diagnosis , because I was convinced that ,in order to isolate the circuit from the high voltage multiplier , it was enough to disconnect the ground wire of the multiplier ( this was also the more easy and lazy way to proceed ,because it does not require the disassembly of the board). I do not know the failure modes of the high voltage multiplier , but I can affirm that the only way to avoid interference on the circuit from a faulty multiplier , is to completely remove it. As soon I did the removal , the high voltage oscillator resumed normal operation , the beam came back (with reduced trace length due to the missing post-acceleration and everything seemed promising for a successful repair.Thanks to all the contributors who helped me.