Tektronix455, high voltage transformer - Page 1
Forum section: electronics / testgear

author: doban
author_rank: Newbie
timestamp: January 24, 2023, 03:36:26 pm
content: Hello friends. I want to check the high voltage transformer of the tektronix455 oscilloscope, code 120-0984-00. Any help or information about the transformer is welcome. Thank you.

author: doban
author_rank: Newbie
timestamp: January 25, 2023, 06:32:12 pm
content: Or does anyone know how I can check if it's good? Thanks.

author: adam4521
author_rank: Regular Contributor
timestamp: January 27, 2023, 10:20:00 pm
content: I have a working 455 oscilloscope, I guess I could do some ohms measurements on the windings (in-situ) if you wanted to compare. You have a copy of the service manual PDF? There is a lot of information in there with test point voltages etc.

author: doban
author_rank: Newbie
timestamp: January 28, 2023, 08:22:36 am
content: The oscilloscope didn't break for me. That's how I got it from a friend. Yes, please make measurements on the windings for comparison, when you have time. I have documentation, with measurement points. The defect is, the transistor Q552 burns - short between the base -collector-emitter and Q556-short only between the base and the collector. I only have one pair of transistors. When starting immediately, the +32VDC fuse blows and both transistors burn immediately. I can start the measurements if I remove T550 and U550 from the card? Thank you very much.

author: adam4521
author_rank: Regular Contributor
timestamp: January 29, 2023, 08:42:21 am
content: I wouldn't remove the transformer unless you have higher certainty that it has failed -- at that point it is curtains for the scope unless you have a parts/donor unit! I would be suspicious of the zener diodes VR552 and VR553 and diode CR552. Are you happy that they are ok? They are holding up very big voltage swings -- if anything fails there I could easily imagine them breaking down your transistors.Unfortunately it might be a few days before I get bench space and time to open the cover of my unit and take ohms measurements -- will do my best to find some time this week.

author: doban
author_rank: Newbie
timestamp: January 29, 2023, 08:59:54 am
content: All right, I'm waiting. Until then, I'm also checking the diodes. Thank you adam4521!!!

author: jonpaul
author_rank: Super Contributor
timestamp: January 29, 2023, 11:39:22 am
content: hello have long experience on similar Tektronix scopes 246xB 465, 475the decades old scopes eventually fail bad HV.Most CRT/HV issues are the HV transformers winding insulation breakdown due to old age, and HV multiplier ( HV caps, HV diodes)The transistor failure is a secondary effect, probably not the cause.HV transformers are not simple to replace as rare to find a good one.Search eevblog threads on Tektronix scopes CRT,HV transformers rewindSearch also on tekscopes @ groups.io and tekscopes2 @groups.ioBon chanceJon

author: doban
author_rank: Newbie
timestamp: January 29, 2023, 01:05:19 pm
content: Thank you Jonpaul!!

author: jonpaul
author_rank: Super Contributor
timestamp: January 29, 2023, 04:59:32 pm
content: Doban RebonjourEasier to check but not as likelyClean intn, focus controls, beam finder buttonHigh Meg resistors in HV section may get increased valueFinally BE CAREFUL of the HV and CRT aquadag stored energy even when offBon courageJon

author: doban
author_rank: Newbie
timestamp: January 30, 2023, 05:14:18 pm
content: Thank you Jonpaul.After many searches I found information about transformers. I found what I was looking for. Page 422.Please comment.I found a pdf with 435 pages. It won't let me post here, only a maximum of 4000kb. If someone can modify it, I can send it by email.

author: David Hess
author_rank: Super Contributor
timestamp: January 30, 2023, 10:20:18 pm
content: The 455 power supply section should be identical to the more common 465M.

author: jonpaul
author_rank: Super Contributor
timestamp: January 31, 2023, 09:55:13 am
content: Doban good work on the transformer spec sheet. You can then test the part if removed from the PCB.But just resistance tests on the windings.Most issues are inside the HV insulation and reveal only when running.Unfortunately without the manufacturer core, gap size, imprégnant and bobbin part numbers, you cannot rebuild it.But if its dead you can depot/remove varnish and ty ro reuse the cors and bobbin.Wire type, insulation and build/lay is important.Bon ChanceJon

author: doban
author_rank: Newbie
timestamp: January 31, 2023, 04:07:13 pm
content: Does anyone know what I circled in red means? Thank you.

author: adam4521
author_rank: Regular Contributor
timestamp: January 31, 2023, 05:31:52 pm
content: Number of turns and wire size, for each winding. What a great diagram!