Tektronix455, high voltage transformer

[doban]

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.

[doban]

Or does anyone know how I can check if it's good? Thanks.

[adam4521]

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.

[doban]

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.

[adam4521]

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.

[doban]

All right, I'm waiting. Until then, I'm also checking the diodes. Thank you adam4521!!!

[jonpaul]

hello have  long experience on similar Tektronix scopes 246xB 465, 475
the 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 rewind

Search also on tekscopes @ groups.io and tekscopes2 @groups.io

Bon chance

Jon

[doban]

Thank you Jonpaul!!

[jonpaul]

Doban Rebonjour

Easier to check but not as likely

Clean intn, focus controls, beam finder button

High Meg resistors in HV section may get increased  value

Finally BE CAREFUL of the HV and CRT aquadag stored energy even when off

Bon courage

Jon

[doban]

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.

[David Hess]

The 455 power supply section should be identical to the more common 465M.

[jonpaul]

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 Chance


Jon

[doban]

Does anyone know what I circled in red means? Thank you.

[adam4521]

Number of turns and wire size, for each winding. What a great diagram!