Tektronix TDS 420 disassembly

[Karlv]

Hi,

I have an error A012131 which I assume is caused by some faulty capacitors.

I am trying to remove what I assume is the low voltage power supply. See photo. it appears that the capacitors have leaked - hard purple gunk covering them.

So far I have been able to remove 2 screws from the board but I cannot remove the board itself. It looks like there are two slotted metal pins holding it in.

Any suggestions on what to do next?

Karl.

[Specmaster]

Before you attempt something like that, its a good idea to arm yourself with a copy of the service manual as very often they provide detailed instructions on the disassembly of the unit as well as a wealth other information.

A copy of the manual can be downloaded here https://isurplus.com.au/manuals/Tektronix%20TDS430A%20Oscilloscope%20User%20Manual.pdf it says user manual in the link, ignore that, it is the proper service manual.

[particleman]

That is the A26 board. A25 which is the LVPS is held in with clips and a single screw. I just re-did a TDS420. Every single SMD cap was leaking along with every capacitor on the A25 LVPS board.

[Dacke]

That's the high voltage flyback board to drive the CRT,  it's all one module attached to the CRT - you slide it out as a whole unit.  You're looking for the power supply,  in your picture it's in the upper left.  It will have one or two very large filter capacitors on the input,  can't miss it.  It is held on with one screw and multiple clips.

[Karlv]

Thanks for the link. I have a copy of the manual.

I need to do a quick check and take another photo!

This is the board with the sick capacitors. I don't think that the gunk dripped on it. It is extremely hard like melted plastic.

I have removed the one screw which appears to be the only screw holding the board in place.

I can see two other pins securing the board.

Is it possible to just remove/release these two pins without first removing the whole CRT assembly?

Thanks,

Karl.

[particleman]

That is glue holding the caps from vibrating. If you are having issues with the scope it is most likely SMD capacitors and SMPS caps.

[Dacke]

This one -

[particleman]

Yep. That is the LVPS. I bet the big caps are good. The smaller ones that have the glue snotted on them are most likely leaking along with all the other small ones. (if you are having PS issues). The SMD caps usually cause acquisition errors and such. There are several SMD capacitors  on the acquisition board several on the front panel board a few on the attenuator section and a few on the video board. Make sure you clean off the leaked gunk as well as you can. I used IPA and a fiberglass scratch pen. This is not a super hard job but having tools will make it easier IE. desoldering gun, braid, good flux/solder ect. Good luck.

[Karlv]

Thanks to all of you for your advice.
All the boards look good, caps look ok.
Cleaned most of the boards.
Re-assembled but put the back-up batteries in reverse(swapped them over - one battery is flat).
Re-boot and only cpu failure.
Re-booted again - same result.
Disconnected both batteries and re-booted. No errors!!
Rebooted and straight to the menu saying all is ok.
Probe hooked up to test point and got a nice square wave.

Once again, I really do appreciate your help as this is the first time I have pulled one of these apart.

Oops - a little too soon.
Everything was working ok until I put the outer metal cover back on.
Now I am getting the same errors but the scope appears to be functioning.
I have removed the front panel but I can not remove the knobs. They are very tight against the inner panel.
Any suggestions?

[particleman]

Take a few clear, close up pictures of the main board.

[Specmaster]

Sounds like a short to ground somewhere when the metal case is fitted. Check to sure the screws aren't to long and thus touching something. Also check the condition of any cables running close to the metal case incase there are cracks etc that maybe touching the case.