Tektronix tds 640A...with fuzzy crt screen - Page 1
Forum section: electronics / repair

author: Talli
author_rank: Newbie
timestamp: November 12, 2022, 03:09:04 am
content: so i bought this oscilloscope and the seller indicated that after 10 minutes the screen goes fuzzy ...so these units from reading are popular with bad caps ..leaking ..so how hard can it be??got home took it apart all the caps look dry no leaks ..on the upper main board(not sure the name maybe VGA processor board...it has many 33uf caps .. changed them all ...put the board back on .. the scope screen is bad... Fuzzy as in the picture.. ..hooked up external monitor ..works fine ...so what went wrong???some parts on the CRT board were semi bad ..and know 100% bad.??or i damaged the main board.???any one with experience ...can guide me what components to replace ??

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 12, 2022, 05:55:54 am
content: Did you thoroughly clean the main board before you installed the new capacitors (a dishwasher is a good idea to make it really clean).The original fault taking time to appear makes it look like a heat related fault, when it gets warm, it fails. It could be a dry solder joint or a marginal component somewhere.The fact you have video out on the VGA port indicates the main board is mostly ok all the way to the CRT video chips on the front left corner, so I'd be first suspecting something on the CRT board first as that image looks like a lack of Horizontal sync.Luckily, schematics are available, so you can trace it down to a likely group of suspect components.https://w140.com/tekwiki/images/c/c9/TDS540_Schematics.pdfPage 72 is where I'd start looking. It could maybe just need a tweak of a few trimpots if you are lucky...I think I have a spare CRT board here so I can take measurements if you need, or sell it to you if you can't fix yours.

author: Talli
author_rank: Newbie
timestamp: November 12, 2022, 06:15:07 am
content: Thank you so much for making the effort to respond... Indeed.did not wash the board because there was no leak from the capacitors... It was in perfect condition... However I decided to change them because they will go bad... And as I mentioned the screen on the CRT is all fuzzy...after 10 minutes... I thought it had to be a cap.Before I change the capacitors and took out the board the crt was OK... For few minutes....or sonever touched the CRT unit... can I test the VGA output from the board to the CRT if it's good... Process of elimination..... If it's good the ic... Etc,then I'll go to the CRT unit board

author: james_s
author_rank: Super Contributor
timestamp: November 12, 2022, 07:21:49 am
content: Did you bump the H-sync pot in the monitor? That's what it looks like to me, either that or the horizontal sync signal is not connected properly.

author: Talli
author_rank: Newbie
timestamp: November 12, 2022, 07:45:25 am
content: I didn't touch the pots yet... Indeed it looks like something with the sink.. The H or Vertical.... But I don't know it's from the crt board or the out put from the mother board to the crt... When the scope is booting.. At some point you can see that round waiting clock... In bad resolution.... You can tell it's the clock.... Is there a way I can test and how... To test the out put sync... Horizontal... Vertical.. Video... Etc...

author: james_s
author_rank: Super Contributor
timestamp: November 12, 2022, 07:54:30 am
content: Plug an external monitor into the VGA port.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 12, 2022, 08:07:00 am
content: I'd just tweak the Horizontal related adjustment trimpots a little and see what happens.