TDS540a vertical on display is short - Page 1
Forum section: electronics / repair

author: manicdoc
author_rank: Unknown
timestamp: October 18, 2022, 06:11:40 am
content: Hi,Picked up a TDS540a in good condition (no sign of cap leak, everything shiny & clean, passes self-test, and displays 4 channels). The only problem is the display ends short. See attached.This might be a bad joint or reduced voltage, just wondering if anyone has seen this before and can point me in the right direction...I thought I might be able to tweak the vertical joke adjustments, to see if they are a factor, but they appear out of reach. I suspect a disassembly is on the cards with being careful of the high voltage on the anode lead (ground it to chassis).Update:Looking at the display, it seems 'shrunk' horizontally as well compared to my 540. That makes me think the LM317 circuit regulating the 24v down to 20.5v that powers the whole display driver circuit could be worth investigating. Either the adjustment is out, a bad solder joint, or we have a bad cap or two. or the input 24v is down and it's out of headroom. Start from the input...

author: TERRA Operative
author_rank: Super Contributor
timestamp: October 18, 2022, 10:46:09 am
content: Looks stretched at the top then squished at the bottom..After checking the voltages, I'd take a look a the capacitors.Incidentally, the schematics are available here:https://w140.com/tekwiki/wiki/TDS540https://w140.com/tekwiki/images/c/c9/TDS540_Schematics.pdf

author: manicdoc
author_rank: Unknown
timestamp: October 18, 2022, 08:37:45 pm
content: yep, got those. I'll carefully strip it down this weekend. My money is on it being a cap

author: james_s
author_rank: Super Contributor
timestamp: October 19, 2022, 01:37:55 am
content: I would check the capacitors around the vertical output IC. That was a very common fault with CRT monitors, usually they'd start to fold over at the top first as the voltage of the charge pump starts to sag.

author: manicdoc
author_rank: Unknown
timestamp: October 24, 2022, 09:18:41 am
content: Update: got the board and CRT out of the chassis and sitting on the side. Did an initial check of the 1000uF inline with the vertical deflection Yoke, slightly out on the ESR, so replaced that. Nothing else jumped out at the initial inspection.Powered up, 24.4v was seen at the input before the fuse, was reading 20.3v after lm317, and turned up to 20.5v via R444. Adjusted the vert size and vert lin. It's better but the squeeze hasn't fully gone yet at the bottom, cannot adjust it out, looks like it's cutting out on the scan early.I might just replace C422 100uF that's across amp_y and flyback pins of the vertical output IC. I'll also pay special attention to caps around the osc and rampin pins and replace them with modern equivalents.

author: TERRA Operative
author_rank: Super Contributor
timestamp: October 24, 2022, 12:04:25 pm
content: I think I have a spare black and white driver board here I can take measurements from if you need anything, if I can find it...Or if it comes to it, you can have it for a reasonable price.IIRC, the HV flyback transformer can fail, so check that (try taking a look for corona discharge with the lights off if you suspect it).

author: manicdoc
author_rank: Unknown
timestamp: October 24, 2022, 10:56:23 pm
content: Quote from: TERRA Operative on October 24, 2022, 12:04:25 pmI have an older 540 with a good display, but the acq board is playing up. The goal is to get the 540a going and then see if that can help in analysis to get the old 540 working, then I'll sell off the older one.Worse case I can do a display or acq swap, and sell the old one for parts.. but its closeUpdate: I've checked more elec caps out of circuit - no issues. This might be ceramic cap aging, but I doubt it, although cannot get a measurement on any of them in circuit... I'll take them out and measure... I've also given the board a good clean and resoldered the circuit, in case something was cracked...