Warmup/Pressing on PCB Squishes Edges of Screen - Tek 2252 - Page 1
Forum section: electronics / testgear

author: SynViks
author_rank: Contributor
timestamp: April 13, 2023, 08:00:04 am
content: New to me Tektronix 2252 has an issue where, after it's been on for a a few minutes and the unit has warmed up, the stuff being displayed on the horizontal edges of the CRT squish inward somewhat. Not just the trace but readout too.After some experimention I found that applying pressure to part of the PCB does the same thing when the unit is first turned on.Here's a couple videos which demonstrate the issue:https://youtu.be/Df9vEXnNqhohttps://youtu.be/0zxDuJ5ESrkHere is a schematic of the area I'm pushing on. U701 is the vertical output amplifier. Seems pushing on the PCB or components near this chip causes the issue. Also my theory that heat expansion after warm up results in the same condition as applying pressure.Unit successfully completes self diagnostic tests. I cleaned and went over the power supply to check for bad caps (looks fine), checked PS rail voltages (they're all good). The vertical + - output wires are 40 and 41v respectively and don't change when the screen edges squish.

author: Nusa
author_rank: Super Contributor
timestamp: April 13, 2023, 08:11:15 pm
content: To me it suggests a cracked trace or solder joint that changes resistance with thermal expansion or mechanical pressure.You could try reflowing all the joints in the area to start. See if that changes anything.

author: gabiz_ro
author_rank: Regular Contributor
timestamp: April 14, 2023, 06:26:52 am
content: But that seems to be related to horizontal deflection, not to vertical deflection.So don't focus only on vertical sections, you press on area with U701 but how PCB move who can know? Check horizontal section too.editSince this appears on both edges, may be a problem of insufficient power supply on horizontal output amplifier, driver amplifier sections.

author: SynViks
author_rank: Contributor
timestamp: April 14, 2023, 07:27:45 am
content: Looked for cracked solder joints or points of delamination/trace damage. Nothing stood out, though I can't easily see the bottom of the motherboard (removing it is rather involved). Indeed it was somewhat confusing why an area involved in vert deflection was involved with this horizontal side effect.Did some more experiementation and found the following:Before the unit is fully warmed up, if I press anywhere in the red area the screen output has squished in horizontal sides. If I press on the blue area, the effect reverses and the screen is normal again.I think what's happening is the horizontal part of the beam find circuit that shrinks the display is getting turning on some how. I think that because if the screen is squished and I push the beam find button the horizontal width doesn't change. Can't be a coincidence that the squish just happens to be the same amount as what the beam find button does.This makes it a bit more confusing because we have thermal expansion turning the horizontal component of the beam find on, and physically pressing on parts of the board turning it on and off. I don't see how a solder joint or trace issue could cause that?

author: gabiz_ro
author_rank: Regular Contributor
timestamp: April 14, 2023, 07:38:23 am
content: On 070-7838-00.pdf service manual on page 295 is horizontal schematicMonitor each voltage in both caseseditArea in blue is part of B sweep and generate signal B ramp for horizontal. page 292Check solders in that area.

author: SynViks
author_rank: Contributor
timestamp: April 15, 2023, 07:56:53 am
content: So I think I solved the issue.Spent a long time trying to narrow it down. When the screen is squished the horizontal +- voltage levels are about the same as what they'd be when the screen isn't squished but the beam find is on. Problem is, if my probing was right, the horizontal pre amp didn't change it's output when screen was squished vs not. I tried to narrow the issue within the circuit but kept hitting dead ends.Eventually I just started reflowing solder for various pieces in the horizontal circuit, including the preamp pins. I'm not sure which pin specifically did it, but it seems to have fixed it.