Tektronix 2465 scope - shrunken display - Page 1
Forum section: electronics / repair

author: Bunny5708
author_rank: Newbie
timestamp: July 11, 2022, 09:53:17 pm
content: Display of calibrator signal is shrunk to 1/3 normal both Vertical & Horizontal. all DC voltages at J119 are normal, HV module P4 should be -900 but is -600. So cathode supply is only about -1300 - should be -1900. DC bias of PNP Q1981 f=rise from 1.09 to 1.18 with no change is HV module P4 value. Suspect bad 2SA1077 PNP not responding to changes in Vbe bias point. Could the Volrage regulator LM1458 be bad instead or even the transformer with a bad winding. Any ideas on how to check for other causes of low Cathode Voltage. See picture of scope trace

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2022, 11:00:48 pm
content: Low cathode voltage should *expand* the display and not shrink it, so my guess is that the post deflection acceleration (PDA) voltage is missing because the high voltage multiplier is shorted and dragging down the cathode voltage.

author: MarkL
author_rank: Unknown
timestamp: July 11, 2022, 11:09:42 pm
content: Check the beam finder. It looks like it's stuck on to me.

author: Manolo Mos
author_rank: Contributor
timestamp: July 12, 2022, 06:36:35 am
content: I think the problem is U800. It's bad news. I've solved mine, removing and applying on it a lot of heat with a 100w solder. This solution works because the fault are bad connection in the IC inside. Bad quality construction IC.Don't worry if you think the IC is gonna break for the high hot.

author: tggzzz
author_rank: Super Contributor
timestamp: July 12, 2022, 08:06:55 am
content: Quote from: Manolo Mos on July 12, 2022, 06:36:35 amDoes U800 affect the Y axis?Only fiddle with U800 if it is certain it is the source of the problem. There are subtle ways to destroy perfectly good U800s.

author: Testtech
author_rank: Regular Contributor
timestamp: July 12, 2022, 11:17:55 pm
content: Quote from: MarkL on July 11, 2022, 11:09:42 pmI agree, this should be the first test.

author: Manolo Mos
author_rank: Contributor
timestamp: July 13, 2022, 09:56:10 am
content: Yes, U800 malfunction affects Y axis.

author: chris_leyson
author_rank: Super Contributor
timestamp: July 13, 2022, 11:54:21 am
content: If U800 has failed then there is a replacement for ithttp://www.davmar.org/TE/Tek2465/lafay.html

author: MarkL
author_rank: Unknown
timestamp: July 13, 2022, 01:48:08 pm
content: There is another U800 replacement developed by "Dr. Ram", built on Lafay's work and has several improvements. He calls it the RU800. Search for it on the TekScopes group on groups.io.But it's very unlikely it's U800 and it would be unwise to mess with it as a first troubleshooting step. U800 is the horizontal output driver and the screen shot from the OP clearly has an issue on both axes.Really... Start with the simplest possible causes before diving deep. The screen shot is what it looks like when the beam finder is pressed. Check the beam finder button and associated circuitry.If you press it and nothing happens at all, it's stuck on. First thing to check is if the button is mechanically stuck or broken internally.

author: tggzzz
author_rank: Super Contributor
timestamp: July 13, 2022, 02:15:32 pm
content: Quote from: Manolo Mos on July 13, 2022, 09:56:10 amHow does it do that?As far as I can see it is only on schematics 6 and 11 (PSU only). I see only outputs +- horizontal sig.

author: Manolo Mos
author_rank: Contributor
timestamp: July 13, 2022, 02:18:57 pm
content: Quote from: tggzzz on July 13, 2022, 02:15:32 pmI don't know, but it happens. I've seen different kind of problems with the U800 IC. I remember one U800 that the problem was removed momentary pushing "beam finder".

author: jonpaul
author_rank: Super Contributor
timestamp: July 13, 2022, 02:52:04 pm
content: Most likely HV PSCRT BiasCRTTry groups.io tektronix or tektronix 2Get serv manual at BAMA Archive, etc.Follow troubleshooting flow chartJon

author: Bunny5708
author_rank: Newbie
timestamp: July 16, 2022, 08:46:11 pm
content: wiggling the BEAM FINDER switch can restore normal traces. but another wiggle causes it to shrink again. Is this part soldered in or socketed ? My earlier view about HV supply was wrong - forgot 900V X4.73/14.73 is about the 616V I measured - DVM has 10M input resistance loading the 9M . HOW can I replace the switch ? or is an IC not responding to the switch and stuck- if possible what IC is that

author: bdunham7
author_rank: Super Contributor
timestamp: July 16, 2022, 09:16:26 pm
content: Quote from: Bunny5708 on July 16, 2022, 08:46:11 pmTry operating the switch 100X and see if anything improves. The switch is soldered into the board right behind where the pushbutton is, but it takes a bit of disassembly to get at it. You could reflow the solder with a lot less disassembly and you might try that first, especially if exercise doesn't help or makes it worse. I don't have one--its missing from my parts board and I don't recall why--but QService has it in stock. It is p/n 260-1421-00. Maybe someone else has one handy.https://www.ebay.com/itm/TEKTRONIX-260-1421-00-Switch-Push-Beam-Finder-2400-Series-Oscilloscopes-/332406631170

author: Bunny5708
author_rank: Newbie
timestamp: July 18, 2022, 04:06:11 pm
content: Thanks so much for pointing me in the right direction. Contact cleaner solved bam finder switch problem no need to replace it. Scope now GOOD.