Tektronix 2465 scope - shrunken display

[Bunny5708]

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

[David Hess]

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.

[MarkL]

Check the beam finder.  It looks like it's stuck on to me.

[Manolo Mos]

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.

[tggzzz]

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.

Does 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.

[Testtech]

Check the beam finder.  It looks like it's stuck on to me.

I agree, this should be the first test.

[Manolo Mos]

Yes, U800 malfunction affects Y axis.

[chris_leyson]

If U800 has failed then there is a replacement for it

http://www.davmar.org/TE/Tek2465/lafay.html

[MarkL]

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.

[tggzzz]

Yes, U800 malfunction affects Y axis.

How 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.

[Manolo Mos]

Yes, U800 malfunction affects Y axis.

How 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.

I 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".

[jonpaul]

Most likely HV PS

CRT Bias

CRT

Try  groups.io tektronix or tektronix 2

Get serv manual at BAMA Archive, etc.

Follow troubleshooting flow chart


Jon

[Bunny5708]

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

[bdunham7]

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

Try 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

[Bunny5708]

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.