Tektronix 2232 Repair Help

[TrooperN]

Hello, I recently got a Tektronix 2232 with a interesting issue, the trace is very thick about 2 inches, the volts/div is working correctly so is the time base and triggering, if I feed a signal in it is displayed correctly but the trace is still very thick, both channels are having the same issue. I have measured all of the voltages coming out of the power supply, here is what I got:

+100v = 102.0v
+30v = 30.0v
+8.6v = 8.50v
-8.6v = -8.47v
+5v = 5.30v
-5v = -5.69v

I have also noticed high heat coming from RT901 and RT905 and the vertical amplifier and preamplifier. Also when it is turned off all of the power rails go to 0v except for the -5v rail which slowly drains to 0v.
Here is the service manual for my instrument (there are two different ones): https://archive.org/details/tektronix_2232_Service_Manual/

Any help would be greatly appreciated!

[grumpydoc]

Switching frequencies superimposed on trace?

Check ripple and all caps in the PSU

[TrooperN]

The focus control does not seem to change anything

[bdunham7]

If you look in your manual for the schematic page that shows the CRT and DC Restorer circuits, you will see a string of 510K resistors with the focus pot in parallel with one of them.  There will be one 180K resistor as well.  Check all of those resistors with an ohmmeter (power off for a few minutes first...) and you will likely find that some or all of them are higher than they should be.  If so, replace them all with an improved HV resistor.  I use Vishay HVR37 for the 510K and HVR25 for the 180K.  It's a common issue.

[TrooperN]

They all seem to be out by about 10 to 20k could that still be an issue?

[bdunham7]

No, not if they are that close.

What do you get in XY mode with both inputs set to GND?  (Non-storage mode, of course...)

[TrooperN]

I get a box the height of the trace and about 1cm in width

[TrooperN]

Also when re-measuring those resistors it seems R890 is completely open or too much resistance for my multimeter...

[bdunham7]

Does the focus knob have any effect on the sharpness of the edges of that rectangle?

If any of those resistors are significantly high or open, they should be replaced en masse before you go further.  If you confirm that the one is open, you can swap in another 510K of almost any time just to see how the rest of the scope works.

[TrooperN]

I was unable to find a 510k one but I found a 485k one which should work?

[bdunham7]

Close enough to test with.

[TrooperN]

Got the resistor replace, that was the problem! However I noticed some back and forth wave thing in the trace...

[josh132]

That wave thing is likely power supply noise. Try adding higher capacitance caps to the out put rail. Or maybe one of the caps are dead and are not effectively filtering power supply noise. A tek 2235 had a similar problem and all that had to be done was add higher capacitance caps that are build to filter supply noise.

[Mike P]

I had the same issue with the string of resistors in the focus adjustment on my 2235. I replaced them the first time with carbon comps (as original) and after a couple of years the problem reoccurred. The second time I replaced them with higher wattage metal films and it's been good since.

[TrooperN]

The issue was R980 for the focus control, I might look into re-capping the power supply. Also I think the wave in the trace went away after reinstalling the digital board.