Tektronix 2235 - working, but so so.

[TerribleToll]

I have an old Tektronix 2235 scope that works, although the trace is a bit fuzzy and the controls a little iffy.
(s/n: 202176)

I have checked over the PSU voltages which seem OK.
The unit is pretty clean and tidy with a little scorching around the focus chain resistors (now replaced with 1w metal film as some were out of range) and R266-271 (not replaced, as these meter up OK and look pretty good). I have replaced the 3 RIFA caps in the PSU that were in various stages of breakdown.

No visible improvement to the display yet - so I was tending towards replacing the 4 SPRAGUE 3000V plastic capacitors (10nf - C855,C975,C976 & C979): my question is: would these be suitable replacements, or does anyone know of better:

KEMET C4C Metallised Polypropylene Film Capacitor, 3kV dc, ±5%, 10nF, Through Hole (C4CAYUB2100AA0J)

Many thanks for any pointers on this.

[bd139]

The focus resistors are always a problem. I just did a 2215A the other day. All resistors will need replacing with suitable ones with no substitutions on value or chaining. The 1W metal ones are probably not good enough. Some of them are probably compromised at high voltages. Also you will need to clean the board with isopropyl after to stop creeping etc.

Vishay VR37 metal glaze 510K ones are fine. Good to 3Kv. Mouser sell them. Don't forget the change the mains filter - they blow up big time!




Also with respect to trace, I would suggest that you stick it in X-Y mode and see if you can get a sharp focused dot first and play with astig and focus. That will eliminate / isolate any problems with the deflection amplifiers and the power supply.

Also here's a cursed one I tried to fix a while back https://www.eevblog.com/forum/testgear/tektronix-2235-repair-thread/

[TerribleToll]

Thanks for the response - will check and report back.

[TerribleToll]

Right - back on the job.

I have already replaced the RIFA cap behind the mains switch - same as the other ones, but forgot to mention it.

The 510K resistors in the focus chain (1/4w), I replaced with good quality metal film 510K 1W and gave them a bit more 'air' as they obviously run hot. Do you think these will do? Also cleaned off the board although the scorch mark still shows.

Will test and report back next.

[TerribleToll]

Well - the trace controls were a bit iffy before I started. Now the Y position control for either channel is not working at all and the trace is flatlining for both channels.

When I press the beam find button, the test square wave is just visible, but flatlines again when released. I suspect this may have been happening intermittently before I started, but now is happening all the time. I'll start from the beginning and run through the troubleshooting guide again.

[bd139]

Ok looks like it's working reasonably well on the HV side at least like that. I'd carefully use the diagnostic charts from here. Check the power rails first though for both correct voltage and ripple.