New guy, Tektronix 2211 Scope will not focus

[gbkd80]

Hi -

I'm trying to diagnose a problem with my Tektronix 2211 scope.  I noticed recently that the focus knob stopped working and the CRT was blurred.  I wanted to try fixing it so tonight I took a crack at it.  I can't find a service manual (free) though based on the research it seemed like there are 6 510k resistors in the focus circuit that go out of range and need replacement.  So I replaced them all tonight, using 510k carbon film resistors that I had on hand.  I made the focus problem worse.  Now it's very blurry, and again the focus control does not work.  I'm sure this could lead me down the path of recapping, etc... but I really don't want to do that.  In the meantime the scope basically does what I use it for and until I can get a Rigol or something digital I'd like to keep using the 2211... any 2211 owners have this problem and have issues with the ficus other than those resistors?  Or did I possibly use the wrong wattage resistors?  I don't even know what they are... maybe 1w?  I used what I had...

[tautech]

First you can't just go replacing HV resistors with what you have on hand, they must be at least the same wattage or better !
Like any piece of gear that's misbehaving the first thing to check is the PSU rails, for exact to spec voltages and ripple. Then you will know if you need to recap the PSU.

There doesn't seem to be a service manual in the Tek wiki so you might have to wing it with a SM from a similar scope from the 2000 series:
http://w140.com/tekwiki/wiki/Category:2200_series_scopes

This can be useful too:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

Good luck and welcome.

[james_s]

These are ordinary carbon comp resistors, they look like 1/4W rated. I replaced a few in a 2215 recently to fix exactly the same problem. In my case there's a loose bit of metal inside the CRT which occasionally falls into the wrong place and blows the focus divider.

Not sure why replacing them made the problem worse, are you sure you used the right value parts and didn't make a solder bridge or something?

[jaunty]

came across THIS page a few years ago as i have a 2445 as well ... seems it might be on the right track with the resistor thing mentioned above


https://cromwell-intl.com/radio/tek2445a.html

[David Hess]

The original resistors in the focus chain were carbon composition. Tektronix later replaced them with *high voltage* metal film resistors.  Do not use standard carbon or metal film resistors.

Check that the focus potentiometer is not damaged.

[gbkd80]

I can probably disconnect the wires from the pcb under the pot and test the pot itself.  If not though, is the focus pot removable from the rear panel or do I have to pull the entire unit apart to get the rod out etc.? 

I have another scope similar to this one that probably has similar parts.  I might just try robbing Peter to pay Paul...

[floobydust]

Careful the focus is a symptom, not the problem.
I use an ohmeter and first check the resistors in circuit. The HV divider, focus potentiometer and 510k parts. Less damage than pulling parts that are probably fine.
Your PCB has to be spotless so no leakage current from flux when you replaced those 510k resistors. Use a q-tip and IPA to clean up, let it dry.

The HV multiplier might be weak and your HV is low, so the focus control can't get in range. Or the CRT emission is down from power supply problems.
Have you replaced any of the eletrolytic capacitors in the PSU?

[gbkd80]

No, but I found the problem and it was simple... one of the wires in the molded ribbon coming from the pot PCB to the main board was broken... I ended up replacing with new wires.  All is good now.  In fact I put back most of the resistors I replaced, less 2 that went south.  The problem was the focus pot had absolutely no effect, and when I tested it with my meter it was working fine. 

[floobydust]

Don't forget to set the astigmatism, it goes hand in hand with the focus control.

I put on a trace (sine) that has both H and V visible  (not a square wave which has no vertical line drawn) and one hand turning astig trimpot and the other hand on focus control, set them for best focus in X and Y direction.
Makes a huge difference in trace clarity.