Tektronix 2235A Ch 1 No Reading Below 1 Volt

[gmcjetpilot]

This should be easy for you experts. Blew to dust off my trusty Tek and it's all working except channel 1, setting 0.5 volts or less, either DC or test signal zip, nothing, some noise. Otherwise works fine 1 volt up. Channel 2 fine. Thanks

[tautech]

Welcome to the forum.

This thread should've been in the Repair board:
https://www.eevblog.com/forum/repair/

If you use the Report, you can ask a Moderator to shift it there. 

It is not uncommon for the front end of a scope to be hurt with an overvoltage event and maybe the V/div attenuators have been damaged.
You'll need a Service manual and there's several manual repositories linked in the sticky threads in the Repair board that you can download from.
Here's one to get you started:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

Come back to us and link the manual you're using so we're all on the same page. 

[David Hess]

The 2235 service manual is almost as good for this.

As far as the repair, the x100 high impedance attenuator is switched in at 1V/div and above so the problem is with bypass switch 2 or 3; the switch contacts need to be cleaned or maybe the leaf springs need to be bent a little.  Gaining access will require removing the A2 attenuator board which is accessible from the top of the oscilloscope.

Clean the switch contacts by wetting some card stock or clay free paper like vellum in alcohol and drawing it through them while they are closed.  Do not use anything abrasive to clean them as it will remove the gold plating.