Re: Tektronix 2245a Flickering Trace

William Schuler
 

Well I fear the worst... While self diagnosing power supply ripple, I
touched 130 V test point, saw a bright flash on screen, then everything
went black. Unit powers up, LEDs on front light up, relays click, but there
is no display on the screen. The beam find button does nothing. Bummer! I
realized I didn’t have probe on 10 X. Is there any hope you guys?

On Sat, Jan 4, 2020 at 11:27 AM Siggi <siggi@...> wrote:

On Sat, Jan 4, 2020 at 8:00 AM William Schuler <guitardad1967@...>
wrote:

That makes me wonder – can you use a mostly functional scope to diagnose
itself?

I guess it depends, though a large section of this document (
https://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf) is
about "Front-Panel Diagnosis" which largely interrogates the instrument
about its own faults. You can - to a point - use a scope to look at its own
power supply ripple and the like, and *some* of its internal state, like
some portion of the sweep ramp(s). I guess it can be hard to know what to
believe when the instrument itself is faulty.
This scope also has a service mode with some self-diagnosis capability. You
can get the service manual from here (http://w140.com/tekwiki/wiki/2245),
see page 6-10 for the service menu.

Also note that every scope has two built-in "signal sources"; the
calibrator and GND coupling, and most scopes have an additional trigger
signal which is the line. You can also think of AUTO trigger mode and the
holdoff as trigger signals. Then there's X/Y mode, which allows you to
introduce a calibrated signal to the horizontal circuitry. In your case, it
might be interesting to inject DC on the X, and e.g. a sine or triangle
wave on the Y. It might also be interesting to play with line triggering to
see whether the glitching is line-synchronous, which could happen if e.g.
you have excessive line ripple on a supply ... time passes ... these scopes
have a switch-mode power supply, so more likely power supply noise or
ripple will be at the switching frequency.

If it comes to it my friend has a scope for use.


That'll certainly be an easier way to go :). If you poke around in the
power supply, be very, very careful, as everything except the final
low-voltage supplies are line-referenced.

Good luck and much fun,
Siggi



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