Fix for scope problem

Fix for scope problem

Post by clifford wrigh » Wed, 29 Feb 2012 06:12:35



G'Day!
For those few who still use the old Telequipment/Tektronix D63 or 65 scope
like me I have a fix (at last) to a very common problem.
These scopes were built with some less than perfect components so high
value resistors in the EHT and other circuits can do with replacing.
The other problem looks like a sync or focus problem, but it isn't.
You get an "fuzzy" of unsynchronised looking image on the screen.
It syncs, but phase realtionships are impossible to read as the image
appears multiple in the horizontal axis.

After a couple of days of hunting around, I noticed that a DVM probe on the
timing capacitor cleared everything up! Obviously the strays introduced by
the probe. However I then noticed that the capacitor C17 (200pF
polystyrene) on pin 2 of IC2 also produced the same effect with a probe
across it.
This is involved in a feedback loop from the timing circuit.

Now, of course, it is well known that the older polystyrene caps have some
self inductance, so I replaced C17 with a modern 200 pF ceramic plate
capacitor.
Perfect sweep at all speeds!!!
So my conclusion is that the special Tektronix chip IC2 can get into some
type of parasitic oscillation. This has the effect of creating multiple
mistimed sweeps (in effect) and ruins the display.

If correct this means that the circuit was NOT well designed in the first
place. It is clear that C17 HAD to be a high quality, low inductance
capacitor for reliable operation. Which it was not!

Hope this helps someone with a similar problem. I have heard of one or two
others myself already on the web.

Regards Clifford Wright Helensville New Zealand ZL1BDA ex G3NIA NZVRS