Tektronix 7603 Focus Issue

[wlzed279]

I am having trouble with getting the trace to focus on a Tektronix 7603 Mainframe. I recapped the power supply a few months on this scope and it was working perfectly. I recently fired it back up only to find that the trace had lost focus. I went through the focus calibration procedure and had to adjust the Focus Preset control fully clockwise to its limit to bring it back into better focus but it is still a little soft.

Adjusting the front panel focus adjustment, Auto Focus bias and the Auto Focus Gain have no effect on the focus. The astigmatism is properly adjusted.

I am not sure where to start looking for this fault. What does fully CW Focus Preset and no response from the other controls tell me?

[BlownUpCapacitor]

Check voltage across R1193. Should be 180v. Ground referenced, -50v, +130v

Check voltage across R1045 too. Should be 100v DC. From ground reference, ±50v.

If they check out fine, check voltage at pin 2 and pin 3 of P1194 ground referenced. Should be around +25v at pin 2. Pin 3 will vary between 0-130v.

If those still check fine, measure voltage at anode of CR1255. High voltage will be present here!!!!! The voltage should vary depending on the focus setting. Report back.

[richnormand]

On my 7603 I had a fuzzy trace that would not want to focus appropriately although the scope was usable (but always a bit more fuzzy than expected from Tek).

It turned out to be due to a bad resistor in the "find trace" circuit.
When you press the "find trace" button it will reduce the x-y scan amplitude but also go out of focus a bit.

I noticed when slowly pressing the button that right at the transition I was getting a good focus for a fraction of a second.

That was a long time ago but I could dig up the scope from the garage if you get stuck. I think the schematics were on the both anchors site at the time.

Best of luck with it.