Tektronix 475

[VK5SRP]

I have a Tektronix 475 and I cannot get full screen linear deflection from the Y amplifier
I have just finished fixing the vertical mode switch, the locking bar in the switch had jamed allowing two buttons to stay down.
Cleaning that area of the switch and lubrication it with tiny drops of plastic compatible oil has fixed the problem.
It has not been used for several years and now I the two Y position controls cannot move the trace to the bottom of the screen and I tried the proceedure in the

manual adjusting R176 and R235 and this did not allow me to move the trace up and down the full screen.
When I feed a clean sine wave signal into the Y inputs I can only get a clean sine wave display when the amplitude is less than one line delow the top and one line

above the bottom.

MY guess is I have to carry out some of the other alignments !

[dave_k]

1st rule of fixing 475's - thou shalt check power supply rails for correct voltage and ripple (as specified in the manual). 

Next question, is the problem applicable to both input channels, or just one? This will tell you if the problem is with one of the pre-amplifiers or output amplifier.

[tggzzz]

When I feed a clean sine wave signal into the Y inputs I can only get a clean sine wave display when the amplitude is less than one line delow the top and one line

And what happens in the top division?

I presume you have carefully looked at and around each and aluminum and tantalum capacitor to see if there are any signs they have failed. Tek sometimes used 15V tants on 13V rails, which is pushing it - in my 485 one failed short circuit and destroyed a nearby resisitor.

As dave_k mentioned you should check the PSUs for voltage and ripple. Be aware that usually one voltage rail is the "master" and others are relative to it.

If you have to replace a PSU cap, consider the Nichicon PW series, or any other marked for "switching PSU". Search on this forum for suitable parts; 465 and 485 scopes have similar issues.

[tautech]

A little gem that many have found useful:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

[MrSlack]

A someone said, check the voltages. Do this on the vertical amp board rather than the test points on the A board. They are marked on the edge connectors. If they are out of tolerance, check the A board voltages. If they are fine, check the tant capacitors on the amp board for low resistance - they snuff it all the time. If the A board is out, check the main filter caps on the 55v and 110v rails first. There is a nasty wet tantalum near the bottom of the A board that blew up on one of my 465B's.

These things don't tend to drift a lot so something is knackered somewhere. Divide and conquer but always start with the power supply.

Also don't use oil on any contacts, just IPA.

[Andy Watson]

Have you checked the operation of the beamfinder contacts? It requires the contact to be closed for normal operation.

[tombi]

Not sure (have to check schematics) but I wonder if the trigger view feature will tell you if it is the front end or the deflection amp?

My thinking is if trigger view is clipped then it is a deflection problem but if not then it is a front-end problem.

I could be dreaming.

Also my manual seems different to yours - did you try going through the calibration procedure? In particular step 23 where you adjust the vertical gain (R449).

R176 is just centering and R235 is used to adjust the vertical positioning between the 2mV and 5mV ranges. I think in other ranges these will have no effect.

It is worth going through it in order however.

Tom

[VK5SRP]

Thanks to all who have replied to my post, I have not had time to get back that project but I am sure your posts will be a BIG help.
The first thing I will do is to check the supply voltages and have a closer look at the Tant caps.

Oh and yes, the problem must be in the output stages, as the symptoms are the same on both channels.
The wave form when displaying almost the full size of the screen is peeky at the top and rounded at the bottom !

Will get back to it soon, thanks again.