Tektronix 475 Repair - No Horizontal?

[Delekhan]

Hello together,

seems like I got lucky and got my first scope straight from the dumpster. Sadly that poor Tek 475 seems to be in need of repair . It powers up fine and I can see the beam in the center of the screen.



But here the problems begin. If I want to move the vertical position of channel 1 it does not behave like I would expect it. Instead of moving the dot vertically it moves diagonally across the screen from bottom left to the top right.

When I attach a probe to the calibrator output the same happens. Now I see two dots which I expect to be the peaks of the square wave outputs. Changing the Volts/div works as I would expect it. But changing anything in the timebase does nothing as well as changing anything in the triggering section.

Does anybody here have experience with those old scopes and can point me into the right direction? From seaching the forums I already found out that the main caps on these scopes tend to fail. I measured all voltages and they are spot on. With my cheapie Aneng multimeter I also see no ripple in AC Volts mode (except a couple mV). Sadly I do not have another scope to check the ripple


Thanks for the help
Markus

[tautech]

Is the scope in X-Y mode ?

[Delekhan]

Hi,

I guess you were kind of right with the assumption of the scope in X-Y mode. After searching around I found out that the V/div knob was the culpit. One part became disconnected from the shaft and only the back part of that huge selection drum was rotating. Screwed everything back together and now I have a signal! 

The only thing I am noticing is, that the buttons and dials are quite dirty. I sometimes I only get garbage on the scope and the trace looks washed out. I guess I have to clean all those contacts. Also there seems to be some high frequency ripple on the calibration signal. Guess it is the decoupling caps?

Cheers
Markus

[tautech]

Good, looking much better. 
Yes the contacts will need some exercise with just working them but if you are to physically clean them be very careful and just use IPA.

For any unknown ripple head for the PSU first and check all power rails are to service manual spec.
Often with these old scopes just getting the PSU rails to spec and adjusted exactly within spec bring the scopes accuracy back near to the day it was new and therefore there is no need to adjust anything else.

[jdragoset]

 :-+WOW!
Someone threw a scope that intact in the dumpster?
Crazy!

[Gyro]

The trace looks ok on the photo, but one reason that it may look washed out is that it is fitted with a clear screen filter rather than a Blue (or Grey one). You ought to be able to pick one up on ebay.

[tautech]

The trace looks ok on the photo, but one reason that it may look washed out is that it is fitted with a clear screen filter rather than a Blue (or Grey one). You ought to be able to pick one up on ebay.

I suspect more a CRO newbie mistake with Intensity set too high which makes the trace bloom.

[BrokenYugo]

Yes, keep intensity set only as high as is needed for comfortable viewing.

I don't see any ripple in that cal signal, the probes aren't compensated quite right but that's a probe problem and easily adjusted.

[bdunham7]

:-+WOW!
Someone threw a scope that intact in the dumpster?
Crazy!

I have to find one of those magic dumpsters.  When I was a kid, literally every electronic item we had in the house -- console stereo, TV, radio, shortwave -- was something I found at the dump and dragged home to fix.  Now it shows up on eBay or at Goodwill for 100X what I'd like to pay.