Tektronix 475 Repair - Weird Trace and Focus Issues

[wlzed279]

Hello All,

I am a relative newbie to repairing test equipment. I secured a clean Tek 475 last year from a garage sale to help me learn to align am and fm radios. Is is a beautiful scope and after just a little cleaning, it worked perfectly.

I recently fired it back up agin to work on another radio. I let it warm up for a good hour and when I returned it was dead. No power, no lights, no trace...nothing. Turns out the 50v low voltage power supply had a dead short. With the help of you guys on this forum, I tracked it down to a shorted tantalum capacitor. Replaced it with ease and fired it back up.

Now it powers up, I have a trace but something is off. The trace is distorted and the focus control is acting up. It used to defocus on either side of the centerline of the control but now at will only come close to focusing by turning the control all the way in one direction. The trace too is distorted. I am getting a vertical signal the changes deflection with controls but it is very noisy on distorted.

All the LV power supply voltages are within range with the exception of the 15v. It is measuring 18.9v.

I read on another post that the resistors around the focus control can be suspect. I am at a loss as to where to begin looking.  Can someone point me in the right direction?

[tautech]

All the LV power supply voltages are within range with the exception of the 15v. It is measuring 18.9v.

The same rail the tant was on ?
Tants don't like ripple so check the 15V rail for ripple and expect to have to replace electrolytics.

[wlzed279]

No the replaced tant was on the 50v rail. Can I measure the same test point on the A9 board and flip my meter to AC to test for ripple?
 It is just an average meter.

[tautech]

Can I measure the same test point on the A9 board and flip my meter to AC to test for ripple?

Sure you can.

[wlzed279]

So I am getting .5 mV on all the rails with the exception of the 110v rail. I am getting 2.0 mVolts on that one. Is that enough ripple to cause this kind of problem?

[tautech]

So I am getting .5 mV on all the rails with the exception of the 110v rail. I am getting 2.0 mVolts on that one. Is that enough ripple to cause this kind of problem?

Not IMHO.
RTFM
https://w140.com/tekwiki/wiki/475

[andy3055]

Try to get that 18.9 volt line down to the correct 15 volts. That is too high.
Download the manual from Tek wiki,  if you don't have it.

[wlzed279]

Looking through the manual now. Thanks all.

[wlzed279]

Thanks all for you assistance. I tracked it down to a bad transistor in the 15v low voltage power supply. Replace it and I now have 14.98 volts and a happy in focus trace.