Tektronix 465 Repair (CRT Not Working)

[nicalitz]

Hi All,

I was fortunate enough to get my hands on an old Tektronix 465 oscilloscope. It's been standing in storage for quite some time now. Powered it on, did a few basic measurements with a signal generator. It worked, but intermittently. It would measure just fine the one time, and the next the CRT would display something that looks like white noise.

After using it for a while, 15 minutes or so, I was measuring a few basic voltage levels when the line on the display started losing focus, and then just went blank. I haven't managed to get the CRT working since. The unit seems to work, all the light and indicators on the front panel displays correctly, just nothing on the screen.

Any suggestions on where to look first, or what the most likely causes of failure would be? Any help would be appreciated. I've never attempted a repair project of this scale, especially working with/around a CRT.

[dave_k]

Search the eevBlog forum .. there are many threads about repairing the 465.
Also join the TekScope group on Yahoo, good advise there too.
Manual is available at http://w140.com.

[bd139]

Fixed a few of these now.

Grab the service manual first and check the power supply voltages. They are all conveniently accessible on the main “interface board”. The power supply capacitors or tantalum capacitors dotted through it are always the first things to go wrong. I think about 80% of the problems I see on scopes that old are capacitors!

With respect to safety, the interface board has a few hazardous voltages around it. If you are unsure about measuring these safely, power it down, wait for at least a minute, attach your test leads to the device and only then power it up. The HT section is underneath a big metal shield that is labelled clearly - don’t futz with anything in there (yet).

[nicalitz]

Thanks for the feedback!

I've managed to find quite a few suggestions on which caps tend to fail most often. Was just hoping to get some confirmation from someone who has some experience with the 465.

Will see what I can do tonight. Hopefully an easy fix!

[nicalitz]

I've tested all the low voltage power supplies. The +110V supply is slightly low, around +105V. From the service manual, the supply limits are +107.8 to +113.4. Would diagnosing the +110V supply be the best way forward?

Assuming the +105V is not my problem, the next step as per the service manual is to 'Check Z Axis TP1486 for an unblanking pulse (+15V to +75V, depending on the setting of intensity control). Excuse my limited understanding, but what exactly is an 'unblanking pulse'? I've attached my measurement, as well as the expected pulse as per the service manual. I'm unsure of the constraints of such waveform. Is the measured response 'acceptable'? I've also noticed that if I turn the intensity knob down, the square wave disappears well before the knob reaches a minimum value (as the high voltage [cursor A] drops down to +15V [cursor B]).

I've measured various caps around around TP1486, none seems to have been shorted.

Any suggestions?

[nicalitz]

Also, is my understanding correct that the pulse should be between +15V min and [+15V to +75V] max? Or should the signal be between ground [min] and [+15V to +75V] max?

[bd139]

To confirm, are all the other voltage rails within tolerance?

Unblanking pulse is one you need another oscilloscope to look at. Basically that fires during the sweep of the bean across the screen from left to right and controls the intensity. 

[nicalitz]

All other low level voltages are within specification (i.e., the +55V, +15V, +5V and -8V supplies). I unfortunately do not have the means to verify the high voltage supply.

[nicalitz]

Regulated voltages:
-8V  =  -8.01
+5V  =  +4.98
+15V  =  +14.98
+55V  =  +55.0
+110  =  +105.1 (+107.8 to +113.4 allowable)

I'm still struggling with the +110V line. I've measured the +55V unregulated line, it seems to be approx +80V. This is reasonably close to the +78V from the schematics. I would therefore expect the +120V unregulated line to be at around +160V. I however only measure +137V or thereabout.

Anyone have any suggestions on why this could be? Any suggestions would be greatly appreciated!!

[bd139]

Odd. Could be half of CR1511 dying or C1512 high ESR (C1512 is notorious for dying). Do you have another scope at all?

I'd look at the 55 and 110V rails (AC couple this!) and see if there is a ton of ripple or not. You might be able to get away with this with your DMM set on AC volts for an approximate reading.

[nicalitz]

Measured both lines, AC coupled. The +55V line is clean. The +110V line, not so much. Any idea why this would be?

EDIT: I'd assume a problem with the rectifier?

[bd139]

Ah yeah. I’d replace the rectifier and cap. High ESR cap causes the rectifier to commit suicide usually.

Get a couple of rectifiers in. They do die with alarming regularity. You should be able to replace it without having to extract the interface board.

[nicalitz]

I guess that would be a good start! thank you so much!!

Any other 'notorious' components that I maybe should replace while I'm at it?

[bd139]

Yes C1534. It’s usually an axial tantalum unit. I had one of them blow up on me and make a mess. A 10uF 100v electrolytic will do the job. Vishay sell suitable axial leaded ones.