Tektronix 465b looses +15 and TEK 466 shorted -8, help!

[BenKeith]

Hi.  My Tekronix 465b, after many years started loosing it's display after being on, sometimes several minutes, sometimes 30 minutes.   Checking the LV supply, the +15 is dropping to 4.6VDC.  Need to isolate it to see if something is loading it or just overheating and dropping out.  I've downloaded the manual and it seems to just be a matter of trouble shooting it but if someone knows of common failures of this circuit it would be appreciated.

The pisser is I got out my Tektronix 466 and it would not give a display and then it gave that funky smell of a smoked part.  A Tantalum on the +15 smoked.  However a bigger problem is the -8 is shorted to ground.  Unlike the 465b manual, the 466 manual does not give you any info on how to isolate the -8 and the interface board looks like it's almost impossible to trouble shoot in the unit.  If anyone has any words of wisdom on how to chase this rabbit, it too would be greatly appreciate.

[tggzzz]

There is a lot of information on this forum and the TekScopes group; it would be a waste of time for me to repeat it.

Look for smoked components; the damage is usually visible.

The switching PSU measures the balance between the various power rails. If there is an imbalance, it shuts down. That's all described in the downloadable service manual.

If one tant has blown (typically because the voltage rating is too low) then similar ones are likely to blow. Decoupling tants on an X volt rail should have a >2X volt rating (some Tek scopes have 15V tants on a 13V rail, oops).

Large electrolytic caps can "dry out" and exhibit high ESR. Replace those with new caps; there is much info available as to how to choose replacements.

[tautech]

From the first post in a sticky in this Repair board:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf
 

[EPTech]

Hi There,

I think this is the exact same problem I had with mine. I remember replacing a tantalum cap. It was somewhere on the top PCB in the corner. Just follow the power rail coming from the supply. I remember it being one of the first tantalum caps I came across following the path of the supply. If you have a thermal camera or IR thermometer. A semi-shorted tantalum that causes that kind of voltage drop will surely get hot.

Happy hunting.

[bd139]

Capacitors capacitors capacitors. Grab the service manual then work your way round all the tantalum capacitors on the 15v rail with a 4.5 digit or better DVM and look for the lowest resistance, snip that capacitor out and try powering it up. Look around the bottom of the interface board (the large one) where the power supply is. It's ALWAYS those damn capacitors...

Some typical problems here which I had with my 475 and 465B's:

https://www.eevblog.com/forum/repair/tektronix-475-repair-assistance/

https://www.eevblog.com/forum/repair/tektronix-465b-repair-97327/