Tek 465 bright dot in center

[murphy-10332]

I was given a Tek 465 dated 1972 that had worked for a while and then only a bright dot appeared in the center. Nothing done with the knobs would affect the beam. I have a service manual but this has exceeded my knowledge and skill. Below is a bit of a list of what I observed.
Filter caps were bad and replaced
After the caps were replaced I had a bit of a trace that was x,y adjustable and time adjustments would effect the beam width. Then there was a buzz/hum around tubes ds1425 and ds1426. Also noticed r1431had broke solder joints and I re-soldered them. Fired it back up and the buzz was still present and then r1428 went up in smoke.
Q1413 had a shorted base and collector (legs were touching) separated them and it tested good. No affect on the trace after.

It burns up r1428 faster than I feel comfortable trying to get TP voltages.

Any help would be greatly appreciated. Thank you.

[tautech]

Welcome to the forum.

Sounds like a bit of a curly one so my advice would be to start isolating parts of the scope circuitry in an attempt to home in on where the problem is. I thought these scopes had something like that described in the manual...I'd better check.
Another document that's handy is this:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

http://www.sphere.bc.ca/ is also a good source for Tek bits if needed.

[tggzzz]

The neons fire when there is excessive grid-kathode voltage.

Have you ever been able to adjust the spot's intensity? I had a CRT which only did a good impression of a torch; see https://entertaininghacks.wordpress.com/2016/03/09/rescuing-a-broken-tektronix-465-crt/for how I "resolved" that.

If r1428 continually burns up then it suggests there is

• a short around c1427/r1427/vr1428/crt mesh, or
• cr1427 is faulty, or
• the 300V transformer output is faulty

I would disconnect the CRT at its neck (don't worry about the other connections) and see if that changes any of the voltages.

Be very careful about the voltages around there; they bite.

Ensure replacement resistors have appropriate specs for resistance (doh!), power, and voltage.

There are many bits of guidance available about 465/475 HV supplies - some on this forum, others on the Yahoo Tekscopes forum.

[murphy-10332]

Thank you for the responses.

I have attempted to follow the isolation/troubleshooting steps in the O-scope troubleshooting. I will continue to review that and the service manual as a I get more bits of information to apply to the text.

Yes the intensity is adjustable.

a short around c1427/r1427/vr1428/crt mesh, or

All components seem to be good. However the voltage across vr1428 is about 80v on start up and drops to 30v after a second or two.
I did not get a voltage going from r1428 to ground. I would have expected 300v there. Correct?

I disconnected the CRT at the neck and no change in voltages.
TP1518 is low at 107.9v
TP1568 is high at 0.79v

What are the chances I will blow up my Fluke 179 DMM poking around the HV supply?

I will take the new found info and go back to the manuals.
.

[tautech]

Thank you for the responses.

I have attempted to follow the isolation/troubleshooting steps in the O-scope troubleshooting. I will continue to review that and the service manual as a I get more bits of information to apply to the text.

Yes the intensity is adjustable.

a short around c1427/r1427/vr1428/crt mesh, or

All components seem to be good. However the voltage across vr1428 is about 80v on start up and drops to 30v after a second or two.
I did not get a voltage going from r1428 to ground. I would have expected 300v there. Correct?

I disconnected the CRT at the neck and no change in voltages.
TP1518 is low at 107.9v
TP1568 is high at 0.79v

What are the chances I will blow up my Fluke 179 DMM poking around the HV supply?

I will take the new found info and go back to the manuals.
.

Very high. Restrict measurements to OTHER THAN EHT (~2kV) and PDA (7+ kV).
That you have as trace indicates you need not check these at this time. PDA never really needs to be measured, it's either working or it's not.
There maybe a point where you need to measure EHT but again it is not required at this time.

[tggzzz]

What are the chances I will blow up my Fluke 179 DMM poking around the HV supply?

Look at the rating of the meter and probes. Make absolutely sure the voltages that might be present are well below those values. Ensure no surface contamination. (Hint: don't try it!)

If you want to see what a 12kV scope probe looks like, see http://w140.com/tekwiki/wiki/File:P6013A.jpg Compare the size of the light gray plastic with the BNC connector; it is large enough to kepp >15cm/6" between the hand and probe it.