Repairing the Tektronix 465 Oscilloscope

[BrownOut]

My Tek 465 Oscilloscope that I bought off of E-Bay a few years ago has developed a very annoying problem. About half the time I'm using the instrument, the sweep is not visable. It just goes out for no apparent reason. Nothing I can do from the front panel infulences this behavior, although it comes and goes and fools me into thinking I have some control over it. I've switched to a brand new B&K precision 2120B for most of my lab work, but while trying to develop a 100Mhz transmitter last night, the limited bandwidth of the new scope became a problem. So I have to fix the Tek. I began to look for technical info on the web last night, and to my great surprise, I was able to download a service manual, with all schematics, assemply drawings, etc. It came as 15 seperate compressed files, and required me to download special decompression software to extract it. I figure to start by looking at three possibilities.

1) Z-Axis signal intermittency

2) Sweep signal out of spec

3) Trigger signal missing

I've printed the sweep and trigger circuits. Just to make things more difficult, these schematics were originally long, fold out drawings, which were scanned piece by piece. I've studied them late last night, and think I have a good idea how they work. I'll began troubleshooting later this week. I'll be using the B&K scope to measure signals.

Open to comments or suggestions. Any cautions about signals that should not be measured. I'm very familiar with electronics and troubleshooting; just never tried to repair a scope before, though I have repaired CRT circuits. I'd like to hear from anyone who has tracked down this kind of problem before.

I'll keep this thread updated for those who are interested.

 

[bountyhunter]

If the entire display goes away, that is the HV section of the power supply that creates the high negative voltage to fly the electrons from the gun to the screen. Don't measure it unless you have an HV probe.

It's possible that the retrace blanking circuits could screw up and blank it continuously, but the HV supply is the most likely culprit. If the numerical displays remain on screen, the HV supply is working and the horizontal sweep circuits are failing to sweep or they are not being triggered..

 

[BrownOut]

Do you know of an alternative way of verifying the high voltage than buying a probe that I might use once in the remainder of my life? I remember in High School electronics shop, we used to draw an arc off the HV lead of TV sets with a screwdriver. Any other method?

PS: The scope has only the sweep displayed, no numerical display to help isolate the problem; however, when the display is not visable, the "beam find" doesn't work either.

 

[bountyhunter]

I think the HV would be in the ballpark of maybe 5kV to 10kV (?). You could build a divider ladder using some 1W or 2W carbon resistors like 1MEG ohm each. I recall the larger size resistors can handle about 1kV each? I think if you stack ten of them in series with a smaller resistor on the very bottom of the string (like 100K) you could make a 100:1 divider. I'd solder the thing into a string and use PVC shrink wrap to insulate them. Use a piece of HV wire out the top (probe) end and hook the other end to ground. The ground end should have the 100K resistor to ground to hook your meter across.

 

[BrownOut]

I'll hook up my cheapest meter first I also remembered while I was thinking back that we used to tape a small neon bulb next to the HV connector, and sometimes it would light. I'll give some of these techniques a try on a night I have more enregy.

 

[mvs sarma]

I'll hook up my cheapest meter first I also remembered while I was thinking back that we used to tape a small neon bulb next to the HV connector, and sometimes it would light. I'll give some of these techniques a try on a night I have more enregy.

Get hold of the circuit schematic.
In general, the high MEG resistors that are used in EHT section show up open circuit.
Try to replace then with proper value and wattage, as measuring them may not be so simply possible. Perhaps it restores.
Last but not least, at the time the trace becomes invisible, just check whether the CRT filament is glowing. It gives a check of power supply condition.