465m shorted 95V problems

The scope is a 465M, S/N is B056721 but it looks like that may not be original, assemblies have been swapped etc..

The problem: the scope had been working fine until one day the crt was dead. Fuse F558 was blown, (it goes from the 32V unreg to the HV oscillator circuit). All other supplies are fine.

I first unsoldered pin 10 from T550 (feeds the 3x multiplier), but the

95V supply still wouldn't come up. I then checked Q556 & Q552, the transistors driving T550, both are fine. I then checked the caps and transistors in the 95V circuit, all fine. I then unhooked the jumper leading from L582 to the rest of the scope, still no luck. I then took out the crt (both the plug on the back and the anode lead), and the supply came up with no problems. 94.5V, .07A, negligible ripple.

My readings are as follows. Anode off & Rear Plug off: 94.5V @ .070A Anode on & Rear Plug off: 64.2V @ .54A & climbing Anode off & Rear Plug on: 26.7V @ .427A & climbing Anode on & Rear Plug on: 21.6V @ .460A & climbing

Is this probably a bad tube? I don't have a whole lot of experience with scopes, as this post would indicate, and don't want to get a new crt if it's not going to fix it.

Also, just to make sure, there isn't any alignment procedure when swapping scope crt's, is there? I don't figure there is, just want to make sure.

I have another 465m, but the same fuse is blown, and I haven't gotten into it yet.

All thoughts appreciated.

Thanks, Steve

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Steve
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Well, I replaced the tube with the one from the other scope, and it works just fine. So, my CRT must have gone bad, and the other scope must have some deeper problems (I'm not even going to attempt to fix it, it's best suited to a parts unit). What causes a CRT to just die? It was working fine one day, the next day it's causing fuses to blow. Now I'm curious. I always though they just wore out, lost brightness, filament burns up, etc..

Again, all thoughts appreciated.

Thanks, Steve

Reply to
Steve

check the heater to cathode, it may have shorted.. that is a common problem in CRT's Years ago, we use to isolate the heater circuit so that it wouldn't ground any more and be able to still use the TUBE>

it's also possible a grid wire could have broke in side.

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Jamie

Steve wrote in news: snipped-for-privacy@4ax.com:

Yes,the whole scope has to be recalibrated.

The CRT should not draw any appreciable current from the 95V supply.

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Jim Yanik

Steve wrote in news: snipped-for-privacy@4ax.com:

You can get CRT elements shorting to others in the electron gun structure. I've seen CRTs with the glass rods cracked or broken,the mesh lens collapsed,glass particles stuck to the screen phosphor making dark or intensely glowing bright spots. I've seen geometry extremely distorted,so when you put time marks up,the straight lines are warped terribly.

Sometimes people stand the scope on it's end while using it(field service),and then accidentally tug on a scope probe,and the front of the scope goes crashing down.

BTW,the 465M was a "ruggedized" scope to use in lieu of a regular 465,but NOT the same circuitry,being more similar to a 455A2B2.

My experience was that they were not as rugged as a regular 465,the plastic case/chassis flexed and caused problems.

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Jim Yanik

Jamie wrote in news:Bzu1i.51$A snipped-for-privacy@newsfe06.lga:

you WANT the heater to be at the same potential as the cathode,so it will not arc over.

Most likely,his geometry or astig elements shorted to another element. IIRC,the +95 went to either one or the other.

The scope probably took a hard shock somewhere.

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Jim Yanik

You are right, the new tube only draws ~30mA extra. In the schematic I have, 95V goes to geometry, 32V to astig. I am sure this scope has probably seen many bumps. I didn't check the date code but I'm sure it's at least 20 years old, probably older. However, it was sitting still on my bench when it stopped working, so I guess something might have just given out after being previously broken.

I agree with you, the case isn't rugged, I simply bought the pair because they cost $20.00 each. I would much prefer a 465b, but I already have seven scopes and have to stop collecting sometime.

Just out of curiousity, the supply was still drawing too much current and the 95V was loaded down when I only had the anode lead connected. Why would this be if the astig or geom elements were shorted? Is this due to the absense of the anode at the other end? Just curious.

Luckily I work at a cal lab so I can probably put it on the bench for cheap. Unluckily, I will be the one calibrating it and this procedure looks a bit lenghty. Oh well, good experience.

Thanks for your help, Steve

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Steve

Steve wrote in news: snipped-for-privacy@4ax.com:

the CRT is not drawing that 32 ma from the 95V,the geom element is an electroSTATIC element,and should draw only ua.

Makes me wonder if you don't have a HV multiplier problem. If the rear socket is off,the geom and astig elements are unconnected,and the 95V is not going to the CRT at all.Nor the negative cathode V,just the

12-16KV anode supply.

Or the anode connection inside the bad CRT has some sort of problem,drawing too much current from the multiplier,and that is why the HV was loading. I suspect this is the real problem.

Well,if you work at a cal lab,then you have all the proper cal equipment! The procedure is not that hard. You want hard,try a 7844 dual beam 7000 series mainframe,or a bistable transfer storage scope like a 464/466. I HATED those 2 scopes.

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Jim Yanik

The 30mA I was speaking of was just the difference in draw on the 32V unreg supply with no tube in vs tube in, so really just an observation I guess.

I've never tried those two scopes. We see a lot of the TDS scopes, I love those. Usually received in great condition, cal takes about 20 minutes, can't complain about that. I used to hate cal'ing the analog scopes when I started, I was spoiled with the easy DSO's. I'm starting to come around though, the amount of engineering that went into them is amazing and they're interesting to work on.

This has sparked my curiousity. When I pulled the jumper off the 95V rail, it didn't fix the problem. If the schematic is correct, that should have isolated everything on the 95V supply, leaving the other items sharing the transformer to blame (tripler, cathode, etc). With no tube in, everything worked fine, so I would figure the HV module was ok (maybe a bad assumption). So it must be in the tube (again, maybe a bad assumption). With just the anode connected, the supply is loaded down. With just the rear plug connected, the supply is loaded down. With another tube, it has no problem. If it was just a anode lead problem, shouldn't the supply have come up with just the rear plug on? I may be beating a dead horse here, I just like to learn.

I think I'm going to pull the bad tube out of the bad scope I put it in and see if I can't find the problem. Would doing continuity measurements show anything useful, or would the likely problem be too high in resistance for say, a Fluke 87?

Thanks for all your help. Steve

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Steve

Well, not sure if it means anything at all, but I couldn't measure any resistance with my meter between any pins. I know the meter puts out small DC voltage, and things become a different story at kV, but I did it anyway. Visual check of the tube looks fine. I was going to hook up a little 8kV transformer I have lying around between anode and cathode, put a couple volts on the filament and see if I got a trace, but then I started thinking about XRays. Would doing this out of the metal shield be a bad idea? Would this produce XRays or radiation? I don't want to give myself cancer or cause damage. Also, would this even produce a trace or am I just wasting my time?

Thanks again, Stev

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Steve

And by trace, I mean dot. I know there would be no deflection or focus or alignment or astigmatism or anything else applied. And, the heater/filament measured about 13.4ohms.

Steve

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Steve

Steve wrote in news: snipped-for-privacy@4ax.com:

You might want a diode in there and maybe a cap to smooth the dc a bit.

Be sure and tie the cathode and filament together, or at least keep the voltage between them fairly low... 8kV might not be good for the insulation.

I doubt that you are likely to get xrays until you get up above 20 kv or so. You might get some nasty UV or very soft xrays at lower voltages, but the crt's glass will stop most, if not all.

But you can sure kill yourself with the voltage.

The metal shield is probably mu metal, to keep stray magnetic fields from messing with the crt beam.

You have a strange problem!

When I pulled the base off of my 545 scope's crt, the HV supply didn't like it at all. With no load, it started snapping and sparking from the multplyer chain to the HV supply's case!

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bz

That would make sense if there was no "good" return path for the hv. Interesting. I think I may just move on to bigger and better things, the scope's working for now, and I have a spare with some really serious issues (I was debating trying to get two working, now I really don't think it's worth it)

Steve

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Steve

Yes, quite. I suspect a short from final anode to something else in the tube is possible, though its not a common fault. If youre lucky the short may be external, and a careful clean of glass round the anode connection should fix it. If less lucky, it'll be an accumulation of crud inside shorting it. Internal CRT shorts can often be cleared by hitting them with moment of of high V high i, and flicking the tube at the same time.

NT

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meow2222

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