Intensity ripple on Tek 465B

Maybe check the LV first. The bridges on this scope tend to go half-bad, turning the bridges into half-wave rectifiers. Also the filter caps can lose capacitance. Put the scope on a variac, turn the variac down to 100 volts and put a scope on the PS test points. If you see 60 Hz ripple, then it's the diodes, if it's 120Hz ripple, its the capacitors.

The HV power supply has like a voltage quadrupler. Maybe a diode or two has shorted there?

Oh, there's also a big voltage divider resistor (the white ceramic thing) Any moisture, dust, or leakage there and you tend to get unstable HV.

"Ancient_Hacker" wrote in news: snipped-for-privacy@e56g2000cwe.googlegroups.com:

I agree;first,assume low voltage power supply problems. EVERYTHING works off them.

(Sam,you oughta know this!)

Not likely,as reduced anode HV would result in a much dimmer and enlarged displayed signal(in both dimensions),and the cal would be greatly affected. Also,I don't think the HV would continue running with a shorted diode in the multiplier. I believe the HV fuse would blow first.

BTW,at some point,increasing intensity will definitely affect focus. The CRT cathode emits from a larger area,and gets worse as the CRT ages.

--
Jim Yanik
jyanik
at
kua.net

"Ancient_Hacker" wrote in news: snipped-for-privacy@j52g2000cwj.googlegroups.com:

The HV thick-film resistor network(TEK-made) has the HV feedback for regulation on one side,and the CRT focus divider on the other. Either one can go out-of-tolerance,and the metal pins can crack and be intermittent.I've never been successful in resoldering them,either.

I do not think this part would be the cause of his intensity modulation.

--
Jim Yanik
jyanik
at
kua.net

Followup: The frequency of the chopped line does NOT correlate with line frequency.

And I should note that at low intensity, it is worst going full off. With the intensity turned up, it's not visible. There is minimal if any geometric distortion of the trace (outside of the focus problem, which doesn't always appear) so I doubt any LV power supplies are involved that also feed the amps.

It's almost as if the Z input has some oscillation on it. When I first noticed, I thought maybe the chopped blanking was misbehaving, but this occurs on all vertical modes.

It's also related to warmup. At power-on, there is no problem. It takes a of couple minutes to show up. The fan is running.

And even then, the severity varies randomly.

So, what could affect the Z modulation AND sometimes focus, but not geometry?

If it was the HV divider I guess a parts unit is the only source for a replacement?

Thanks.

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I only have the manual for the 465 , not B variant. But assuming ps similar I would suggest a problem leading to ripple on the unreg 120V supply rail (bridge, caps )feeding the EHT oscillator/driver circuitry or problem in that oscillator circuitry or of course, wallectomy time, problem in the EHT step-up transformer

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Sam Goldwasser wrote in news: snipped-for-privacy@saul.cis.upenn.edu:

Perhaps HV oscillator freq?

Then you're down to decoupling caps.

A thought occurs to me;there's a 1uf/150V(IIRC) electrolytic on the wiper of the CRT grid bias pot,to ground;that could be going bad.

Yes,it was a TEK-made part,from their defunct hybrid/ICO in-house plant. Removing a good one from a motherboard without damaging it is not an easy task

--
Jim Yanik
jyanik
at
kua.net

I don't think so. It's order of a few hundred Hz and not constant.

I'll check that.

Now it really doesn't like even normal intensity.

But the geometry (beam stiffness, etc.) is not affected. So, I'm leaning toward a problem in the grid circuit, not the HV itself.

Does anyone know of a free source for decent quality schematics? I have several complete manuals downloaded from various places but the resolution of the schematics tends to be terrible and only marginally useful.

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I think we have the 465b manual at work, I'll have to check on Mon. If we do, I'll only be able to copy a couple of pages so I'll just be able to get the power supply and maybe a few others, but I'll give it a shot.

Thanks. That would probably be adequate. There's no problem with the text of the manuals, just the schematics. And this problem would seem to be associated either with the power supplies or the CRT bias/intensity/Z input circuits.

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***

I found the 465B manual from bama.sbc.edu is nice: I can read the schematics easily, and quality is alright for my repair purposes. Sam, did you try this free source? don't forget about bama mirror at edebris, for faster download.

Only thing you have to do extra is that you have to install the free reader, for .djvu format. Even if it is old, it still works adequate.

hth, Andreas

That looks much better than the ones I have. Thanks. I knew about the bama Web site but never bothered to download the djvu reader before.

Any hints on getting pages to print with reasonable quality?

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Check your mail.

I've got the schematics if you still want them. I was only able to get horizontal amplifier, crt circuit, power supply, and the power supply troubleshooting sections. Let me know...

Steve

text

Thanks but I think what I was able to get from the bama Web site is quite legible, if a pain to print. Printing from within the djvu reader results in poor quality, at least with my printer. I now export the page to a .bmp file and print it in sections. I think all I should need is the CRT circuit and I've printed that.

I'm just surprised that with all the 465Bs out there, no one has come forward to say they've seen this problem before.

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text

Glad you got what you needed. It is a strange problem that you have, I hope you can track it down. My T-935A has a problem sort of similar to what you're describing, but it is by no means periodic noise and seems to be much more like a dirty intensity pot. It has a chopped appearance, but just looks like dirty pot noise instead of what you're describing. I haven't had any desire to take it back apart, too many other things to do. Best of luck.

Steve

It may be.

New followup: I've now got the thing open. Since it appeared heat dependent, I used a rubber tube to blow on various components in the HV area. Blowing on the thick film resistor network even immeidately after power-on and the beam appeared (already messed up) seemed to make a big difference causing the symptoms to disappear entirely for awhile. This could not have been heat related because it was within 10 seconds of powering up so nothing really could get hot. Then, after touching the thick film assembly perhaps with a few wiggles thrown in, the problem has disappeared entirely and has not reappeared. I've since cleaned it with alcohol and I will be running the scope off and on for a few hours over the next few days on the bench to see what happens.

Of course, the instant I put the cover back on and replace the scope in its hard-to-reach spot, it will screw up again. :)

So the working hypothesis is that indeed either contamination or cracked joints to this part.

If cracked joints turns out to be confirmed with the problem reappearing, I would probably try conductive silver Epoxy first at each of the connections to the thick film itself rather than solder. If that didn't last, then what about fashioning a replacement from discrete parts? It might be a bit of a challenge to get high voltage resistors to fit in the space but seems like it could work. It's only 4 resistors, though admittedly 3 of them are high resistance (6.57M, 25.6M, 24.5M, and 550K) and at least 2 of them need to withstand almost 3 kV.

Any comments?

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dependent,

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Some woven glass sleeving , cut to required lengths and formed into a compressable and insulated pad or more pads if you cannot localise better and cautiosly compressed by cable tie/s around the thick film ? I've only done similar on low V SM ICs that are covered in black epoxy , boards as well, so impossible to re-do the SM solder , to the ceramic boards, and is a throw away job otherwise.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Sam Goldwasser wrote in news: snipped-for-privacy@saul.cis.upenn.edu:

I've wondered if low melting point solder like what comes in the ChipQuik desoldering kit might work on thick-film connections.It wasn't available to me at Tek.The conductive silver epoxy may be a better idea. Trouble if you have to resolder the thick-film back into the motherboard.

Alternately,you could fashion a small PCB to fit where the thick film goes,and solder your resistors to the PCB. I've not tried this,though.

--
Jim Yanik
jyanik
at
kua.net