Oscilloscope CRT problem

[etnietering]

Re: Oscilloscope CRT problem

Where might this resistor be? Here's the link to the schematic again:

[Wizard]

Re: Oscilloscope CRT problem

fading to the right is bad filter or weak resistors for the "screens" (like G2) that let emissions to droop with each sweep.

Cheers, Wizard

[etnietering]

Re: Oscilloscope CRT problem

Well here's a kinda bad picture of it working. The trace on the screen is brighter than that, it's just my camera can't capture it properly....oh well


that signal is the 12v output from my logisys with a 4A load on it...almost 150mV @ about 2kHz! Yikes!

The problem of it getting dimmer on the right isn't too bad - I'll be able to live with it. Thanks everybody here for your help!

[i4004]

Re: Oscilloscope CRT problem

btw. how old is that scope?

i have pretty old hameg and it doesn't have those trimmer oxidation issues...
http://picasaweb.google.hr/i4004b3/I...su_2008_10_19#
its schematic is dated 22.5.1972.
yay...older than me!

[etnietering]

Re: Oscilloscope CRT problem

I found the 'trace rotation' pot and fixed it, so now the trace is level. I'll have a look at the schematic and play with the other pots to see if I can fix the fading out problem...I can change the X position setting, but that just chops it off at either end. I'll see if I can take a decent picture of it too...

Thanks for you help everybody!

[i4004]

Re: Oscilloscope CRT problem

>1) the screen fades out from left to right

interesting.
brightness(ie beam current) shouldn't be modulated at all, but it seems something is changing it in your case....
could it be that another trimmer(or potentiometer) is borked from too much waiting...heh

or the deflection is doing something it shouldn't (some extra wobbling or ringing when beam reaches right side) and that makes beam look dim...
or is the same circuit(h. deflection) accelerating beam while it shouldn't...

you know, that scope should have all lytics checked with esr meter.
lytics drifting can cause such weird behaviour. better to say they're the first suspects.


btw. could we wee some images of the trace now that you got something?
don't put them inside the post, leave them as thumbnails if you're gonna post big pics.
(ie just upload the pic to the post and that's it)


oh yeah, well done!

[xtraelectronics]

Re: Oscilloscope CRT problem

Awesome! There should be a"tilt" control on the unit somewhere, that copmensates for the earths magnetic feild and other stay feilds in the vicinity. I had to adjust mine while it was on the bench, and again after it was back on its shelf.

[etnietering]

Re: Oscilloscope CRT problem

Well I have an update! I checked the -12v and +12v going into the high voltage circuit, and they're both there, and pretty close to their expected values. I was then looking around the schematic for where the 2kv line is (it's different from the 1.9kV line) and I found it. And the intensity knob is adjustable with 2 pots on the circuit board, so i fiddled with them, and now I get something on the screen I can see! First it was just the little blob moving back and forth, but then I fiddled with things more and now I can see waveforms! There are still a few problems though...
1) the screen fades out from left to right
2) there is a definite angle upward on the screen. I'm using the 2v p-p square wave it has built in, and if I set it so on the left side it goes from -1v to 1v, then by the time it gets to the right it looks like it's from about -0.5v to 1.5v

any further thoughts?

[xtraelectronics]

Re: Oscilloscope CRT problem

Jeez I hope the schematic is wrong- the 100uF/50v cap across the +12/-12v is backwards! The IC is a common OP-Amp. (741). Looks like the corresponding resistor to the 2120 is R952, 51ohms. Check it and the zener. H.V. for the anode is off of junction of R950, and HR959.

[i4004]

Re: Oscilloscope CRT problem

>I don't think the HV is from the mains transformer look at that yellow high frequency switching transformer in one of the pics.
To the right of the transformer is probably it's switching FET. The socketed IC nearby probably delivers the pulses to gate of the FET.
This is probably how the HV is developed.
Devleoping HV at 60Hz would require more copper and a bigger transformer.


yes, that's what i think too, but indeed in the el.diagram there is "from pwr trafo" caption.
and pwr trafo is just the mains trafo.
that other trafo has different "name".

.
wait...ah yes...i see it now
https://www.badcaps.net/forum/attach...chmentid=11561
small trafo under the mains socket. where his blue line ends.

yes, that's where HV should be coming from and that circuit should be checked.

that voltage is probably missing or heavilly dampened.

that circuit itself shouldn't be too hard to check...
it has + and -12V supply that feeds op.amp and then that drives the transistor and the trafo primary...



page6

[xtraelectronics]

Re: Oscilloscope CRT problem

The circuit boards are different in the 2120. The circuit itself is very similar. Test for voltage @ the IC, then check the device on the heatsink for voltage. It may trace back to a resistor elsewhere on the board that supplies this circuit. That bulging cap could have caused a failure upstream. Just my 2 cents.
Dan

[Krankshaft]

Re: Oscilloscope CRT problem

I don't think the HV is from the mains transformer look at that yellow high frequency switching transformer in one of the pics.

To the right of the transformer is probably it's switching FET. The socketed IC nearby probably delivers the pulses to gate of the FET.

This is probably how the HV is developed.

Devleoping HV at 60Hz would require more copper and a bigger transformer.

[i4004]

Re: Oscilloscope CRT problem

about further prospects: you need something to measure that 1.9kV voltage, and that trick with bulb that xtra mentioned i doubt would work here as you don't have typicall flyback trafo, but it seems HV is derived from mains trafo...

also, i don't know if that method would work when secondary of the trafo is open, ie you would have magnetization, and lamp maybe lit, but that wouldn't mean trafo is ok.

are you sure HV is coming from mains trafo, and there is no other trafo there?
inspect pages 6(power) and 11(crt section) of pdf.

about the outlook for repair; well yeah, if the trafo itself is not producing HV then it could be a problem to find such trafo elsewhere...

as for this "new" scope on ebay, well it surely is better as it has some return policy..heh...
but i dislike that housing, no legs etc.
i would get something that wasn't taken out of some industrial rack....

[etnietering]

Re: Oscilloscope CRT problem

Well guys, I had an idea. I'm thinking I should just forget about this one, and try to get a different one that actually works. All this one will do is frustrate me, and chances are I'll never end up with a working scope....which doesn't do me any good. I'm still going to try to get my money back, and I'll either scavenge it for parts (although I won't get many), or sell it as 'needs repair'

What do you guys think about this one instead? http://cgi.ebay.com/Tektronix-2213A-...d=p3286.c0.m14

[etnietering]

Re: Oscilloscope CRT problem

Well I found the wire that's supposed to have the -1.9kV on it, which I assume is to drive the anode. It comes straight from the power transformer, apparently. That's where the wire comes from, and where the schematic says it should come from....weird.
Also, I think I do in fact have a neon bulb. I bought an ion ray gun from amazing1 last year, and it came with one, to use as a test to see if your gun is working. How would I go about using this to test the circuit?

Thanks!

[i4004]

Re: Oscilloscope CRT problem

>well for the lazy bums I've attached the picture again, with the capacitor in question circled and labeled.

i ment in the electric diagram(ie schematic) you linked earlier....

details like this:


where's that focus voltage coming from again?
to work on this schematic one would need it printed, this way it's too torturing.

it seems it comes via P-803 connector, but i can't find matching stuff on power section...uh...

the circuit next to mv950 should be the one producing HV (page 6 of pdf, p-805a is connecter that carries HV)..but where is it plugged in?
huh...


either way, after checking cnnections like toasty said you'll need to trace back the HV and see where does it dissapear.
or gets so damped that you only get such a faint trace...

Unfortunately, I never tested it in as dark a room as this before I replaced the cap, so I don't know if that helped or not.

yeah, i doubt it helped.
darkness did, though..heh...

[etnietering]

Re: Oscilloscope CRT problem

Progress! (maybe)
Well I replaced that capacitor with the 2.2uF 350v samwha I had, and now I can get something very miniscule to show up on the screen sometimes. I have to have the intensity and focus knobs cranked all the way to the right, and the var sweep all the way to the left, and I get a very small blob that starts on the left side and moves to the right side, moving up slowly in the process. The speed in which it moves changes with the TIME/DIV knob, as would be expected. However, the frequency with which it appears also changes. At 50ms/div it shows up about every 1.2 seconds. At 100ms/div it shows up about every 5 seconds. At 20ms/div, it shows up 2.5 times per second.
To see them, I have to shut off all my computer monitors and wait a few seconds for my eyes to adjust, and then I can barely see it.
So it seems that the tube is working, it's just not being fed the correct signal. Unfortunately, I never tested it in as dark a room as this before I replaced the cap, so I don't know if that helped or not.
Any thoughts/advice? Thanks!


edit: oh I forgot to mention, this is only when there's nothing plugged into the inputs, and with them on 'add' mode.

[ratdude747]

Re: Oscilloscope CRT problem

no, i mean the kind where you take out vert/hor cable from the gun and lenthen them, then a d a sweep generator.

[Krankshaft]

Re: Oscilloscope CRT problem

The scan rates on a TV would be too low so it's use would be limited.

The fixed horizontal and vertical deflection rates wouldn't help either.

[ratdude747]

Re: Oscilloscope CRT problem

this sounds too stoop!d, but are old tv's turned into scopes worth anything?