My oscilloscope just did something strange

bountyhunter

Joined Sep 7, 2009
2,512
I have always been impressed with the way you guys deal with some of the questions that crop up but please allow me a little rant.

The OP explained (in his first post) very well that the pot is OK and that he can now adjust the intensity. Are you guys not reading what he is saying.

Sorry, will now crawl back under my rock.
YES, which is why I asked:

Are you saying you can no longer adjust it down to a normal level of intensity?
 

Thread Starter

adam555

Joined Aug 17, 2013
858
Something still wrong with it: the other day it just went back to the previous low level while using it, and after a minute it gradually increased the intensity on its own over a span of 3 seconds. You can also see some quick flicks some times, but always returns to the new high levels... until today: it dropped the intensity levels quite a lot; though not a low as when I bought it. So I decided to open it.

As soon as I opened it, it completely dropped to the lowest levels of intensity; as low as at the very beginning. I took out the circuit where the intensity potentiomenter is connected to, and now it's back to high intensity levels.

I pocked the components as #12 suggested but I can't seem to replicate the problem. All the soldering looks fine, I see no burns or loose pins, and nothing happens when I move the cables; so at this point I don't know what else to do.
 

Thread Starter

adam555

Joined Aug 17, 2013
858
MrChips, I really need your help. :(

I closed it, after testing it was working well. It continued to work well for a while, but sudently went back to the very low levels.

I opened it again, but now there is no way to bring the bright levels back.
 

Thread Starter

adam555

Joined Aug 17, 2013
858
I left it rest for a while, and now it's back to the high intensity levels.

I measured the voltages on the control panel diagram (the one in post #15), and they all seem the same when it fails as when it's fine.

So with the intensity set to a normal viewing condition, measure the voltage at P1302 pin-1 wrt GND.
For example: at P1302 pin 1 I got -8V with the intensity pot at maximum and 0V at minimum; both when it's working well and when it's not.

I also got the same results either way with pins 2 and 3; nothing at pin 2 (because it's disconnected) and -7.5V to -26.8V on pin 3.

EDIT: corrected pin values
 
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Dr.killjoy

Joined Apr 28, 2013
1,196
I left it rest for a while, and now it's back to the high intensity levels.

I measured the voltages on the control panel diagram (the one in post #15), and they all seem the same when it fails as when it's fine.



For example: at P1302 pin 1 I got -8V with the intensity pot at maximum and 0V at minimum; both when it's working well and when it's not.

I also got the same results either way with pins 2 and 3; nothing at pin 2 (because it's disconnected) and -7.5V to -26.8V on pin 3.

EDIT: corrected pin values
Do you know what the voltages are suppose to be????
 

Thread Starter

adam555

Joined Aug 17, 2013
858
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THE_RB

Joined Feb 11, 2008
5,438
Have you sprayed some electronic cleaner (100% evaporative type) in the brightness pot and worked the pot for a couple of minutes? It could be dust or corrosion in the pot.

After that I would get a real strong magnifier and go right over the PCB looking for dry joints and circle fractures on joints, especially on any power components or anything that runs hot.
 

Thread Starter

adam555

Joined Aug 17, 2013
858
Have you sprayed some electronic cleaner (100% evaporative type) in the brightness pot and worked the pot for a couple of minutes? It could be dust or corrosion in the pot.

After that I would get a real strong magnifier and go right over the PCB looking for dry joints and circle fractures on joints, especially on any power components or anything that runs hot.
Thanks,

I haven't cleaned the pot yet, but I don't think that's the problem. The pot seems to be fine because it graduates the intensity really smoothly, no jumps, no flicks... just perfect. Also, when the intensity fails, I moved the pot... well, practically in every direction, pocked the pins and solder joints, and there was absolutely no change.

Checking the PCB with a magnifier is going to be a real problem. I already did that for the small one where the pots are connected to, and I saw nothing wrong with it. but the big one with the main circuits is well hidden under a lot of cables, screws, and things, and I would need to dismount the whole thing before I could reach the bottom side to see the joints.

In any case, I moved the components on that part of the circuit and also got absolutely nothing.
 
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THE_RB

Joined Feb 11, 2008
5,438
Thanks,

I haven't cleaned the pot yet, but I don't think that's the problem. The pot seems to be fine because it graduates the intensity really smoothly, no jumps, no flicks... just perfect. Also, when the intensity fails, I moved the pot... well, practically in every direction, pocked the pins and solder joints, and there was absolutely no change.
...
OK, that makes it less likely the pot is mechanically faulty.

I would still do some testing around there. Try measuring the voltage on the pot pins, as you adjust the brightness. Maybe one pin will have a DC voltage that moves when the pot is turned. If so, measure that voltage again when the low-brightness fault happens. That might give some more information. It might be something as simple as a bad connector where the pot wires plug into the PCB or something.
 

MrChips

Joined Oct 2, 2009
21,818
Take a look at the HV and Z-AMP circuit schematic you provided.

You want to take as many voltage readings as possible at nodes that you can identify while the scope is working.

Something is flaky. So the next time the scope goes dim you can compare the voltages with what it should be when working properly. Hopefully this will allow you to narrow down which component is acting up.

There is a CRT BIAS pot shown as RV1011. Make a note of its setting. Try to measure the voltage at the center pin. Turn the pot and put it back to its original position.
 

ian field

Joined Oct 27, 2012
6,539
Try using switch cleaner on the controls to get rid of dirt and debris.
You have to be a bit careful with switch cleaner - some plastics parts of electronic components are ABS (like motorcycle helmets with loads of dire warnings not to clean with solvents). When I was in the TV repair trade, I remember squirting some switch cleaner on the mode switch in the bottom of a VCR - and watching in amazement as it turned into tiny granules before my eyes!

Last CRO I had open, the intensity pots were all at high voltage! - not a lot of current, you're as likely to damage the PSU as yourself if you stick your finger in the wrong place.

One old CRO I got from the tip and had repaired (bodged!) many times, was getting a bit weak on cathode emission - the diodes in the multiplier were actually long selenium stacks (many selenium pellets stacked in a long thin insulating tube) - the combined Vf of a single diode stick was enough to give you a shock. Yet another atrocious bodge - I replaced each stick with a series pair of TV line output stage flyback diodes to reduce total Vf and so boost the HT.
 

Thread Starter

adam555

Joined Aug 17, 2013
858
Too late.. :(

I just came back from buying the cleaner and I already used it on all pots. One of them gave me problems afterwards, but then it went back to normal.

I read the warning after buying it, which says it should be tested before use on some plastics. I was a bit worried about the small pots (like the one I needed to clean) as they are completely covered in plastic; but I don't think they were damaged.
 
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