Oscilloscope - chasing an open circuit - Page 1
Forum section: electronics / testgear

author: calin
author_rank: Regular Contributor
timestamp: August 07, 2013, 06:00:14 am
content: OK .. i got a while ago a really nice Leader LBO-516 scope from bay ... for 19$. Was dinged on one side etc. Funny enough i fixed the ding with a hammer and eliminated the short and the machine got back to life , all fine and OK worked for a few months until yesterday.Here is what is happening, the CRT sometimes turns on and sometimes turns off. Clearly is not a short because I measured all power supplies and voltages are all spot on. I could not measure the -1950V HV supply because I don have a HV probe and I dont want to get a 2000V kiss ... I will end up being pretty pissed off if I do. Sometimes the CRT just comes on because I do a move somehow and stays on , then I do a wrong move and goes off again. Everything else works like a charm ... I can see that it triggers etc, even with CRT off the trigger light is on. Just few moments ago I was testing to see what is happening if i shake a bit, i just left the scope to drop like from a half in on the bench and it went off. Sometime just the shake from plugging a BNC cable or pushing a button on the front panel turns just the CRT OFF or ON. SO the scope does not go ON/OFF, just the CRT.Most likely there is an open circuit somewhere ... or cold junction or something that has its origins when the scope was dinged. Tried to find it with a continuity test .. but is like finding a needle in the haystack, plus the damn thing is intermittent.Does any of you has any idea how to chase down an intermittent open circuit - shorts are easy .. but this ?

author: Rerouter
author_rank: Super Contributor
timestamp: August 07, 2013, 07:21:58 am
content: if its a dry or intermittent joint that is that picky, freeze spray is your friend, equally, if its only around the power supply section you suspect, give it a re-solder, if it was around the more high speed analog stuff i would avoid but power supplies shouldn't need high silver solder,

author: free_electron
author_rank: Super Contributor
timestamp: August 07, 2013, 07:23:09 am
content: check the brightness controll pot ... you may have a loose wire there...

author: PA4TIM
author_rank: Super Contributor
timestamp: August 07, 2013, 07:37:27 am
content: indeed, check intensity pot, but also the CRT connector. I have seen two scopes with problems there. One had so much dirt there because the fan was there it had to much leakage current. The other had a loose wire.But it can be a real problem, my Tek 212 has a problem inside the tube. If I tab the crt, the trace lenght decreases.

author: Marc M.
author_rank: Regular Contributor
timestamp: August 07, 2013, 08:25:11 am
content: Quote from: calin on August 07, 2013, 06:00:14 amIt is not advisable to drop a CRT based scope in such a manner, especially an electrostatic type! Rerouter's advise of using freeze spray is spot on. Another technique that I've often used in localizing a mechanical intermittent fault is simply tapping on the board and/or nudging wires and components with a non-metallic alignment tool looking for any changes in behavior. Once I narrow down the general area I'll use freeze spray to try to further isolate it. If that doesn't help, I'll inspect all solder joints in the area under magnification or just grab the iron and reflow all the joints in the area. It doesn't take very long if you have a decent iron. If none of that works I typically do a lot of swearingthen repeat the process.

author: calin
author_rank: Regular Contributor
timestamp: August 08, 2013, 12:51:58 am
content: May no be supposed to drop it .. but... I had to confirm somehow that indeed is something that at least looks like an bad contact. After that I left the thing sleep in peace and went to bed. This morning I went and turned the scope on and is working .. and i can't make it stop again !!! Literally ...??I guess I have to give it more time ....

author: PaulAm
author_rank: Frequent Contributor
timestamp: August 08, 2013, 01:05:03 am
content: I had a Tek scope once that did something like that. The CRT would go blank and it would come back, mostly, when I tapped the side of the case.Turned out it was a cracked lead on a ceramic resistor network in the HV circuit. You could see it with a microscope. Drove me absolutely crazy tracking that one down.Get an insulated rod about a foot long and use it to lightly tap and stress the parts in the HV supply. Freeze spray is good too. Maybe you'll get lucky. Just because a part looks OK, don't assume it is. You probably have a bad solder joint or a lead has cracked right by, or even inside, a component body.