Faded CRT picture in scope, scary HV crap? - Page 1
Forum section: electronics / repair

author: MT
author_rank: Super Contributor
timestamp: August 13, 2018, 02:28:15 pm
content: In one of my old scopes CRT intensity have faded over the years(30) into almost invisibility in daylight can there be anything else then scary stuff like HV fail , Tube depletion/resistance garble, tube leak, HV resistors, etc, thats at fault?I rather ditch the scope then try repair scary stuff else it's a very simple and safe thing to repair , intensity knob have zero effect , but crt picture still sharp and focus knob works. old GW Instek scope.Thanks for your thoughts!

author: james_s
author_rank: Super Contributor
timestamp: August 13, 2018, 03:35:26 pm
content: There's no scary stuff inside a scope, there are some modestly high voltages but nothing crazy. The most dangerous area is the mains input which is capable of delivering substantial current.Lots of things could cause a dim trace, troubleshooting a scope is like troubleshooting anything else. If you want more detailed advice it would help to know the make and model of the scope.

author: MT
author_rank: Super Contributor
timestamp: August 13, 2018, 05:18:28 pm
content: There are scary stuff inside 30 year old analog scope like high voltages stored in the tube and caps, there are always special procedures with HV, besides all the dead rats! Your diluting my thread! Buy a cow!. Its not only mains input, its not fun getting multi zapped by 2-15kv, and why zapp your self if you can buy a low end Rigoletto or Sigletto avoiding the zapp, i also dont have any kitchen HV gas igniter to zapp the CRT so it have to be a simple fault to even bother with this old crap? At the same time if its a dead simple fix its always neat to stretch "one more" year out of it.https://www.eevblog.com/forum/beginners/crt-displays-dangerous/

author: james_s
author_rank: Super Contributor
timestamp: August 13, 2018, 07:16:18 pm
content: My point was there's no reason to be scared of the HV, it won't kill you and if you have any sense you won't even get a shock, just be careful. I've been repairing CRT equipment for ~25 years and haven't hurt myself yet. 240V mains is far more dangerous than 15kV at a milliamp or so for a CRT.

author: Gyro
author_rank: Super Contributor
timestamp: August 13, 2018, 07:32:19 pm
content: The first thing to do is see if you can find the service manual for it - you haven't given us a model number yet.If it has steadily got worse over the last 30 years then something has aged rather than suddenly failed. Itmightbe the tube, but it could also be something as simple as a resistor changing value. The fact that the brightness control has "zero effect" is interesting though, was that sudden? Does it not make any difference at all, or is it just too dim at maximum brightness?The observation that the trace is still in focus is interesting too - the brightness and focus controls are usually part of the same resistor chain.Does the focus control have any effect?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2018, 11:08:07 pm
content: Y'mean like Tek 475s? They're notorious for apparently running the cathode too hot, wearing out the tube. Replace it and it's good as new (I did this once).Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: August 13, 2018, 11:29:40 pm
content: Yep, if everything else seems OK, your cathode has probably lost emission.CRTs are just like any other thermionic tube,---- the cathode only has a finite number of hours before the emission falls off enough to become unusable.If it is possible, you could try increasing the heater voltage a bit, which might give a bit more life.

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2018, 04:03:55 am
content: If the horizontal and vertical deflection has not changed, then the high voltage has not changed either.Low cathode emission can be tested by increasing the brightness and watching for "double peaking" where the brightness decreases before increasing again. This is accompanied by widening of the trace.Sometimes the CRT grid bias adjustment drifts and all that is needed is a z-axis calibration.The early Tektronix 2213/2215 also had the problem with excessively high heater voltage causing low CRT life. Tektronix released a service bulletin about it which described adding a reverse turn to the transformer to lower the heater voltage.

author: MT
author_rank: Super Contributor
timestamp: August 15, 2018, 11:16:03 pm
content: Quote from: james_s on August 13, 2018, 07:16:18 pmIm 92, anything can kill me, like pokeing around the HV while having Parkinson hands! Surely you would not advice such unless you wanted cause a death covered up as death by Parkinsson?! I'v actually zapped my self with a HV capseveral times , it's not nice, but i do lick 9V batteries as to se if they contain charge!I do not doubt your experience with CRT's.Quote from: Gyro on August 13, 2018, 07:32:19 pmWell GW GOS622 i had assumed most old analog scopes is more or less same in the back end drive circuitry.QuoteMy fault there, what i should have said is 30 year old scope that faded past 2-3 years, sorry!QuoteNope faded for 2-3 years then just stuck in one level. Somethinmg silly like solder joint/cable off etc? Last time at least 10 years ago when i repaired H amp driver i recall the wave soldering in the beast look properly.QuoteNo difference at all, trace is visible at evening light and scope can actually be used. No max intensity only low.QuoteYes over it's range dim focus dim again like when it was new.Quote from: David Hess on August 14, 2018, 04:03:55 amOK interesting to hear.QuoteI have the schemo for GOS620 630 i assume 622 is similar i presume.QuoteSome trim pot?Quote from: vk6zgo on August 13, 2018, 11:29:40 pmIm aware of that and also though that could be the case, but since its still usable in evening light (not daylight)and focus works, well?QuoteOK, il keep that in mind.Here are the GOS620-630 schemos for Z amp and supply.

author: tautech
author_rank: Super Contributor
timestamp: August 15, 2018, 11:34:46 pm
content: 92At least there's no PDA in this CRO only normal EHT of a KV or two.Just check all the LV supplies for value and ripple and then suspect the high value resistors around the CRT EHT have drifted high thereby disturbing the biasing of the grids to get the required trace intensity.Often there's an intensity bias preset that's normally set to have the trace just disappear at minimum front panel intensity settings. You could tweak that but I'd still be checking all the M ohm resistors associated with EHT.The only ones visible are R627 and 632 however some of the EHT associated with them travels off that schematic and you need check any others that might have drifted too.