Oscilloscope display is dim even if intensity is set to maximum. - Page 1
Forum section: electronics / repair

author: bjcuizon
author_rank: Frequent Contributor
timestamp: April 25, 2017, 10:45:09 pm
content: Hi, my analog oscilloscope is a bit dim even if I set it to maximum intensity. And the trace will vanish on the fastest time base setting. I opened it and found an internal intensity adjustment pot.Can I adjust it to brighten the trace on the screen? And does it shorten the crt's life?Thanks.

author: tautech
author_rank: Super Contributor
timestamp: April 26, 2017, 01:02:41 am
content: Quote from: bjcuizon on April 25, 2017, 10:45:09 pmYou haven't mentioned the model but in short yes you can. The internal pot sets the bias and normally at ~1/4 brightness for the main brightness control you then adjust the internal bias pot until the trace isjustvisible.This procedure should be described in the Service manual.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: April 26, 2017, 01:48:23 am
content: Hi, sorry I forgot to put the model number. It is a Leader LBO-522 analog scope. By the way I already found the manual and schematics so I guess I will tweak it a bit. Thanks.

author: tautech
author_rank: Super Contributor
timestamp: April 26, 2017, 01:52:50 am
content: Quote from: bjcuizon on April 26, 2017, 01:48:23 amPost the link to the manuals so we're all on the same page and use this thread as a running documentary on the adjustment....write it up well and it can help others with the same problem.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: April 26, 2017, 02:01:17 am
content: The manual helped very much. It even has the layout of adjustment pots.Here is the link:https://www.google.co.nz/url?sa=t&source=web&rct=j&url=http://www.leaderamerica.com/marketing/manual/LBO522-523-Instruction_Manual.pdf&ved=0ahUKEwiek4-ZgsHTAhUENpQKHWayCDMQFggbMAE&usg=AFQjCNEefRLJS5CrQ_AbN_sciyXQz3MtPw

author: tautech
author_rank: Super Contributor
timestamp: April 26, 2017, 03:02:07 am
content: Quote from: bjcuizon on April 26, 2017, 02:01:17 amThat's just a Users manual but at least it describes the simple adjustments.I'd want to ensure the LV supplies were meeting spec and low ripple. Always check the PSU first.Have a hunt for the full Service manual with schematics and PSU specs then link it in the thread.Now you might ask why don't I.....how do you learn about these things if everything is handed to you on a platter.

author: carl_lab
author_rank: Frequent Contributor
timestamp: April 27, 2017, 07:31:10 am
content: Have you cleaned the filter screen in front of the CRT?Once I had a HP oscilloscope with a grey dust film at the inner surface of the filter screen - resulting in insufficient brightness.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: April 30, 2017, 02:09:27 am
content: Well, I think it is still clean. But i'm just going to double check it anyway.

author: ebclr
author_rank: Super Contributor
timestamp: April 30, 2017, 02:20:48 am
content: Scopes tubes aren't forever, and the 1st tip is a low bright before you need to buy a new tube,Bright in an analog scope is function of several things1) High voltage, power supply ( most problems come from here ), or disconnected cables2) Filament power supply, very few problems comes from here, but if your tube is on near end of life, you can get some extra time adding 10 20% tension to the filament and pray that they did open, But you are already in a situation that the tube is end of life, the maximum you will get is a total blank and need to a new tube, It's the last try before buying a new tube3) Grades control system, Several trim pots are involved on this part for bright, focus and retrace blank, this can be another point of low bright problems.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: April 30, 2017, 02:28:26 am
content: Yeah thanks, I already did your third tip. I found the intensity adjustment part in the manual and tweaked the pot in the scope a bit.

author: james_s
author_rank: Super Contributor
timestamp: May 01, 2017, 08:03:10 pm
content: Has the brightness always been low while you've had it, or did it suddenly lose brightness? Normal tube wear will cause a gradual decline but if it suddenly changed I would be wary of making adjustments as you may well be trying to compensate for a fault. If the tube really is worn out, turning up the brightness will speed the process, but at that point you have little to lose anyway.

author: tautech
author_rank: Super Contributor
timestamp: May 01, 2017, 08:33:47 pm
content: bjcuizon I think has just got this scope off our local online trading outlet Trademe, so I guess it might be his first and maybe has higher expectations of a CRO trace brightness.He started a thread about CRT failures so maybe there's something wrong with the scope or the attempted adjustments didn't work.More feedback from bjcuizon required for this to progress.

author: james_s
author_rank: Super Contributor
timestamp: May 01, 2017, 08:38:09 pm
content: That's a good point, the trace may be as bright as it's supposed to be, cameras tend to make CRO traces look much brighter than they appear to the eye. It should be bright enough to clearly see though, and at least visible showing a waveform on the highest horizontal rate.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: May 02, 2017, 06:41:31 am
content: Quote from: tautech on May 01, 2017, 08:33:47 pmYeah, you are right tautech. I just got it off from trade me last week. And also I thank you for some replies with advices about it. I opened it up and some parts dated 1986 so I thought it would be 31 years old by now. I was just wondering if CRTs ever die because there are already some minor screen burn in marks on it. By the way, I found out that it was from the University of Wellington's physics department so maybe someone could have left it on for quite some time.

author: tautech
author_rank: Super Contributor
timestamp: May 02, 2017, 08:52:51 am
content: Quote from: bjcuizon on May 02, 2017, 06:41:31 amDo you have it operating to your needs or must we work through some repair ?It may be getting old but it could operate for some time until you can upgrade.They are OK CRO's and good to learn on.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: May 03, 2017, 03:20:03 am
content: Well, it is working all good and fine right now. I might upgrade it later to a dso. Thanks.

author: gadget73
author_rank: Regular Contributor
timestamp: May 03, 2017, 03:21:38 am
content: If its got screen burn, thats usually a clue that its got a lot of hours on it, and quite probably a lot of hours with the brightness turned up too much.

author: tautech
author_rank: Super Contributor
timestamp: May 03, 2017, 03:37:45 am
content: Quote from: gadget73 on May 03, 2017, 03:21:38 amYep, CRO's brightness setting needs be used with some care for longevity.Bright enough to see all you need to see and no more.