CRT Monitor Issue (Advice/Recommendation Sought) - Page 1
Forum section: electronics / general

author: Unknown
author_rank: Unknown
timestamp: February 21, 2014, 06:31:29 am
content: Hi everyone,I thought I'd reach out to the EEVblog community for some assistance.I'm in the process of expanding my vintage computer collection and managed to score a bunch of old Apple Macintosh hardware for $10. Absolute gold! Part of my acquisition were two Apple 'Multiple Scan 720' 17" CRT monitors. But both have identical problems.Whilst my expertise lies in IT/Data forensics, I'm not a CRT/Television/Power Supply expert. I'll try to explain the symptoms as best as I can:Both power up and display a signal without any problems. Overall, the picture is pretty good; Sharp, good colour, good shape, no burn-in.ISSUE #1:The maximum brightness/contrast settings seem a bit low. I mean the image is fine, but I feel that I shouldn't have to crank the settings all the way up to achieve a normal picture.ISSUE #2: Perhaps the more serious issue. Intermittently, the monitor has a "glitch". The image will distort for a fraction of a second and an audible "click" or "tick" can be heard. My guess is that there is an issue with the power supply.Could anyone with some CRT expertise point me in the right direction? I'm even happy to pay someone to diagnose/fix my problem (I live in Sydney). Poking around CRT picture tubes has always made me nervous so I'm hesitant on working on it myself.Michael.

author: Monkeh
author_rank: Super Contributor
timestamp: February 21, 2014, 07:29:47 am
content: #2 sounds like it's degaussing. Have you not had a CRT before?

author: justanothercanuck
author_rank: Frequent Contributor
timestamp: February 21, 2014, 07:37:23 am
content: The brightness issue is probably capacitors.As for the clicking noise, if it is indeed the degaussing coil, that shouldn't happen during normal operation. It should only degauss when the monitor is initially powered up. That might also be a capacitor issue, but don't hold me to that one.

author: Unknown
author_rank: Unknown
timestamp: February 21, 2014, 08:26:17 am
content: Quote from: Monkeh on February 21, 2014, 07:29:47 amIt's definitely not degaussing. It degausses for a few seconds on power-up, which is expected. Even the manual degauss function works as expected. The behaviour I'm describing is abnormal. I'll try to capture it on video and upload.

author: G7PSK
author_rank: Super Contributor
timestamp: February 21, 2014, 09:00:54 am
content: It could well be just dirt build up on the outside of the crt. leaking off the ht. with the odd arc over. Try cleaning the crt's first by blowing or vacuuming the dust of and then clean any residual grease ect off with alcohol swabs. I have often found that is all that is required, especially if the monitors or TV's have been used by smokers or in a kitchen or workshop environment. That is where I would start, but remember to turn off and unplug the monitors some hours before opening them up preferably the day before to let the tube charge leak off.

author: Unknown
author_rank: Unknown
timestamp: February 21, 2014, 09:06:15 am
content: Quote from: G7PSK on February 21, 2014, 09:00:54 amI'm thinking you might be right on the money here. I've powered it up for a while and low and behold I can't reproduce the problem (maybe the CRT is camera shy?). The only thing I can put it down to is the recent high humidity along with the build-up of dust might be enough to cause the issue.I'll pull the cover off, discharge the CRT and give it a good clean over the weekend.

author: nihilism
author_rank: Regular Contributor
timestamp: February 21, 2014, 09:26:36 am
content: faulty flyback transformer. Insulation is breaking down somewhere.

author: justanothercanuck
author_rank: Frequent Contributor
timestamp: February 21, 2014, 10:53:21 am
content: Quote from: nihilism on February 21, 2014, 09:26:36 amFlybacks don't usually fail, in my experience. If it did fail, you wouldn't get an image, or you would have a loss of horizontal sync (or deflection, I forget which), as the HV circuit also drives the H-sync.edit: speaking of the HV circuit though, if the X-ray protector kicks in, that can cause a clicking sound, but it also cuts the power to the tube, and it usually doesn't come back on until you fix the fault.

author: tautech
author_rank: Super Contributor
timestamp: February 21, 2014, 11:03:10 am
content: Quote from: n45048 on February 21, 2014, 09:06:15 amIf its been "sitting" for a while your usage will have dried out the leakage paths and as you have pointed out a clean up is probably required.Years ago I had a CRT TV that crackled away every Autumn with the dust loading tracking.Just be careful not to let it bite you!

author: justanothercanuck
author_rank: Frequent Contributor
timestamp: February 21, 2014, 12:48:20 pm
content: Just throwing it out there, if you have a few hours to spare, check out this 3 part video series from Randy Fromm. It goes over all the basics of CRT operation and repair...He has a few more of these series, but I think this one is the best.

author: Anks
author_rank: Frequent Contributor
timestamp: February 21, 2014, 01:14:18 pm
content: Having to have the brightness way up normally points to a low B+ voltage. This can also cause lots of other faults (normally with the range of the control's) that could be triggering xray protection or just dropouts. The arcing over I have seen many times from either the crt or the fly back as well at the HT lead.Ive worked repairing arcade machines for a large portion of my life and a lot of these issues do come down to a bad capacitors or two. I wouldn't just go shot gunning capacitors like a lot of people do with these things as it annoys me that I never find the proper culprit and the next time I see it will be able to go straight to the problem.

author: Macbeth
author_rank: Super Contributor
timestamp: February 21, 2014, 06:18:25 pm
content: Spray the A1 and Focus pots on the back of the LOPT with switch cleaner / iso-propyl alcohol. Remember the initial positions and give them a few full back and forth wiggles with your phantom twiddler tool while unpowered. Return to stock position.If still poor, you can try over-driving the A1 slightly. It will increase brightness, but also start showing flyback lines if you go to far.ETA: Make sure you resolder all the circle of joints on the bottom of the LOPT PCB too. Bad joints are fairly common.

author: VK3DRB
author_rank: Super Contributor
timestamp: February 22, 2014, 12:49:03 pm
content: Quote from: justanothercanuck on February 21, 2014, 10:53:21 amEHT tripplers can arc on the surface over dust and sometimes the plastic breaks down and you can get arcing within carbonised tracked in the plastic. Take the back off, turn it on (being VERY careful not to touch anything - stand clear), turn out the lights and see if you can see any blue arcing. If there is, power off and disconnect from the mains for 24 hours to allow any HT build up to dissipate, use isopropyl alcohol to clean the arcing part or if necessary clean out the carbonised (burnt) material and use RTV silicone to seal the crack or hole. Sometimes the CRT can arc inside and you might be able to see this in the dark.The dim display could be due to an number of causes. Sometimes if the CRTs have not been used for years, sometimes the cathodes can oxidise slightly (despite having "getter" compounds in the neck of the tube) which can limit electron emission. Old, well used CRTs cna also have weak cathodes. You can "boil" the cathode using a picture tube rejuvenator from a TV serviceman, but good luck finding one. This machine will put a higher voltage on the heater and apply grid voltages that will encourage excessive electron emission for a shot time, quite often giving a CRT a new lease of life.

author: justanothercanuck
author_rank: Frequent Contributor
timestamp: February 23, 2014, 10:08:56 am
content: Regardless, have you had a chance to look at it n45048? I'd be interested to see how this turns out.

author: cprrw132
author_rank: Newbie
timestamp: March 03, 2014, 09:44:55 am
content: With your brightness issue on the fly back transformer there are 2 adjustable pots painted { glued } into position 1 pot will affect focus and the other is for brightness that is if my memory serves me right the focus pot usually has focus written on it pots will need to be re glued after adjusting John cpr Albion Park NSW

author: justanothercanuck
author_rank: Frequent Contributor
timestamp: March 07, 2014, 11:21:39 am
content: Quote from: cprrw132 on March 03, 2014, 09:44:55 amCranking that pot will increase the brightness, usually at the expense of the phosphor in the tube. I'd only do that as a last resort, but that's just me.