Help on CRT distortion troubleshooting - Page 1
Forum section: electronics / general

author: robrenz
author_rank: Super Contributor
timestamp: January 25, 2012, 05:07:54 pm
content: This is the CRT of my CNC mill. It still works fine but I just started this compression of the right side of the display. Ignore anything on the screen pics except the actual orange phosphor display. It was hard to get a decent picture, you are seeing flashlight glare on the screen. The lines at the bottom of the screen are usually evenly spaced but you can see the last space on the right is compressed. At the top the "INCH" is also compressed. on other screens the trace on the right edge is a little brighter than the rest of the display.Any guidance on generally what to look for would be greatly appreciated. I don't have schematics. I do have most electronic test gear, I just don't know squat about CRT driver circuitry.I can't stop right know to open it up to give pics of the insides but I can later.Thanks in advance for the help

author: Zad
author_rank: Super Contributor
timestamp: January 25, 2012, 05:50:38 pm
content: Three things spring to mind:1. The side panel has become magnetised and isn't being shielded internally - it needs degaussing.2. A capacitor coupling the horizontal scan signal has become non-linear. This is usually an electrolytic drying out.3. The voltage feeding the horizontal scan amplifier is too low, restricting the scan.It could be a transistor that has gone a bit "iffy" and low gain or a dry joint causing (2) or (3).

author: vk6zgo
author_rank: Super Contributor
timestamp: January 26, 2012, 01:36:57 am
content: In CRT type monitors there is a trick they used to call "Boost HT" back in the tube days,& which is also used with solid state circuitry.On the Horizontal Output transformer,there is an overwind that produces a hundred or so volts at the horizontal frequency.This is then rectified,filtered,& added to the normal power supply voltage to the Horiz & Vert outputs.The filter capacitor is usually something like 10 to 20 uF at 400v ,& MUST be a low ESR cap--ordinary ones go bang after a few minutes!The (half-wave) rectifier is a fast-recovery type --again,ordinary types go bang!If the cap has become old & lost capacitance, the expected nice filtered dc is not smooth anymore,plus it's regulation is poor.The result is the effect you have noted.With the kind of display you are looking at,you wouldn't notice it,but the vertical scans will also be distorted.Monitors used in industrial equipment are usually made by the same companies who make them for arcade games,etc.If you look inside,you may find a manufacturer's name,& part number,& possibly you will be able to find a schematic on the Net.if not,trace around the circuitry associated with the Horizontal Output transformer,& you should be able to find what you are looking for.Take great care,as this area can BITE you!You shouldn't need to do that part with the monitor energised,anyway!If you have identified the circuitry,measure the "Boost HT" with a DMM,it will probably be something like 150volts dc.If it is much less,you have probably found the fault.If your DMM reads around 100v,so italmostlooks OK,be suspicious,as some DMMs will give false readings with fairly high frequency rectified ,unfiltered dc.The best thing to look at this with is an Oscilloscope.If you have determined that this is the fault,change the capacitor (low ESR),& all will be well.Good Luck!VK6ZGO

author: robrenz
author_rank: Super Contributor
timestamp: January 26, 2012, 01:35:02 pm
content: Thanks guys, That really narrows it down to get me going in the right direction.

author: robrenz
author_rank: Super Contributor
timestamp: January 26, 2012, 03:30:33 pm
content: I opened it up and found the 400 volt cap in the horizontal section. It is a SBE 715P 273J 9810 the best I can determine it is a .027 microfarad cap. It looks like a Y5V 102M 1KV (I think 1000 picofarad +/- 20% ceramic)maybe in parallel with it.I can only find .022 and .033 microfarad in the 715P caps. is it better to be higher or lower? those numbers are the high and low of a .027 at 20% tolerance.should I suspect the ceramic also?Thanks again in advance for the help.

author: zaoka
author_rank: Frequent Contributor
timestamp: January 26, 2012, 10:52:58 pm
content: Just follow yoke connector, it usually have red green blue and yellow wires. Red and blue are for horizontal deflection,thats where this problem coming from.THere you will find several capacitors and coils. Test all of them, replace electrolytic capacitors if you find one and make sure inductors are not burned or shorted.I usually replace all capacitors that are more than -+ 5%.You can also switch red and blue wires, it will move INCH to the left side, if its still distorted than the problem is with electronic, if distortion remain on the same side this means that electronic is OK and problem is with deflection yoke and/or magnets attached to it.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 27, 2012, 04:32:24 am
content: Quote from: robrenz on January 26, 2012, 03:30:33 pmWrong cap!I very much doubt that there is anything wrong with the actual deflection circuitry.The "boost" circuit I described,although it is driven from a winding on the Horiz out/EHT transformer,is a separate circuit,which has, as its only function,to provide the Horiz & Vert output amplifiers with extra volts so they can produce the scanning levels required without distortion.The cap associated with this circuit will almost certainly be an electrolytic.VK6ZGO

author: robrenz
author_rank: Super Contributor
timestamp: January 27, 2012, 08:15:27 pm
content: Thank you all for your help. I could not stand any more down time in trying to fix this since it has my CNC out of commision. so I had to bite the bullet and get a exchange unit sent out.Just for reference, the only cap anywhere near 400 volts was the 715p polypropylene. No electrolitics on the board were over 40 volt.

author: johnwa
author_rank: Frequent Contributor
timestamp: January 30, 2012, 09:15:32 am
content: If you just need to get it going again, you will probably find there is a standard CGA/EGA/VGA video signal going to the monitor. It is easy enough to hook this up to an external monitor, though you may have to try quite a few before you find one that will sync to such a low horizontal frequency. (Failing that, it is a fairly simple exercise to display video on an analogue scope)I did this as a 'temporary' fix to a CNC machine tool once - instead of the standard monitor, I had a 20" widescreen LCD perched on top of the control cabinet! It was eventually repaired 'properly', but the little CRT cost about three times what the LCD monitor was worth...