My Panasonic CT-20S12S (20 inches, not 12 inches) has some pretty bad geometry in various parts of the screen. The edges of lines bow in or out, distorting shapes.
How can I adjust the geometry on my CRT to fix this bowing?
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do you got any screen shot ... ideally of a test screen (distorted and undistorted image)?– SpektreCommented Jan 15, 2018 at 8:47
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Try borrowing, begging, or stealing a service manual for that monitor - geometry adjustments (especially convergence, which can be nontrivial for colour monitors) should be described in detail. If the monitor is actually defective (see Spektre's post), you will need to do these adjustments anyway after repairing it.– rackandbonemanCommented Jan 15, 2018 at 9:46
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If the pincushion setting of the monitor does not help to adjust those bends, the high voltage is leaking out of the main transformer body. That's the usual reason for extreme distortions and marks the EOL of the whole device. If you don't want to fix it for museum purposes, throw away. Other parts of that monitor are going to fail soon, too.– JankaCommented Jan 21, 2018 at 19:30
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@Janka Oh hell, I should have clarified. It's only minor geometry issues on my TV (NOT MONITOR), but they're still a pain in the butt to see.– user7645Commented Jan 21, 2018 at 19:32
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With a TV set, it's even worse because the pincushion is treated automatically by the so-called "east-west circuit". That's usually some diode/thyristor magic around the medium voltage (about 800V) base of the main transformer. Most likely one of these components is blown and will blow again because the transformer has internal HV-short-circuits.– JankaCommented Jan 21, 2018 at 19:36
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2 Answers
Fault you describe implies problem with electron ray sweeping...
Triangle wave generator
this circuitry is used for x and y sweeping across the screen area. If the shape is not triangular but curved it can create distortions like you describe. There are few common reasons what could cause it:
electric fault of some kind
like bad electrolyte capacitor, faulty transistor or tube... You need to identify and change the faulty part. That is usually done by oscilloscope probing specific test points and according to shape of the signal you can decide what is going on. Some docs for old TVs and Monitors contain this along with the circuitry schematics. Try to look for service manual for your monitor all the info should be there.
too old adjustment trimmers
they tend to oxidize and starts to be not reliable after ~10+ years. If you hit the device a bit and situation changes it is the most likely this (or cold (dead) soldering on the PCB). There are solutions for this. You need to lubricate the part resistive area with something oil-like and not isolative too much. In my region is the best to use this:
its called DIAVA Politúra Červená There are also pure electrical solutions like CONTOX or what ever but I do not have any experience with trimmers using it. Applying a drop of Diava at the trimmer resistive surface and moving the adjustment a bit along the way and then set again to original position will usually help for 1-2 years then you need to apply it again. Before changing the setting make a mark where it originally was.
The circuitry might got different parameters over time and need a different settings. So you could locate the linearity trimmers and try to adjust them until your lines are linear again. Doing this blindly can do more harm than good. So the best is use an oscilloscope. If You do not have one then use a test screen like:
and pay attention to the vertical grid lines they should be equally spaced along whole screen. If the distribution changes with
Yaxis (on top is different than in the middle) then you need to adjust horizontal linearity too.CRT magnets
on the vacuum screen tube (in the thin cylinder part on the back) there are sweeping electromagnet coils and adjustment magnets. The magnets are used to compensate for Background Earth Magnetism (use those only as last resort) when nothing else helps. Some tubes also have magnets for focusing do not touch those !!!
Do not forget to mark any adjustable parts original position so you can reverse back to original settings !!!
Safety
As Ralf Kleberhoff pointed out There are high and very high voltages inside your CRT monitor so you need to be extra careful. Do not touch any metal stuff or parts directly. Use only insulated tools.
The most dangerous part is the Voltage multiplier (big ugly transformer like but almost without cables part sometimes enclosed by plastics but most likely covered by dark burned dust)
It is usually connected with single thick extremely isolated cable to the CRT (somewhere on the side closer to the front of the CRT with sucking disc like insulation).
The DC voltages used are usually between 12.7 - 32 KV which means that it can throw an electric arc through you if you just near (without touching it). Usual air has insulation of 3.6 KV/mm but near this stuff the air can be partially ionized so it can be dangerous even deadly up to distances around 1-5 cm !!! from the live parts. Also be careful it tend to suck in also insulators so if you got any free hanging sleeves, hand watches etc it can be a problem.
Also generally is a good idea to remove things like rings , piercings etc that could come into proximity to live parts of device as they tend to decrease the transmitting resistance of owner causing much bigger current in case of incident much more likely doing bigger harm ...
If using oscilloscope or any measurement device do not probe the High Voltage pins ... that would fry your device for good... See Thought I found serial port - broke embedded device instead! Help? and always check the Voltage range selected (and or cable connector used before measurement). Especially with multimeter devices where you can measure both current and voltage as using current connector/setting for Voltage measurement is performing a short circuit where it should not be ...
Be careful even if the monitor is off as any LC circuit (coil-capacitor) can be charged/oscillating long after shutdown and CRT monitors have quite a few of those (like the sweeping coils)...
If you are a rookie and do not know a bit about electricity do not do this !!! or at least do not attempt to do this alone so just in the case there is someone to shut down the power and perform First aid / resuscitation AFTER POWER SHUT DOWN with main power kill switch for your home or disconnecting the power plug. Do not put your fate just in to current protector (even if they are a good thing to have)
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8SAFETY! Don't forget that CRTs use high voltage parts. Touching them can be VERY dangerous. Commented Jan 15, 2018 at 9:20
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Also, before opening, one should be 200% sure that there isn't current in the capacitors. Commented Jan 15, 2018 at 14:47
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@IsmaelMiguel that is not possible as there is always some charge in not short circuited capacitors. That is why the big ones are stored with a short-circuit cable (especially the very high voltage ones for power distribution purposes as they tend to charge up themselves on their own just from surrounding temperature changes and charge) those where deadly sitting on the shelves in the early electric days ... as they could (and did) suck in personel just walking around and discharge ... But for common capacitors used in electronics is this not a big issue– SpektreCommented Jan 15, 2018 at 15:28
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@IsmaelMiguel The charge is usually disipated throug the circuitry resistance very quicly. Only oscillators and switching power supplies are danger and they can stay powered even hours after shut down ... in some circumstances they can charge up on their own if the circuitry allows that ... so the safest is not touching at all ...– SpektreCommented Jan 15, 2018 at 15:29
The vertical and horizontal deflections in a CRT are not entirely independent. There is a 'pincushion' effect that is commonly adjusted using a 'pin' or 'pin amp' labeled control, which is usually only accessible with (part of) the case removed. Misadjustment results in pincushion-shape or barrel-shape rendering of a test rectangle.
Other geometry adjustments are permanent magnets (often stuck on plastic posts around the neck of the CRT). The symptoms of missing or disturbed trim magnets are not symmetric like 'pincushion'. The steel parts of a monitor can become magnetized, which also disturbs the trim, and a 'degauss' circuit will correct that. Degaussing can be done with a wand, but many monitors have automatic circuitry (every cold-start of the monitor does a degauss), or sometimes a pushbutton. Degaussing with the picture active, you'll see a shiver of the image, possibly with odd colors.
Finally, there are 'color convergence' controls that make red/green/blue colors all land together to make a white line. Readjustment of convergence is required when the picture tube ages.
A few late-model CRT displays had on-screen menus for some or all of these adjustments.
A twenty-year-old book on color TV or computer monitors will fill in details, but it isn't safe to just open up the machine and poke things. Proper readjustment also requires an assortment of odd tools and some test-pattern equipment (and/or software).
