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I have an old Atari SC1224 CRT which has badly lost focus over the years.

After taking the cover off, I can see various focus controls that can be adjusted by screwdriver.

Question: is there a science or method to adjusting focus on CRT monitors? And are there any well known test patterns that can help?

I'm know it's impossible to get perfect focus everywhere, but I'm looking for as good as I can get!

  • 2
    I use a flat-screen VGA monitor with my Atari 1040 ST. I acquired an excellent video cable from BEST Electronics, and I have a scan converter. Works with both color and hires mono. I rarely drag my old CRTs out. Careful storage and very occasional usage seems to prolong their life. – Brian H Mar 25 '17 at 13:27
  • For completeness: bad focus, color rendition problems/nonlinearity ... can also be a sign of the CRT itself failing (contaminated vacuum, damaged cathodes...) – rackandboneman Apr 3 '17 at 15:48
  • My CRT TV only has one static focus control on the flyback transformer. :/ – Black and White Jan 21 '18 at 19:29
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The EHT (extra-high-tension) circuits used to drive colour CRT displays run at 25kV (kilo-volts), and the capacitors contain enough stored charge to kill you stone dead if you touch the wrong thing, and the stored charge does not disappear as soon as you switch the device off. I would take it to a repair shop if I were you. The technicians will have the skills and equipment to handle it safely.

Since the display has lost focus, that is because the EHT circuits are drifting for some reason, probably ageing capacitors. Fiddling with the focusing controls will almost certainly upset the convergence and make the image even worse. Any improvement that you do manage to make will not last long, since the failing components will continue to degrade.

If you want to make the display fully-operational, the best course of action is to let a qualified service technician identify and replace the faulty components.

I appreciate that this doesn't answer your question, but it may just save your life.

  • Thanks for the warning! Don't think I'll be trying this then. Any recommendations for a good technician in London, GB? – jamesfmackenzie Mar 27 '17 at 11:27
  • Most electrical shops will be able to help you, although these days, they will probably send the job out. Basically, you need someone who knows how to repair TVs. – Mick Mar 27 '17 at 11:40
  • @jamesfmackenzie I've left a suggestion for a London-based repair service on Retrocomputing Chat. – Mick Mar 27 '17 at 15:35
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    With old electronics, when in doubt.. inspect the caps and look for corrosion; it is very rare something else breaks by itself, but these two problems are very common. I started to tinker with electronics when I was around 9 years old, helped by my dad; I didn't even understand 1/3 of what I was doing, but I was loving it. It was really fun and very magical at the time! – Thomas Mar 28 '17 at 23:04
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    I'd be even more wary of all the other several-100-volts circuitry around the CRT than the (low current capacity) 25-32kV system... – rackandboneman Apr 3 '17 at 15:46
9

The CRT itself is essentially a high voltage, high capacitance device on steroids and can hold a charge for well over a week. Touching the wrong contacts with your bare hands will throw you across the room or kill you. Years ago I discharged a 26" CRT - melted the ground wire (10 AWG) and about 2" of a screwdriver...right hand paralyzed for almost a week. I regained consciousness about 8 feet away and to this day all I remember is snap, crackle and waking up with an iron grip on what was left of the screwdriver. HDTV's have a PC monitor hookup. This is a probably a better and safer way to go.

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    Some power supplies will keep charges for long as well. Learn from my mistake: never hold one without its case above your legs while wearing shorts! – Thomas Mar 28 '17 at 22:37
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Focus is a high voltage function (but not as high as the main color monitor anode, which is 24 kV or so). Locate the 'focus' knob (which, if you are careful, can be adjusted while the set is under power). Sometimes the back of the monitor has to come off, to get to it. It's always labeled, don't touch anything that ISN'T the adjustment knob.

If tweaking it works, put the case back on and enjoy.

With simple tools, you can discharge the HV, attach a grounding strap, and work safely on a TV or color monitor. The CRT is irreplaceable, and it is NOT the problem, so don't mess with it otherwise.

If focus controls don't help, it's usually just the carbon composition resistors around that 'focus' control. It is very likely that some resistors, values in the range of 100 k to several megohms, have failed. They're cheap, under $1 worth of parts.

Alas, color monitors may also need screen adjustment, and convergence and pincushion and suchlike, tweaked as they age. Unless you find an old guy who used to do this work, it's not gonna happen (there's lots of non-obvious steps in the process).

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