Tweaking Monitor Pots

[Dak-ak]

I just set up my ArcadeVGA into my computer and got it hooked up to my arcade cabinet... it works fairly well (im happy with the results!) BUT:

The screen seems to have the top and the bottom inch squashed to about a half-inch or so... maybe even more (it gets worse the closer to the edge of the screen you get)  which kinda sucks, because that is where a lot of games stick info boxes (for instance, in puckman, the scores become orange lines.. ick!)

what pot would help with this? i hunted around for a horiz. stretch (what i was expecting) but could not find one... hurmph!

i don't know what kind of monitor it is.. its a mutt from an old konami TMNT cab... your guess is as good as mine...

any ideas?

[RandyT]

Look for something called "Vertical Linearity".

Might help, might not

RandyT

[Dak-ak]

bah, no luck... anybody have any other ideas? i hunted and i hunted, but couldn't find a vert. lin. pot anywhere...

[mpm32]

the pots may not be labeled.  But they should be little pots all over the board.  If you have a manual for your monitor you'd be in luck but, if not do what I did.  Get yourself a plastic screwdriver or cover one in electrical tape.  Place a mirror in front of the monitor angled so that you can see it while working in the back.  Sytematically turn each pot and view results in the mirror.  Not the one you're looking for - turn it back to its original position. Keep going until you find the one you're looking for.  If your smart, you'll write down what each one does for future reference.  If you're like me you'll be too impatient to waste time writing things down.  One day I'll sit there with my mirror and plastic screwdriver and write down the location and action of each pot but for now, I'm too busy playing games.

Note; the monitor must be on while doing this so, you take your own risks poking arms and screwdrivers around the back of a powered monitor.

[RandyT]

Be very careful doing this.  Some of those pots are responsible for regulating voltage to certain components in the circuit.  If these are tweaked too far out of range, you might damage something.  I'd generally avoid tweaking anything that you can't identify, but especially if it has a label on it that starts with "VR".

You might also have a bad capacitor in the monitor that is causing it to do this.

RandyT

[mpm32]

Good point,  I only turn the pots 1/8 of a turn.

[MameFan]

Randy --

While you may be thinking "VR" means voltage regulator, it also is usually labeled next to EVERY pot because a potentiometer is technically a "Variable Resistor"  


However, I concur that you need to be very careful before blindly adjusting things.  If you do adjust the ones that affect voltages you may cause the monitor to start producing X-Ray radiation and (hopefully) if the X-ray protection circuitry is working, it will shut down immediately. (If not, you'll end up getting a dose of it, more the closer you are to the neck)


You should first familiarize yourself with monitors in general...where things *typically* are and what they do.  Any pot next to the power input, large filter capacitor or flyback is generally going to be a power regulation.   The rest of the pots will affect the vertical and horizontal and guns and stuff.

I'd first take a black marker and mark the center of each pot to some common point (generally between the two leads from it or something).   Then make a note of EXACTLY how many times you rotate it.  And don't rotate too much each time.

Also there is one that you may think is a pot but actually is a variable choke coil...it's the horizontal width coil.  It contains a ferrite (iron) core and twisting it is delicate, depending on age you can easily crack the core.  Also sticking anything in it that is metal (even if covered in electrical tape) will affect the operation of it and you can't just your adjustment WHILE you're doing it... Use a nylon/plastic tv technicians tool instead. (Bob Roberts and Happs sells a kit of these--side effect is also they are fully insulated so no shocking hazard)


Go to RandyFromm's arcade school page and find the manual for your arcade monitor and find out what each control does first.. before you fiddle.
http://slot-tech-ftp.serveftp.com:8080


By the way... if you mess with the V.LIN (vertical linearity) control, that is the HARDEST to realign (next to the gun controls and yoke magnets of course) since a tiny adjustment affects the geometry significantly.   V.POS and V.HEIGHT are simple to adjust to your liking, V.LIN is not.

[RandyT]

Randy --

While you may be thinking "VR" means voltage regulator, it also is usually labeled next to EVERY pot because a potentiometer is technically a "Variable Resistor"  

Heh  I don't remember seeing them next to everything, but I believe you are correct.

The places I tend to notice these are where the function isn't labeled, which is usually in the power supply section.  Thus the connection.

But a good rule of thumb is:

On a board where you see a number of pots (variable resistors ) and most of them are labeled with the common functions, there's a very good possibility that the ones that aren't labeled shouldn't be toyed with.....period! .  ie. if the manufacturer thought it would be beneficial for these to be adjusted they would have been labeled with their function.

By the way... if you mess with the V.LIN (vertical linearity) control, that is the HARDEST to realign (next to the gun controls and yoke magnets of course) since a tiny adjustment affects the geometry significantly.   V.POS and V.HEIGHT are simple to adjust to your liking, V.LIN is not.

I agree.  But if you have a good crosshatch pattern and an accurate ruler it's not too bad.  Convergence is MUCH worse .   But you mentioning the difficulty involved cleared my memory a bit on this control (it's been a while) and I now know that this wouldn't help the problem Dak-Ak is having.  If his display was crunched at either the top or bottom and expanded at the other end, then yes, but not on both sides.  

Ok, MameFan, you know a little too much about monitors to have learned this in passing.  So, how many years have you been fixing these things (and what is wrong with Dak-Ak's monitor ?)

RandyT

[mpm32]

Excelllent link,  I wish I had it when I needed to adjust my monitor.

[MameFan]

Ok, MameFan, you know a little too much about monitors to have learned this in passing.  So, how many years have you been fixing these things (and what is wrong with Dak-Ak's monitor ?)

Last question first... I didn't read his post closely enough the first time.  Perhaps it simply is in need of a cap kit, if he has vertical fold-over/squishing.

Dak-AK -- Did the monitor have a great picture to begin with or don't you know?

Also, if he has something like a G07, they seem to be very sensitive to switch mode power supplies and line noise and end up bending/curling one side of the monitor. The computer (arcadevga) may be putting out too powerful of a signal for it to deal with and messing it up.  

We truely need more info like model # of monitor, quality before PC/etc.. to give better help.


As far as my knowlege.. I've posted a brief history of my knowledge gained here in another post about a week ago actually, if you bring up my posting history (from my icon) you'll find it.  Summary: Always have been an electronics tinker-er cuz my dad was a science teacher, and then a few years ago started collecting arcade games and made myself learn from tons of googling and web reading and manual reading to understand how monitors work and such.  Nothing magical at all.   Just a happy, knowledable novice... not a pro by any stretch (but probably know more than some or at least can apply knowledge of one to another and not have to refer to a book each time  )

[RandyT]

.... Just a happy, knowledable novice.

A learned hobbyist, perhaps, but not a novice .


BTW, thanks for the link.  I referenced that site a couple years ago which led to my purchase of the Sencore CR7000 CRT Analyzer and Restorer.  Great info.

If anyone is curious, the process of CRT repair/rejuvenation is described in great detail and is a good read for anyone who might have an old monitor with a dim or "smeared" image.  Here's the direct link:

http://slot-tech-ftp.serveftp.com:8080/monitors/crt/sencore/cr7000.htm

Not much use in this particular case, but seeing it reminded me of some of the problems others were having .

RandyT

[Dak-ak]

the monitor, with the original pcb, worked perfect... well, sorta...

it USED to look perfect (i had put probably 20 dollars into this game down at the local video store) but then one day the monitor was dead... so i picked up the cab...

it would just make a loud clicking noise when it was turned on, so i took off the back, and there was a crack in one of the transformers (i believe thats what it was) and was jumping a couple inches to a metal plate on the circuit board (pretty cool looking actually, but sorta dangerous/kept it from working) so i took a plastic lid from one of those coffee tins and stuck it in the space where it was arcing - stopped it completely (yay!)... the game looked fine then (like it did before) i hooked up my arcadevga, and after getting windows about right, it looks fine, no squashing (although on the lower resolutions the bottom line is very bright, perhaps a minor bit of overlaping?)... some games look "okay", some have the squashing problem very very bad... for the most part, fairly playable...

some games also have alittle black box in the lower right hand corner... about 1-1/2 inch by 1-1/2... very seldom shows up, but i don't think its tied to certain roms.. different ones do it, and those don't do it everytime... perhaps the same problem? i don't think so, but im not a monitor wizard...

btw, im not... a monitor technition by any means, but i know what not to touch (i've taken a couple crt's apart to service them before) just.. not sure exactly what to do with this one, and quite frankly, i don't want to pick around til i find it... a bit unsettling, and i might make it worse

if i can score a digicam, ill get some pictures, im sure that will make it 100% easier to identify... i know someone will be able to instantly recognize the problem...

[Ken Layton]

Look for a pot labeled "Vertical Height" or "Vertical Size" and sometimes they interact with the vertical hold control. On some Wells-Gardner monitors there is a pot labeled "50/60 HZ" which will also give you a little bit of compression or expansion at the top and bottom of your picture.

A word of caution: these pots are very delicate so turn them gently or they'll bust.

[jerryjanis]

A word of caution: these pots are very delicate so turn them gently or they'll bust.

Wish I'd heard that a week ago.  

[Ken Layton]

Mouser Electronics (www.mouser.com) has the correct replacement pot for only 61 cents. It's their part number 323-409H-2K.

[jerryjanis]

Wow, thanks a lot.  I just ordered it.  I wouldn't have known where to begin looking.

[Ken Layton]

If you change the "H" in the part number to a "V" you can also order the vertical mount style. The vertical style is used on the neck boards for the color balance adjustments. The numbers after the last dash in the part number I mentioned (in this case -2K) is the resistance of the pot.

[jerryjanis]

for only 61 cents

Hooray, I'm back in action: