Confused about potentiometers (want to control 3 signals at the same time)

[dos]

I'm just starting out with electronics, in the past few months I've taught myself how to solder by assembling the little Velleman kits they sell at Microcenter, etc. I decided it was time to give myself a "real" project and set out to build a supergun because I'm big into arcade games, and actually wanting to repair my own games and hardware is the reason I decided to learn, though I'm finding it's a fun hobby on it's own! Anyway, I'm a little confused about pots and thought I'd ask here since I can't really find the answer I'm looking for with google or asking in the various arcade-specific forums:

When most people build a supergun at home they use 3 linear pots for the analog video signal: one each for Red, Green, and Blue (arcade boards output pure RGB). People  typically use a resistance between 250-1Kohm. These pots are necessary because arcade games never output a standard brightness so the user has to be able to adjust the value when changing the game board, each one is different. This isn't to say some games have higher red values and some have higher green values, etc. The vast majority are uniformly brighter or darker. A professional/factory made supergun has only one brightness knob that allows you to easily adjust the brightness uniformly without tweaking each individual line. I'm just wondering what's the best way to implement this in my build. Do I just stick all three lines on a single pot? Do I need a triple gang pot? I've tried looking for triple gang linear pots with those specs and it seems like they don't exist (I've checked mouser, ebay, etc). Is there something else I'm missing here?

If it makes any difference I'm not integrating any kind of video encoder with this build, I'm working only with the RGB signal (which I send to an external component processor with all my other systems). My build right now is as simple as can be, with a JAMMA harness wired directly to a power supply, RGB SCART connector, high/low audio converter, and DB15 joystick ports. Everything is working great, but there's no control over the video signal yet so naturally many boards look too bright or dim etc.

Here's a JAMMA pinout (the harness that connects to the game):



I have R, G, B, video ground, and video sync connected to the proper places on the SCART connector (which is only being used for video in this build):



The pot(s) would be on the RGB lines between the harness and the SCART. I hope this makes sense!

[Alex Trofimov]

Maybe try Voltage Controlled Variable Gain Amplifiers and one pot to set control voltage for the three of them.
For example, LMH6505, it's 2-channel, 6$ per unit on digikey. There're a lot of such devices from different manufacturers, of course, this is just the first I've found.)

[monkeysuncle]

Building a linear VCA that won't compromise video signals will be a challenge. I recommend you get a triple-gang potentiometer. Upgrade to an active solution only if you find that a triple-gang pot isn't working for some reason.

Note, you may still want to add buffering in order to properly drive 75R video lines.

Will your device be putting out RGB, composite, or some other component variety? If the final output is not RGB, then you need some kind of transformation matrix, and it may make sense to incorporate the image adjustments into that matrix.

[dos]

Building a linear VCA that won't compromise video signals will be a challenge. I recommend you get a triple-gang potentiometer. Upgrade to an active solution only if you find that a triple-gang pot isn't working for some reason.

Note, you may still want to add buffering in order to properly drive 75R video lines.

Will your device be putting out RGB, composite, or some other component variety? If the final output is not RGB, then you need some kind of transformation matrix, and it may make sense to incorporate the image adjustments into that matrix.

The output is pure RGB. Where do I get a triple gang potentiometer with a resistance between 250-1Kohm? I have looked everywhere and they don't seem to exist.

[Richard Crowley]

Where do I get a triple gang potentiometer with a resistance between 250-1Kohm? I have looked everywhere and they don't seem to exist.

https://www.fairradio.com/catalog.php?mode=viewitem&item=4353

[dos]

Where do I get a triple gang potentiometer with a resistance between 250-1Kohm? I have looked everywhere and they don't seem to exist.

https://www.fairradio.com/catalog.php?mode=viewitem&item=4353

Awesome, thanks! Do you know if there's a quick way to tell if a pot is linear or logarithmic (I need lin), or should I just ask them?

[Richard Crowley]

A 3-gang or 4-gang pot is so RARE here in 2016, you don't have the luxury of selecting linear/log taper.
Besides, for something as semi-static as the "brightness control", it won't make that much difference.
If you are REALLY that hung up on taper, you can always use one of the shunt-resistor hacks to change the taper.
I would think that is a linear taper, but I wouldn't bet $$ on it.

It is trivial to measure if it is linear/log.  And at US$4, it is a monumental bargain.
You will probably have to pay more to USPS or UPS to get it to your doorstep.
With 100s of thousands of vintage items scattered about dusty warehouses, I rather doubt they will go out and measure one for you.

[dos]

A 3-gang or 4-gang pot is so RARE here in 2016, you don't have the luxury of selecting linear/log taper.
Besides, for something as semi-static as the "brightness control", it won't make that much difference.
If you are REALLY that hung up on taper, you can always use one of the shunt-resistor hacks to change the taper.
I would think that is a linear taper, but I wouldn't bet $$ on it.

Gotcha, the way they talk about it in the arcade community makes it sound like you absolutely need a lin taper, since I've never messed around with either I was just going along with that. I've asked the seller anyway & am waiting on a response but I'm just going to order it either way to see what it does. I really appreciate all the help!

[txescientist]

You could use 3 regular pots and join their shafts - radiohm pots have slot on their back shaft. Because you use them to trim,  you could also connect 3 bigger trim pots on one shaft. http://www.soselectronic.com/a_info/img_data/a/trim/pt15nh.jpg

Regards, Milos.

[dos]

I just want to update this post in case anyone has the same questions I did: I got the A-4627 linked by Richard and it works perfectly. Couldn't be happier, all three colors adjust in perfect sync. I'm actually glad the site had a $12 minimum order because I ended up getting 3, in case I need one in the future. The pots are NOS from like 1958 and came in some crazy packaging that smelled like 1958. They're absolutely enormous though, here's one with a soldering iron for scale