Hi all, I found in this page: http://geocities.ws/podernixie/htpc/...tml#jammascart that a 1KOhm resistor is needed on the Sync PIN.Now, in my selfmade Supergun I don't have this resistor while I have the three 220Ohm resistors on RBG and I get perfect colours on my SCART TV and also sync is good except for some PCBs (bootlegs mainly) that roll up and down.For example I have a Pitfall II PCB which syncs well in a Sony TV while it rolls in a Grundig TV and in a Amiga monitor M1438s which is connected to the Supergun through a sync splitter (with EL4583 IC) since this monitor accepts only H and V sync. So my question is: Is that 1KOhm resistor really necessary on the Sync pin?What is its utility? Thanks.. P.S. Is it possibile to add some circuit to adjust vertical sync on a JAMMA Supergun?I know on arcade monitor it's possibile.
The JAMMA video spec is quite loose, it can be anywhere between 1vpp to 5vpp at around 1k ohm (which is why you see the 1K ohm resistor being used). Compare this with the SCART video spec of 0.7vpp at 75ohm. Most SCART tvs can cope with the higher voltages but may require a resistor to limit the voltage. I have a 5K pot on the sync pin as I find it does help certain issues with some of the boards I have, I also have a triple ganged 1K pot so I can adjust the RGB lines as if the voltage is too strong it can affect the sync.
OK, Thanks.I had the three pots on RGB but then I removed them and put normal 220Ohm resistors.How do you connect the pots?Did you use also the ground on one leg?
You can arrange the pot as a voltage divider (input at one leg, GND at the other leg, wiper is output), or you can arrange it as a series rheostat (input to one leg, wiper connected to the other leg which is the output). Probably go with the rheostat method since it's simpler. 1K pots should be enough since 1K can attenuate a 5V video signal (no arcade games should be higher than about 4V) to half the 100 IRE (0.714V) consumer standard.
Ok,Thanks.I'll try it as rheostat.Is the "wiper" the central terminal, right? So I have to connect the central and one adiacent terminal each other and this would be the output while the remaining leg is the input?
I have put 1KOhm pot following your indications but I noticed that the voltage on the terminal output doesn't change rotating the knob and I can't understand why. Here is how I wire it: http://img688.imageshack.us/i/potg.jpg/
Wired like that you're not changing the voltage, just the resistance - if your TV has capacitively-coupled inputs (some do this to allow them to use a live chassis with no earth pin) it won't work. Try moving the purple wire to the middle terminal, detaching the link and attaching the end terminal to 0V. Pin1: Jamma sync input Pin2: SCART sync output Pin3: 0V This way you're guaranteed to be adjusting the signal's voltage between the original voltage level and 0V, however your TV is wired internally. Stone
Ok, I will do it .For 0 volt did you mean video GROUND of JAMMA? But If I remember well , I already tried this configuration of the pot and didn't not resolve the sync issue on some boards but I noticed that rotating the knob all PCBs lost their sync at certain point. The strange is that some PCBs have sync problems ONLY on a Grundig TV and a multisync M1438S Amiga monitor while on a Sony TV they have correct sync. My goal is to adjusting the sync of these PCBs on the Amiga monitor since I use mainly this on my jamma test ring.Since this monitor doesn't accept composite sync but only separate H and V I use a EL4583 sync splitter to wire it to my supergun. So, maybe, I can put a pot ONLY on the vertical sync since I think the problem of these PCBs is related to this one and not to the horizontal Sync judging from the rolling up&down picture.
You are changing the voltage (you cannot see this without a load). The series resistor forms a voltage divider with the 75 ohm termination resistor. Turning the pot to 0 ohms won't attenuate. Turning the pot to 1k ohms will attenuate 14X. Since video DACs will run off 5V (which will have a maximum Vpp of ~4) video can be adjusted down to 0.29V, more than adequate. The only non-linearity in this config will be due to the sinking/sourcing capabilities of the DAC with various loads (75-1075 ohms). And it WILL work with capacitively coupled inputs... Dunno why you think that. High input impedances make for high RC constants but using the pot as a voltage divider as you suggest doesn't change that at all, only an active design would. Using the pot as a voltage divider will "work", sometimes, but it's a lot more unwieldy here because you'd only be adjusting the wiper between the top 1/4 of the pot and the response is non-linear, especially for this range. The pot is also in parallel with the termination resistor which innately lowers the video level. If a game did in fact output 0.714V into 75 ohms this configuration couldn't help but attenuate where the rheostat can just be adjusted to 0 ohms.
Can you tell me where t ofind this triple ganged 1KOhm pots?Thanks P.S. Regarding the 5K pot I found a 4.7 one, I think it should fit anyway
It was a industrial pot that I managed to buy via work, it is not cheap being about $45 but it does have excellent accuracy and should have a much longer life span then commericial pots. I wired the pots up so one end is connected to the video out of the board and the centre is wired to the Megadrive RGB wired din socket. It seems to work well regardless of TV I tried but generally needs to be adjusted depending on the board.
Ok, I ask on potentiometers.com which has triple ganged KKK series pots but there is a minimum order of 50 pieces and each costs 50$! Regarding your pot, I don't understand well if you used it on sync pin or RGB lines of the JAMMA.
You're right, I'd forgotten about the termination :nod: I used to have a TV with capacitively-coupled inputs that weren't terminated until they'd been through a cap so only changing the Vp-p changed the received signal amplitude. It was a pretty shite set though Stone
Triple ganged pots for RGB is just a bad idea. You're assuming the RGB levels from the mobo are always going to be equal. If one color is not the same level you CAN NOT correct the picture with a triple ganged pot. You can only make the picture brighter or dimmer with the same color problem. Individual pots for RGB is the best fit. I usually use a 1k pot on the sync line, 5k might be overkill and harder to tune properly because of the high resistance.
I used a triple gang pot as 99% of the time the RGB levels on the board will be fine and it saves having to tune the pots one by one, again the idea is to get the levels low enough so they match the SCART standard. I have about 25 boards and not one has a colour that does not match the other colours, so white looks white. I do not use pirate boards which is likely to be the only case the colours would be out. Also use a 5K pot as it works better then a 1K pot, again most of the time you are only worrying about getting the levels down to the scart standard so fine control is not really needed. I generally find I need to use it a few times on certain boards and a quick twist generally fixes it.
There's nothing wrong with a gang pot as long as it's configured to attenuate linearly; displays themselves all have the same R, G and B termination of course, it's the same principle. If the color components aren't evenly weighed they won't appear even on a proper display so the attenuator should have the same effect. The only time a gang would not be OK is if you are trying to play an ancient game with a 4, 5, 7, 8-bit palette ROM and your pot is arranged to attenuate in a non-linear way. In this case one color component has less resolution and is unlikely to have the same output impedance which will throw off the non-linear attenuator (such as the voltage divider configs mentioned here), so that component (probably blue) will be off. When you have pots for each color component it's sort of analogous to treble and bass controls, by all means adjust the colors however you please but they probably aren't the intended hue. With a gang pot in linear config you'll always have the correct hue and you'll only be adjusting the brightness, and quicker, and with better precision. IMO the only disadvantage is that they're really expensive and if I were going to go that route personally I'd start looking into an active termination and attenuation design.
