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Author Topic: Wells Gardner K7400 - Rolling Display  (Read 1315 times)

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tamnosbig

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Wells Gardner K7400 - Rolling Display
« on: April 24, 2022, 01:29:28 pm »
Please accept my apologies if this topic has been covered elsewhere. I've scoured these forums looking for a solution but I haven't found anything yet. Here we go:

I have an old Arcade Legends cocktail table from Chicago Gaming Company with a K7400 monitor. The PC was missing so I'm attempting to hook up a new one with Windows 10 Pro.

The cabinet uses a USBLinx for an interface between the PC and the controls and CRT. I have an ATI HD 5450 (low profile) card with a DVI/DMS-59 connection port.

CRT Emudriver installed without a hitch and I can see the Windows desktop environment on the CRT. I followed Calamity's installation instructions to the letter. Because the video card only had one analog output, I chose Analog_0 for EDID emulation.

Here's the problem: I can't settle down the horizontal settings to save my life. I've spent hours attempting to adjust the H and V holds and position settings on the remote and also the one H-hold pot on the main board. It simply won't stop rolling.

Before I go to the trouble of installing a cap kit, I wanted to share this issue with you fine people and find out what else it might be. I'm guessing that's the next step because of the problem.

Thank you so much in advance for offering any advice! Cheers!
M
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Calamity

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Re: Wells Gardner K7400 - Rolling Display
« Reply #1 on: April 25, 2022, 02:25:31 pm »
Hi tamnosbig,

I know nothing about the USBLinx, whether it outputs c-sync, etc. You probably don't have v-sync to your monitor. It's either that, or the sync polarities are wrong (if these are switchable on the chassis).
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tamnosbig

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Re: Wells Gardner K7400 - Rolling Display
« Reply #2 on: April 25, 2022, 03:28:21 pm »
Thanks, Calamity. I'll start messing around with it.

Found this PDF manual for the USBLinx:
https://service.globalvr.com/downloads/ultracade/components/040-USBLNXM-UCT_USBlinx_User_Doc_v02H.pdf

Sounds like your thought on sync is on the money. Here's the short entry for video:

Quote
NOTE: No resolution or frequency conversion is done by the USBlinx.  The only processing of the video signal is amplification on the red, green and blue signals and the combination of separate horizontal and vertical syncs into a composite sync signal.  For frequency and resolution conversion, please refer to the UltraCade uVC video converter at www.ultracade.com . The video circuit in the USBlinx is used when the target monitor is either an arcade CGA or EGA monitor.  The USBlinx provides three necessary functions to convert PC signals into arcade monitor signals. 

Firstly, the BIOS has control of the video during the PC boot-up phase and may output incompatible signals.  Secondly, the voltages of the RGB signals are different.  Finally, the synchronization format may differ. During PC boot-up, the application, and even the operating system, has no control over the video mode.  The BIOS uses whatever default mode the motherboard supports.  There is therefore the potential that these modern synchronization signals may damage older style arcade monitors that do not support such fast synchronization signals.  The USBlinx therefore disables the synchronization signals from propagating to the target monitor until the DDC signals enable it. 

DDC consists of clock (SCL) and data (SDA) lines, and is a mostly unregulated method for monitors to identify themselves by sending and receiving configuration information.  The SDA line defaults to high, and the USBlinx needs to see the SDA line pulled low to enable the pass through of the video synchronization signals. Both PC and arcade monitors use +5v synchronization signals, however, the voltage ranges are different in the red, green and blue signals.  The USBlinx therefore provides a video amplifier for converting the PC’s 0.7vpp RGB signals (input into the DB15 connector J11) to the arcade monitor’s 5vpp signals (output through the JAMMA connector).

The type of synchronization signals used by arcade monitors can differ.  All PCs produce separate horizontal and vertical synchronization signals, while some arcade monitors require just one composite signal.  The USBlinx therefore combines separate synchronization signals (input into the DB15 connector) into a composite signal (output through the JAMMA connector).

« Last Edit: April 25, 2022, 04:06:08 pm by tamnosbig »
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Zebidee

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Re: Wells Gardner K7400 - Rolling Display
« Reply #3 on: April 25, 2022, 04:23:18 pm »
I hope you get it sorted out.

Another thing that often goes wonky with old CRT monitors is the pots can wear out, especially the V-hold pots because they get used a lot.

Easy enough to check pots with a multimeter on ohms setting. While powered down, connect one probe lead to one terminal, another to the "wiper" (usually the middle pin). Then you can move the pot slowly through the range and see if the ohms readings move reasonably smoothly. Repeat for the other terminal.

If you get patchy changes to resistance, dead shorts or open circuit readings, that suggests the pot is stuffed.
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tamnosbig

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Re: Wells Gardner K7400 - Rolling Display
« Reply #4 on: April 30, 2022, 05:32:00 pm »
I bypassed the USBlinx unit and the situation didn't change. So that's one thing checked off.

I attempted to use the H-pot on the main board and noticed something: if I apply pressure with the screwdriver while tuning the pot, the picture settled down significantly. That has to mean something.

Going to take a multimeter to the pots and check for cold solder joints...I keep thinking this monitor isn't THAT old but these arcade machines came out in the 1990s (?) so that's close to 30 years if not more...!
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Re: Wells Gardner K7400 - Rolling Display
« Reply #5 on: May 01, 2022, 03:02:31 am »
Yeah - standard pots used in most arcade monitors are not intended for constant use - maybe as little as 100 turns for the cheaper ones. The carbon film layer is fragile and can get worn pretty fast. Hopefully resoldering the joins will help, but if the carbon film layer is worn then not much you can do except replace it.

For this reason, I've been using cermet (ceramic-metal) trimpots for my projects lately, where possible, as they are rated to last around 50k turns, about 500 times longer than carbon film pots! Adjustments are smoother, and there are both single-turn and multi-turn (even finer adjustment) varieties. Cermet trimpots cost a little more but are not hugely expensive. Bourns is a popular brand. You'd just need to find the matching ohms rating, footprint and orientation.
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