My QNIC CGM-2500 monitor problem

[theHaMMeR]

I noticed a post about this same monitor having problems. Mine was blowing the 2.2 ohm resistor but I finally fixed that, it was a bad isolation xfmr, and was able to get video and play. In the middle of a game I lost the picture to a single line about 1/2" horizontally in the middle of the screen. I have the schematics for the monitor chassis so if anyone can help that would be great. Getting very irritated with it...

[grantspain]

sounds like you have a frame collapse-usual causes of this are the frame chip itself,missing supply voltage to the frame ic(safety resistor or cap),dry solder joints,bad connection to satelite remote board-usually the voltage to the frame ic will be 25volts and your schems should show a test point for this,then it easy to trace back the fault

[theHaMMeR]

I was told to check voltage to the vert. sync and I do have 22v DC to that. Would that lower voltage, even though its 2-3v low, cause the problem?

[grantspain]

22volts may be the correct voltage to run your particular frame chip,it all depends on the chassis type.usually when you get problems with the frame supply voltage its actually missing (0 volts)
if you have the schems it should show the frame ic and the test point,thats where you need to read the voltage otherwise you could reading further back in the circuit where the voltage is still present
i am intrigued by your previous fault with the 2.2 ohm resistor-this value usually denotes it as a safety resistor

[theHaMMeR]

I think I'm checking the same IC that you are talkin about. The diagrams I have of the IC's shows 10 pins, 3 of which are not connected. There is the ground, vert. out, out-vcc, input, feedback +b vcc and boost out. I checked voltage at the +b vcc and that's where i measured the 22vdc. Should I measure elsewhere? As for the 2.2 res. problem it was a safety, the bas iso. xfmr was causing a voltage to ground problem. Like I said after I replaced the xfmr everything was fine until the all of a sudden the screen shrunk. Thank for you input.

[grantspain]

i would check for bad solder joints on the scan coil plug and continuity of the scan coil itself,also does your monitor have a remote adjustment pcb?-also look at the vert/hor size pots check its not broken,whats the frame ic number?

[theHaMMeR]

Thanks, I will check all that. The IC number is LA7831.

[grantspain]

hey check this web site out sounds like your fault exactly-www.hamradio-badarc.co.uk/television/tvs/sharptvprt3.htm,and look for model sv287xh

[theHaMMeR]

Sorry I didn't see any help on that page. Maybe I missed something.

[grantspain]

it says,r518 2r2 smokes due to fault on i/c501 la7831-caused by caps c521 and c522 100uf 35v open circuit.so in other words change i/c 501 and caps c521,c522
is this not your fault exactly?

[theHaMMeR]

So you think i blew resistor 518? I will check into that and the caps mentioned. I don't think I have an overvoltage though. I did replace IC 501 with no help.

[theHaMMeR]

I may have confused you with my problems. The burnt resistor problem isn't related to the screen issue. I resolved that on my own. The screen issue is a new problem that just happened in the middle of playing. I thought it may be thermal but I had played for longer periods of time with no problem. Then one day after about ten to fifteen minutes of playing the screen shrunk. I did check the pots on the remote board and replaced two that were cracked, that didn't help any but I will keep working on it. Thanks for your help.

[grantspain]

is that not the same resistor you changed in the first place?unfortunately some faults require nearly the whole of that circuit replacing otherwise you end up going round in circles as you have more than one faulty component thus blowing the others
this monitor is pc type yes,does it have a standby led?if so what is its status?

[grantspain]

yes we are getting confused,sounds like the la chip is not your frame i/c-i think we are looking at the wrong part of the circuit,i would need a schematic to be sure

[theHaMMeR]

No that was a different res. I don't think this is a pc monitor. It is the original monitor that was installed in the cabinet. The cabinet is a dedicated NeoGeo 2 slot. Don't know if that helps any.

[grantspain]

i have never heard of this monitor and i have worked in the arcade industry for 20 years,nevermind we need to find the frame i/c and voltage supply,any chance of pic of your circuit diagram-this ones got me and i don't give up

[theHaMMeR]

I have pictures of the problem if that would help.

[grantspain]

not a frame collapse as such,you need to check your vertical size pot and check the continuity to the vertical scan coil,it looks to me the size pot but this looks a wierd fault

[theHaMMeR]

I gave up on it for a couple weeks to let the frustration pass. I decided to blow the entire cabinet out with compressed air just to get everything 100% clean. Now it works just fine ' '
Clap clap clap! The only thing I can think of is that the dust was causing a short in the windings. Hopefully this can help some one with the mystery problem I had. Thanks to everyone for the help!!

[grantspain]

congrats,maybe there was a sliver of solder shorting out the frame circuit somewhere