QNIC CGM-2500 blowing fuse: SOLVED! Now having color problem...

[S1N3d dW17]

My monitor immediately blows its fuse when the cabinet is powered up.  I noticed that the large capacitor next to the AC input (C905) appears to be bulging out at the top.  Could this cap be bad, and would it cause the fuse to blow?

I used an ohmmeter to test the cap, and it seems pass that test, but the bulging in the casing just makes me think its bad.  Anybody have any experience with this monitor?

[Q*Bert_OP]

Even if it's not bad, when caps start bulging, I replace them.

[grantspain]

bigs caps tend to have plastic cover on the top that distorts due to normal monitor activity,i don't know your model monitor but they all work in a similar way,does it blow the fuse black or does it just part?

[S1N3d dW17]

Blows the thing black.

[grantspain]

all crt monitors have a degauss circuit that basically demagnetizes the tube,the circuit consists of a coil that surrounds the tube which plugs in to the mains input side of the chassis via a component called a ptc(its a back or white three legged plastic package near the mains input),basically when you switch a monitor on from cold the ptc directs voltage(acting similar to a thermal switch)to the degauss coil thus demagnetizing you tube for a very short amount of time-now a short on either the coil or ptc will blow your fuse to hell,first disconnect the degauss coil if no joy desolder the ptc component,then if you still have a problem look at any diodes(bridge rectifier)on the primary side of the psu ,sometimes things like the horizontal output transistor or the lopti(flyback transformer to anyone in the states) can cause fuses to blow but they are more likely to cause a monitor shutdown(trip),happy hunting

[S1N3d dW17]

I already tried disconnecting the degaussing coil from the chassis, same result, blew my last fast blow 3A fuse  .

I located the PTC and will desolder it and test as soon as I buy more fuses.  If I understand correctly, you're saying to desolder it and then try to power the monitor on without this component.  What should I expect to happen?

Also, is there any way to test the PTC with a multimeter?

Thanks for all your advice so far, it's been very helpful.

[grantspain]

first make sure it is the ptc(you can post a pic),if you desolder and repower the worst that could happen is your screen will be gaussed(discoloured),but if you don't move it then it will be fine for a while until you can purchase a new ptc,you know i have never bothered to meter one so i can't answer that question,i just know when they are bad

[S1N3d dW17]

I actually think I may hold off on the PTC, because I (once again) think I found the problem.  (Last time it was a 2.2Ohm resistor that is apparently supposed to read like it is shorted).

I moved to the next thing you said, the bridge rectifier, which in my chassis consists of 4 separate diodes.  I found one that is giving near-zero readings in both directions, so I desoldered it.  I'm getting readings of 1 or 2 on my ohmmeter in both directions outside of the circuit.

Seems to me like this could be the problem, correct?  If so, should I replace just this diode or all 4 that make up the bridge rectifier?

[grantspain]

yes any one diode short(always read out of circuit) on the bridge rec will put a direct short across the ac/dc(no jokes about the band please) which will blow the living crap out of your fuse,as for the 2.2 ohm resistor that would be a protection  resistor to help counter any serious short,i always read diodes by lifting one side out of the circuit board-by my reckoning you have found the fault,best of luck

[SirPeale]

Bulging caps are almost always bad.  Replace it.

Start checking the HOT and the bridge in the power supply circuit.  If it's blowing fuses, likely one of those things is bad.

You don't check capacitors with an Ohm check.  You need to check it with a cap tester.

Be sure to test the diodes and transistor with the diode check on your multimeter as well.

[S1N3d dW17]

Bulging caps are almost always bad.  Replace it.

Even though it may not be my problem, since I think it's the bridge rectifier, I ordered a new one anyway and will install it.

You don't check capacitors with an Ohm check.  You need to check it with a cap tester.

You can test for a short, which I believe can be one of the ways a capacitor can go bad (not that I can see why it would cause a fuse to blow, though)

Be sure to test the diodes and transistor with the diode check on your multimeter as well.

All I have is a cheap multimeter with an ohmmeter, no diode check .  I'll see if the one at my job has it, but I really think I found the problem as I stated in my last post.

Hopefully it only is that ONE problem, not a series of them

[grantspain]

i think peale is referring to the standard electolitics that have metal lid that looks like a hot cross bun,when they bulge then they are done for-the electrolitic you have has a plastic cover-they always degrade(the cover that is)
caps are difficult to read correctly as they charge,so your reading will change-leaky caps are almost impossible to identify which why it is advised to change all when you start getting wierd start up faults,colour bleeding,picture distortion etc.
to be perfectly honest i have only a couple of monitors where i have had to change alot of caps(hanterax funny enough)

[S1N3d dW17]

IT'S ALIVE!

Thanks to all your help, I installed the new diodes for the bridge rectifier (also installed new B+ filter and cap kit), and the monitor powered up and showed a picture!

However, the colors are all totally screwed up.  I tried adjusting the color potentiometers (red/green/blue drive/cut) on the neckboard (bad idea?), but I just can't get it anywhere near right.  It seems like the only color showing properly is blue.  I can try to get pictures if it would help me convey the issue.

I was told it may be a bad transistor on the neckboard, which would be why the green and red aren't showing.  But when I turn up the red and green pots, the screen DOES turn those colors.  It seems like it is a universal coating of red or green blanketing the whole screen, not just where they're supposed to be.  If these transistors are bad, would the screen still turn those colors when those pots are turned up?  Am I anywhere near finally finding a solution?

This cabinet is driving me nuts!

[SirPeale]

Then you need to test those transistors.  You'll still need that multimeter with the diode check.

[grantspain]

best to do a complete readjust on your colours,contrast and brightness,turn all your background,foreground colours to minimum,turn down brightness and contrast and screen volts-turn up the screen volts so you can just make out a picture,turn up the backgrounds to say half way,then the foregrounds,then use brightness and contrast then use colour pots to fine tune,you will probably have to try different pots at different stages(otherwise you may have a plug off or a broken wire or the said transistors)

[S1N3d dW17]

I tested the transistors in circuit, and they all appear fine.  One thing I noticed was that ALL of them appear to be shorted from collector to emitter.  I will have to desolder them and test them out of the circuit to get a more accurate test.  I checked for loose wires as well, thought I had one, but it had no effect.

I tried adjusting the colors, but I just couldn't get it anywhere close to correct.  For example, if I turn up the red drive, the whole screen will turn red.  It doesn't seem like there's any way for me to get red to show up where it's supposed to.  Same goes for green.  The only color showing up where its supposed to is blue.  Anything I did with the adjustments just doesn't affect this.  Basically what I mean is that the colors red and green ARE firing onto the screen, just not where they're supposed to.

Could these 3 transistors be the culprit?  If so, where can I buy replacements?  Even Bob Roberts doesn't list them!

[grantspain]

if two of your transistors are faulty its easy to prove just compare reading to the other good one,its a strange fault you have-it sound like the red and green foregrounds are not working at all.is this monitor a pc issue type if so maybe you have a problem with the colour decoder ic(usually chip on the tube neck card,sometimes its lm1308n).
strange thing is your original fault is the fuse blowing,you have repaired that but now you have a colour issue-the two should not be related at all.i don't want to sound like a ****head but i would take the chassis out again and check connections,also its not digital menu is it???

[S1N3d dW17]

You seem to be spot on with the foreground colors missing.

I seem to be getting the same readings from all the transistors, but I will test again.

I don't believe its a PC issue type, just a regular standard resolution arcade monitor.

I know this machine had been sitting in a storage container for years, so who knows what happened to it during that time.  I don't know what condition it was in when it was put in storage, but I have a feeling it was because of the monitor not powering on and all (blowing its fuse) and the op didn't bother fixing a low-earning Tekken 3 machine anyway.

It's possible I just don't know how to adjust the colors properly... could somebody explain the difference between red drive and red cut off?

If I can find replacement transistors I'll just replace them all; it's just a point of finding them.

[grantspain]

i have never heard of this monitor you have,do you have anything on a label anywhere else on the chassis(or maybe a picture),cut off is the background colours-turn this up too high and your entire screen will turn that colour,drive is the foreground colour act similar to contrast giving depth to each colour-turn this up too high and you get an overdrive of the colour.
if your transistors read the same then it ain't them,depending on the chassis type you will have feed resistors and caps that can go faulty or may have broken track-i have a strange feeling you have a wells garner monitor-best to post a picture though

[S1N3d dW17]

The monitor is a QNIC CGM-2500.  I know, I've never heard of it either.

[grantspain]

is it possible for you to post a photo of the chassis and the componant side of the neck card?

[S1N3d dW17]

I'll try to get one tonight or tomorrow.

[S1N3d dW17]

I've become tired of pulling this monitor in and out of my cabinet to work on it.  I brought it to a monitor specialist.  As much as I would have loved to figure it out myself, I'm just too frustrated with it.

Thank you for all of your help in diagnosing the problems I've been having.  At least I got to fix one problem 

[SirPeale]

You were putting it back into the cab to test?

Here's what you should do should there be a next time.

Open the back of the cabinet.

Get something you can get the monitor to cable height.  Put it in the back of the cabinet (but not IN the cabinet, just outside).

Put the monitor on that.

Hook up the monitor power and video signal.