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M1296 12" RGB Monitor - No High Voltage?

warmech

Well-known member
So I got my hands on a 12" RGB monitor, but it does not appear to have much life to it. The font LED comes on, but there seem to be no other signs of life: no degausser hum (though that can vary from monitor to monitor) and no neck glow. C418 looks like it's leaked some fluid but, from what I've read so far, that doesn't especially seem like it would cause the monitor to go into shutdown; the usual issues with that cap appear to be related to image geometry more than anything, but I'm open to it being a culprit. The fuse is good and no other components look to have blown; there are no bulging caps but, having worked on (much simpler) 15KHz arcade monitors, that isn't always an authoritatively good sign. Before I go testing yanking the chassis out and testing transistors, is there any advice anyone can toss my way? If a cap kit is a likely fix, then I'm more than willing to take care of it; I'm just worried a transistor (or the flyback) has gone that is now unobtanium and would render much of the work in trying to figure out what's happening pointless.

Thanks y'all! :)

 

techknight

Well-known member
Thats actually not a good sign. 

Sure, Caps are an issue in this model. Actually, a Big issue. But, Something has already failed that could have been directly, or indirectly caused by the capacitors, and a simple recap isnt gonna fix it

So, with that said, I am going to hazard a guess: 

One of the main weakpoints in this particular monitor is the cluster of caps next to the horizontal drive transistor and transformer leaking. They sit next to some big heat producing resistors. 

If this capacitor leaks bad enough, (and they do), it can eat the traces or solder joints in that circuit. Which as a result, will produce no horizontal drive. Creating the symptom you are seeing. 

 
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warmech

Well-known member
Thats actually not a good sign. 

Sure, Caps are an issue in this model. Actually, a Big issue. But, Something has already failed that could have been directly, or indirectly caused by the capacitors, and a simple recap isnt gonna fix it

So, with that said, I am going to hazard a guess: 

One of the main weakpoints in this particular monitor is the cluster of caps next to the horizontal drive transistor and transformer leaking. They sit next to some big heat producing resistors. 

If this capacitor leaks bad enough, (and they do), it can eat the traces or solder joints in that circuit. Which as a result, will produce no horizontal drive. Creating the symptom you are seeing. 
I was hoping that wouldn't be the scenario, lol. Thank you so very much for pointing me toward that; I'm going to start looking for damage. I'm assuming the horizontal transistor will need a replacement as well?

 

warmech

Well-known member
So I pulled the chassis and, as suspected, the area under C418 is pretty rough. The good news is that everything except for one trace (between the two resistors next to C418) tests okay; some of them are pretty roughshod, though, so I'm going to jumper anything that looks eaten just to be safe.

IMG_2027.jpg

The two resistors are another story altogether, though. In flipping the board back over, I grazed one of the resistors next to C418 and it just instantly fell apart. I'm going to assume the other is in a similar state, lol. I can't find schematics for this chassis, testing reveals they're toast, and the colors are a little faded on the them, but are those 5.5K ohm (Green Green Red Silver) resistors?

IMG_2029.jpg

Altogether, this looks addressable if there's nothing else amiss. Would it be worth pulling the transistor nearby (D1138) and testing it, or is it unlikely to have suffered aside from trace destruction? Either way, I'm going to order new caps and resistors (once I know the value for certain) and see where that takes me. Everything else on the board looks okay - the flyback isn't cracked or leaking and nothing else looks eaten up or blown out.

 

techknight

Well-known member
Ok so it appears that resistor is broken in your photo, which is likely why your monitor appears dead. This effectively cuts off the B+ supply to the horizontal drive transistor. 

So you should remove those caps and do a super thorough cleaning. Then replace the caps. Also, replace the damaged resistors and patch up any damaged traces.

The value of the resistors is hard to tell, the color bands are burnt right off of them it looks like. Might have to measure the one good one and report back. They are 2W resistors. Ill check my boards if I can find them to see if the color bands are any more readable than that to determine its proper value. 

Anyways, after replacing those parts and cleanup, it should work fine after that. 

 
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warmech

Well-known member
Ok so it appears that resistor is broken in your photo, which is likely why your monitor appears dead. This effectively cuts off the B+ supply to the horizontal drive transistor. 

So you should remove those caps and do a super thorough cleaning. Then replace the caps. Also, replace the damaged resistors and patch up any damaged traces.

Should work fine after that. 
Sweet deal! So those are 5.5K ohm resistors, then?

 

warmech

Well-known member
Check my edit. You caught me in the middle of editing. 
Hehe, sorry to jump the gun on you, there. :)  Both resistors appear to be toast, unfortunately, as a multimeter reading fails on both. I've been searching for photos of the chassis to no avail as well, sadly. Any help is tremendously appreciated, and thank you again for your assistance!

Edit: Speak of the devil! I found a nice, large image, but there's just a little too much glare to tell what the third color is, but it looks different on each resistor.

OriginalC418.jpg.ad98cc724c7c9eacb67a13b78c49654b.jpg


 
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warmech

Well-known member
Okay, after some more googling, these look like 820 mOhm 5% 2W resistors - Grey | Red | Silver | Gold. I'm basing this off of my photos, the recent one above, and (amazingly) an M1296 diagnostic video that I was surprised to find was made by you (screenshot below)! :)  I'm guessing the color bands did actually burn off over time, just like you said - also, I doubt that band between red and gold is white, lest we be dealing with an 8.2 GOhm resistor, lol.

Think that's a reasonable theory?

 Screen Shot 2020-09-29 at 4.44.15 PM.png

 
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warmech

Well-known member
Doesn't look like I can edit my previous post, but, that may be either grey or violet for the first band; after looking at mine again under very bright white light, it is difficult to tell if it is either grey or violet. Would there be that big a difference between .72 Ohms and .82 Ohms? I know it's B+ traveling across them, but, can .1 Ohms even have that big an impact on it? Either way, I've confirmed both of mine are trash after using a second multimeter, so any confirmation will have to be external to these. :(  

 

techknight

Well-known member
I will need to dig out my boards and double check them. 

That silver/white could be a faux paus because of the fact the resistors get really hot in the middle, So the color fades. 

Its definitely Purple, Red, Something, Gold. 

Gold is common for tolerance, that and silver. 

I highly doubt they were .72Ohm, if they were they wouldnt have gotten so burned up. 

Its something else. 

 
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warmech

Well-known member
So, a nearby friend mentioned he had a working monitor and offered his for reference; I'm a bit stumped - it looks like it is .82 Ohms, but that seems so dang tiny an amount of resistance. I don't think the multimeter could measure low enough (I'm assuming the 2 Ohm reading is an error), but neither he nor I have anything more sensitive to measure with. If you're able to take a look at your boards, I'm really curious as to whether these really are what they appear to be.

Thanks again for your help, man - it's greatly appreciated! :)  

 IMG_2036.jpg

IMG_2042.jpg

 

techknight

Well-known member
Well, it might be that value then. Also, yea you cant measure resistances that low with a cheap multimeter like that one. your leads have more resistance than the actual resistor does. 

Dang, those get HOT for such a low resistance. There must be a ton of current flowing through that. Insane. 

 
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warmech

Well-known member
your leads have more resistance than the actual resistor does. 
Hahahaha! Oh physics, you troll.

Dang, those get HOT for such a low resistance. There must be a ton of current flowing through that. Insane. 
That's what had me so stumped, too! I figured I was just interpreting things incorrectly due to the fact that, on pretty much every standard res monitor used in an arcade cabinet from the same era, the resistors that B+ ultimately traverses are all gigantic concrete affairs, and THEY even get hot. Not to the point of failure, obviously - and they're almost always mounted to a chunk of aluminum - but too hot to remain touching for very long.

 

techknight

Well-known member
Well its rare for those resistors to go bad on these monitors, but they are 25+ years old now so anything can happen. 

The trouble is the design of this monitor. Those resistors literally BAKE the capacitors next to it, and boom. Recipe for disaster. 

 

warmech

Well-known member
Alrighty, back with an update! The chassis has been recapped and the two resistors replaced and, lo and behold, it powers up! Except now I have a vertical linearity problem and the screen jumps up and down like a Jack Russell Terrier. Adjusting the V-LIN pot all the way leaves several lines of foldover at the top of the display, and I have been unable to track down the issue. Not being able to find any schematics for these things isn't helping either, lol. In addition to the foldover, the image bounces up and down rapidly, but only as you increase the vertical image size. If you dial it back down to a fairly small (squashed) size, the bouncing goes away and the image stabilizes. I assume there is once again something wrong with the vertical circuit (guessing deflection with the second issue), but I'm so dang frustrated by working on this thing without any kind of roadmap. Has anyone ever run into this kind of thing, or would anyone be able to recommend a good starting point from here? I'm attaching a short video to demonstrate the issue.


View attachment IMG_2198.MOV


 

techknight

Well-known member
Double-check your work. you probably made a mistake somewhere. Please make sure you didnt do any random capacitor value substitutions, or, potentially any mixup of non polar caps with polar caps, wrong value caps switched around, etc. 

If that pans out ok for certain, then change the vertical output IC. 

 
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