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Amber Compaq Monitor

antiquekid3

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Hey guys,

I was demonstrating my Compaq Deskpro to a friend when suddenly the monitor went dead. The trace collapsed to the center with a bright dot that quickly disappeared.

I want to first check the voltages, but I haven't a clue what I should look for. Can someone describe the monitor power jack on the back of the Deskpro?

Thanks!

Kyle
 

Raven

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The Deskpro is a very long series of PCs, you should probably be more specific - if you look up "Compaq Deskpro" it's very muddled..
 

antiquekid3

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Sorry, should've specified. It's the 386/20. The monitor plug has three pins and an outer sleeve.

Kyle
 

Raven

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That sounds like what I assumed when I first read your post, and that's a standard AC jack passthrough. Basically it's the inverse form of the plug that goes from the wall to your PSU on the PSU side. If you doubt the voltage coming from the socket on the PSU for that, then hook a standard PC power cable into the plug and then into the wall, if that doesn't work, then it's your monitor. If it does work, then it's the PSU.

Edit: To make sure we're clear.. I mean the plug at the top of this PSU's rear (the side facing us):

1109552104.600.jpg
 

antiquekid3

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No, this is a DIN-sized connector. I checked out the back of the monitor and it says "12V - 2A" and I found 12.3V at the back of the computer. So I guess this is a monitor issue...crap...

Anyone have a schematic? It's a Type: 12CQM31; Model: Compaq Deskpro DSM.

Kyle
 

antiquekid3

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This thread, now that I think about it, is probably in the wrong section. Sorry about that.

But here's what I've found: the filament is getting power and it lights up. There is NO trace of high voltage. Sounds like a flyback, maybe? Not something I wanted to replace...

Kyle
 

MikeS

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...There is NO trace of high voltage. Sounds like a flyback, maybe? Not something I wanted to replace...
Kyle
Maybe, but not at all certain; usual advice - check capacitors for visual problems, transistors for overheating/explosion, etc.

Good luck; I assume that's the dual-mode monitor, nice to have.
 

Chuck(G)

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The first thing that I'd look for is a blown fuse--sometimes these are soldered in and sometimes they masquerade as other axial-lead components. Sometimes they go just because they're old.

The next thing would be the horizontal output transistor.
 

antiquekid3

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I can't see anything visibly wrong with the board. Everything looks so clean! I also don't see any fuses on the board. I'll upload pictures soon so everyone can see.

I see a few large transistors. I presume one must be the HOT. One is a TZ994, but the other has a different (smaller) heatsink that covers the part number. THe heatsink is soldered to the board...

Kyle
 

glitch

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If you got a bright dot in the center, that indicates your deflection coils weren't doing their job. Does it still produce the bright dot in the center when powered on? If so, part of the deflection drive circuit is gone (don't run it very long like this, or it will burn a permanent dot into the center of the screen very quickly!)
 

channelmaniac

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I don't see anything that's obviously burned on the board.

If the monitor runs off of 12v AND you have high voltage (as evidenced by a bright spot on the screen) AND you have no deflection (as evidenced by the bright spot on the center of the screen)... then you have a power supply issue.

Now, it's not the power supply that supplies power to the monitor. That 12v is working and so is the flyback transformer that gives the high voltage to the suction cup on the side of the picture tube.

The power that is missing is derived from taps off of the flyback transformer. Look around the flyback carefully for burnt resistors or cracked diodes. Somewhere there's a missing voltage being derived off the flyback that gets used for driving horizontal and vertical deflection.

RJ
 

MikeS

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He did say that the bright spot quickly disappeared, so I doubt that there's any high voltage; that's what normally happens when the power goes off and the deflection circuits stop but there's still some residual HV in the CRT.

Any signs of a glowing filament in the back of the CRT?
 

antiquekid3

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The bright dot disappeared after a second or so. The flyback produces no high voltage, and thus, there is no longer a bright spot. The 12V is there, but there is no high voltage from the flyback. I don't think we can say for certain that the deflection circuitry is bad, since there's no high voltage present.

Kyle
 

antiquekid3

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Yes, the filament does indeed glow. I don't want to start desoldering unless I know with some confidence that it's a particular component.

Kyle
 

MikeS

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The bright dot disappeared after a second or so. The flyback produces no high voltage, and thus, there is no longer a bright spot. The 12V is there, but there is no high voltage from the flyback. I don't think we can say for certain that the deflection circuitry is bad, since there's no high voltage present.

Kyle
Quite right, nothing's certain. The high voltage is derived from the horizontal deflection circuitry though, so if there's no horizontal deflection there's no HV.

Maybe follow the 12V supply and see if it and any voltages derived from it are getting to where they're supposed to? And another careful visual inspection?
 

Chuck(G)

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If the screen collapsed to a dot and then faded, it's almost certainly a power issue. If it were just the horizontal, the screen would collapse to a vertical line.

Photos of the PCB, please! I suspect that there's a fuse or overload protector that you're not seeing.
 

Chuck(G)

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Thanks, I missed them, as I usually just go to the last page of a discussion I've visited before. Nothing shouts to me right now, but I'll give it some thought.
 
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