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Need help with an RGB monitor I found at the dump


GoldenWheels

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I saw this little guy on our dump donation table today. Magnavox RE4050. I was impressed that a monitor made in 1984 (I THINK, according to the faded and hard to read back sticker) had UHF, VHF, Composite, AND RGB connections. It was dirty as hell but I had to bring it home.

 

So I get it home and hook up a nes clone. Nada from that input. RGB had a live screen, but I have no RGB devices. So I switch back to composite and jiggle the composite inputs. They're loose, and when I hit them a certain way, there is the picture and sound, sort of. So I figure dry joints, no prob, that's something I can actually fix. I do that, put it all back together, and voila! It works. But I cannot get the monitor adjusted right....see below.

 

Screenie.jpg

 

That section of the screen always ends up crawling left. I can sometimes get it dialed in for almost 2 seconds...but then it just crawls again. To adjust it I am using the vert hold combined with the "level" button that has a leading line to the video port, as seen here:

 

Back%20Controls.jpg

 

The "level" button and the vert hold seem to have to be in certain spots to get the picture to not roll and to be as you see there. Here is the back of that board, showing my (crappy) reflows on the composite ports:

 

IMG_2010.jpg

 

That "H position" (horizontal, I am assuming?) turner on the back does bupkiss. All the way right, all the way left, nada. I don't know if the pot (think that is what that is) is shot or what....joints looked good but I reheated them anyway just for laughs.

 

Does anyone have any ideas here? Would a working "h position" help me here (the image is not correctly centered on screen). Am I missing something? Is the thing just shot? (keep in mind I am not too great with this stuff....I probably should not have even opened it up as I do not know how to properly discharge them and I guess you can get the shit shocked out of you. But I was able to pull the back and work that one board without going near any other guts.

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can you get any numbers off the board? it would help to see a schematic or look up and find a pic of he whole board. I looked and the solder joints appear to be fine on the h position pot like you say. Just from looking at the pic all I can trace now is one line from one side of pot to the 150nF cap, and the other side of pot going to a resistor to the ground plane. I'm not really sure. though. I'd like to see the other side of that cap the board behind it. Also the other pin of the cap if you look at the pic it looks like it may have been hot in that spot by looking at the board but I could be wrong could just be some residue on the board there, possibility could have cracked joint there but the joint looks fine by looking at it but just knowing there was heat there it could still be cracked, may want to reflow that also. Can you pull back that cap and take another pic on that spot, and also get any numbers off the board to look up? Also I'd like to know what IC that is behind the cap if you can take a pic from other side of board.

Edited by SignGuy81
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I've looked all over trying to find a PDF service manual for your TV and turned up zilch but I did find this http://www.ebay.com/itm/Magnavox-Service-Manual-for-the-RE4050-RAD139-TV-Monitor-mp-/201599949577which may help. It is the only service manual I could find for that model, unfortunately couldn't find free but that one isn't too high I guess.

Edited by SignGuy81
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Signguy I am too exhausted to take it apart again right now (just mowed my back 40) but I did take more pictures beforehand, if they are of any help. This one shows the H position adjuster, and the number on the board itself (not visible here) is NEC PWC-1875. The H position adjuster turns just fine but seems to do zero.

 

Back%20Controls%20Inside.jpg

 

And I may buy that service manual on ebay, thanks. Crazy that there is so little out there on this thing, there's not even a pic of it via google. Just that manual.

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I saw this little guy on our dump donation table today. Magnavox RE4050. I was impressed that a monitor made in 1984 (I THINK, according to the faded and hard to read back sticker) had UHF, VHF, Composite, AND RGB connections. It was dirty as hell but I had to bring it home.

 

I had a TV like that, though don't recall the brand. My folks gave it to me for Xmas of 85 (maybe 86). I'd asked for a Commodore 1902 for the 80 column support for my new Commodore 128, so initially I was disappointed when I unwrapped a TV. But then I was excited when I discovered it had an CGA port (I think it looked like the one in your photo), which was compatible with the 128's RGBI output. Then I was disappointed again as it didn't come with the cable needed for the 128 and, of course, all the stores were closed :lol:

 

It worked great, even supported interlaced RGBI video which not all RGBI monitors did (a few friends w/128s couldn't use it). I used interlaced mode to output an 80x50 display from my 128. That was quite useful for editing large programs, though the flicker was pretty bad so I'd have to take extra breaks. One day I came in and discover that wearing polarized sunglasses suppressed the flicker enough that I no longer needed those extra breaks.

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You could try putting a voltmeter on the wiper of the pot and see if it changes while you turn the knob. Try it on both dc and ac. If nothing happens on the lowest voltage range you can use, then suspect a faulty pot, desolder it and test it for resistance.

Hrrmmmm.....I was about at the limit of my abilities just re-soldering it but I have wanted to learn how to use a volt meter.

 

I assume the TV has to be plugged in and on when I do this testing? I'll admit that makes me nervous. I see what I think you are calling a wiper (the arm that swings back and forth inside as you turn the pot) where do I apply the voltmeter tips to get a reading?

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Probably best to leave it alone then if you aren't confident. There are some extremely high voltages in a CRT TV. Or you could just leave the tv to me in your will ;-)

 

With the tv off you could use a multimeter on its resistance range to see what you can find out about that pot, though without the circuit diagrams it's hard to be conclusive, but looking at the solder side of the board we see that the wiper is connected to one end of the track. That tells us the component is being used as a variable resistance rather than a potentiometer (a variable potential divider). https://en.wikipedia.org/wiki/Potentiometer

 

If B5KX means its a 5k ohm device, put the meter on the lowest range above that, probably 10Kohm. Measure the resistance across the two outer connections and turn the control, but that does depend on external circuitry. Change the meter to a lower ohm range if necessary to get a more accurate reading. As you slowly turn the control you should get a smooth change in the meter reading (easier to see on an old analog meter with a needle than it is with a digital meter). If it is at all jerky or doesn't move at all then it is possible that the contact between the wiper and the resistance track is dirty.

Let us know what you find and we can take it from there.

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Probably best to leave it alone then if you aren't confident. There are some extremely high voltages in a CRT TV. Or you could just leave the tv to me in your will ;-)

 

 

 

When I was a kid, probably 12 or so. I took apart an old TV we had in a garage, curious what it looked like inside. I'm looking at a wire going to the picture tube that had the boot on it, was like "Hmm I wonder what this is, pried it with my fingers, and I got threw back on my ass, lol."

 

I know a lot more about electronics now that I've studied it in college and repair them but some times I wonder why I'm here all the stupid things I did back in the day.

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  • 1 month later...

 

 

When I was a kid, probably 12 or so. I took apart an old TV we had in a garage, curious what it looked like inside. I'm looking at a wire going to the picture tube that had the boot on it, was like "Hmm I wonder what this is, pried it with my fingers, and I got threw back on my ass, lol."

 

I know a lot more about electronics now that I've studied it in college and repair them but some times I wonder why I'm here all the stupid things I did back in the day.

Whoa, i thought your parents would tell you not to mess with TV (and why of course)

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