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Author Topic: Help!- Broken Pots  (Read 1701 times)

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Davestar

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Help!- Broken Pots
« on: July 29, 2002, 02:01:57 pm »
I have an Electrohome G07 monitor, and two of the pots came apart when I was trying to adjust in a new video card.  I checked Bob Roberts site, and he has pots, but I don't know how to put these in.  Is this a major job?  Would I need to discharge the monitor?

Also, when the pots broke, the screen filtered down to a single bright blue line all the way across the screen in about the middle of it.  Is this a sign of a major problem, or if I replace the pots will this straighten out?

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Dave
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MameFan

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Re:Help!- Broken Pots
« Reply #1 on: July 29, 2002, 02:39:26 pm »
If you're talking the variable resistors (not variable capacitors---e.g. coils), the small white/cloudy clear ones on the back end of the board..  Then, they're fairly easy to replace.

Basically, you'll need a good 30-35 watt soldering iron with a good clean tip.  The $4 ones in your dollar store might work for this project, though they tend to burn out the tips quite fast, so a better soldering iron with replacable tips and variable wattage might work better.

Invest in a solder sucker.. About $5 ?? From radioshack. A red squeeze bulb with a high-temp nylon nozzle (to keep it from melting). You want this to remove the solder from the old ones before you try to remove them (otherwise, if you try to "wiggle out" the old ones while holding the iron there, you will end up either burning yourself or breaking a wire trace on the board, and have to fix more stuff later.

No, you will not have to discharge the monitor.  You can to be safe, but you don't have to.  If you've never worked on these before, I would HIGHLY suggest you do it.

Also, on the G07's especially, that huge electrolytic capacitor  (adjacent to the 2 fuses) can hold quite a punch in it.  The electronics SHOULD discharge it when it's turned off, but it wont if something is wrong OR one of the two fuses blows.  You can get quite the zap from the board even if disconnected from the tube unless you ensure that capacitor is discharged.

To remove, if you want to for some reason keep the old pots without wrecking them more, then heat each of the 3 leads comming out of them, then sucking off the solder on each one, then move to the next (from the bottom).  Eventually it should be free or near free of solder.  Then pull it out. Not everyone can desolder quite the same so you might have to heat up one or more of the leads really quickly to be able to GENTLY slide it out.  Don't work too hard and NEVER press back down from the top thru the bottom without the solder being hot... you will rip the trace from the bottom of the board and have more problems fixing that with a wire jumper later.

If you DONT care about saving the old pot bases, then to make desoldering easier, take an old nail cliper, and cut off the lead as far off the TOP of the board as possible (as close to the bottom of the pot as you can). Clip off all 3 and the pot will fall off leaving 3 leads sticking up from the board. This then lets you remove them much easier, no need to rock out 3 partially soldered leads at once.. just pull each one out with a needle nose plier after removing the solder from them.

Once removed, ensure you can see thru each of the 3 holes, and press in the new pot gently. If it meets resistance, you may not have got all the solder out of the hole. If it's only one hole, heat it up and press it thru while the solder is warm. If more than one, you'll need to try to remove more solder. (A larger needle, pressed thru the bottom towards the top can help "spread" the cold solder enough in most cases. Again never press from the top down as you'll rip the trace off)

Once seated, heat each of the 3 leads and use new electronics grade solder, flowing enough around the lead and trace.  Remember: 1st rule of soldering. Heat the metal, NOT the solder.  Otherwise you will end up with "cold solder joints" where through a bit of vibration the solder will separate from the metal and/or trace, and then cause intermittent problems and have to be reflowed.  Dont heat the leads too long, as you might end up melting the plastic disc on top or something, and when flowing the new solder, press the solder against the heated lead...NOT the iron's tip.

Make sure you match the OHM resistance of each pot... They may not be all the same. (I've never replaced them  on a G07 myself).. Putting the wrong one in will result in very very bad control over the screen, or worst, frying some component due to too much resistance.  

To double check.. Get an OHM meter, and set it on the 20Kohm or 200Kohm setting or so.  Using the OLD pot from the board, touch the black lead to the left most post, and the right lead to the right most post (skipping the middle post). Note the resistance. (Make sure you have fresh batteries in your meter too, otherwise it might read wrong).  Now compare that against the new one you're going to put in. Ensure it's the same or within reason.

E.g. If the old one showed 4.6 Kiloohms resistance end to end, the replacement one should be the same or can be SLIGHTLY more (e.g. 5.0, 7.0). Dont go less, other than by some small .0Kohm (e.g. 4.5 might work, 2.0 likely wont). The reason is, if you go less, then you restrict the amount of resitance you can set, and you may find you can't adjust the screen "as tall" as it once was.   The reason you dont want to go to high is that you now lessen the "exactness" you can adjust.  Getting one that is twice as much will result in you turning it half as much to adjust it, and trying to find the "sweet spot" might end up with you jumping from "too small" to "too much"... Pretty simple.. Think of a volume control only having 10 positions, or 5 positions.  10 is MUCH finer control! :)  
<continued>

MameFan

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Re:Help!- Broken Pots
« Reply #2 on: July 29, 2002, 02:39:51 pm »
<continued from previous>

If you have a triple decker one (3 stacked together behind each other)--I cant remember of the G07 or a Hantavex has this, where turning one pot turns 3 in line with it, then it's the same principal, just a lot more leads to solder.


Regarding the single blue line: If you broke the vertical height pot, then it lost all resistance, and therefore collapsed the vertical deflection into a single line in the center of the screen.  Make sure you DO NOT RUN your monitor more than a few seconds with this picture.. You will end up burning in that line on the screen quickly.  (Think of it as 525 lines etching each 30th of a second into a single line, never being able to "die off"--it will burn in quickly)

As far as it being only blue.... If the signal (e.g. picture from the gameboard connected) was missing or a normal screen (e.g. not blue tinted overall), then it sounds like your broke a red and green bias or cutoff control on your tube neck board, not just the vertical/horizontal/brightness/contrast on the main board.   But you'll need to replace your vertical pots to get a full picture before you can go back to testing the rest.


Davestar

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Re:Help!- Broken Pots
« Reply #3 on: July 29, 2002, 03:00:01 pm »
Mamefan,
     Wow! Thanks for that very detailed response.  I'm gonna have to think about this abit as I'm not the worlds greatist solderer, and I'm real nervous about working with something that holds as much energy as a monitor.  Thanks for the tips as well on soldering, lead protection, etc.  I'm glad you explained it so well for someone with limited electronics experience.
      Also on the line, I think your right on.  That was the pots I broke while adjusting.  The line I believe was actually white, with a slight blue tint.  I think if I do replace the pots, it should fix it.
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Dave
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Re:Help!- Broken Pots
« Reply #4 on: July 29, 2002, 09:40:15 pm »
I always discharge and pull HV lead from the monitor. If that wire's insulation has the smallest crack in it, and you are grounded (touching the metal), and you get near the crack, it will arc to you. Ouch! Not that it ever happened to me. I went to TV school for a few weeks. I heard all the stories. I learned from them.  ;)
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Davestar

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Re:Help!- Broken Pots
« Reply #5 on: July 30, 2002, 09:16:59 am »
Jay,
     How do you ground your HV out?  Do you use the screwdriver to ground (chassis) method, or the transformer method?  I'm not feeling too confident about either.  Should I discharge if I'm swapping out monitors?  I was thinking of just getting a new monitor, and swapping it out using a thick pair of welders gloves for insulation.  Any thoughts?
     Also, is installing a new monitor a major headache?  It looks like 4 bolts, and a few wires, to unhook, and then reconnect.  Is that right?

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Dave
« Last Edit: July 30, 2002, 09:21:09 am by Davestar »
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Re:Help!- Broken Pots
« Reply #6 on: July 30, 2002, 01:19:12 pm »
The screwdriver method is the only one I know. I use a high quality Xcelite screwdriver that is 12" long with a very narrow (#1) tip. The a "Heavy Duty" jumper lead from Radio Shack.

I also take the HV lead and stick it's metal tip in a screw hole on the chassis. I have heard of that recharging itself without power being applied. Capacitors... Also the picture tube will almost always recharge itself. You can simply and carefully stick the screwdriver in the hole to redischarge it. Don't jam it in or you could damage something. The idea is to simply touch the metal inside with the screwdriver.

I never bother discharging a monitor to swap it out. BUT while holding the metal chassis, DON'T touch the HV wire with your arm or chest or whatever.

Any decent leather glove will do. I use them, not to protect from voltage, but to protect from sharp metal.

It pretty much is 4 bolts and a few wires.

Other safety concerns:

Wear eye protection! If you drop the monitor it could implode. And that SOUNDS ok. BUT! Glass will probably still fly around abit. I have seen people that work on TVs wear full face shields. Use your own judgement.

The neck on the picture tube is EXTREMELY FRAGILE! I have seen many broken. Sometimes the slightest bump, will break the glass due to the leverage you have at the end of it.

Disposal. If disposing of a picture tube or whole monitor, you should break the neck. The prefered way of doing it is to throw an old blanket over it whole thing and the place a 2x4 or other suitable board on the neck, then turn your head as you step on the board. Otherwise kids in your neighborhood will be throwing rocks at it and will some how manage to implode it.

I hope this helps without making you too paranoid. I seriously doubt a monitor will kill you, but I KNOW they will send you to the hospital! Just Be Careful!  ;)
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Davestar

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Re:Help!- Broken Pots
« Reply #7 on: July 30, 2002, 01:53:36 pm »
Thanks for the reply.  I'm really leaning towards replacing the monitor.

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Dave
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