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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: x56h34 on February 24, 2004, 08:10:37 AM
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I wasn't happy with the display quality of the A3000 scan doubler output to my VGA monitor (picture was jumpy on 640x400) and I've decided to fine-tune it by rotating the little tuning screw a bit with a screwdriver (the hole next to the VGA port in the back of A3000) and to my misfortune the screen is now royaly #%*$!! up. I just touched it a little, that's all, and I can't get any normal display anymore, just garbage on screen. I've tried to tune it back but it just won't work anymore. What happened here? How do I fix it?
Thanks.
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Are you in fact turning the pot correctly?
It can be tricky to get at if you don't have the right shaped screwdriver.
Don't forget it's a 10 or 20 turn pot, so try the video throughout the complete adjustment range of the pot.
Obviously you need to have the rear switch in the "enabled" position for the scandoubler to work.
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Isn´t it a coalbased pot? (sorry don´t know the right name in english). What I´m going at is that maybe you have to trim it up by turning it left-right a couple of times to make it work? Just remember how it was in original setting before you start. :-)
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Well it seems that the screw is turning. I have a small screwdriver that fits ok. I pointed a flash-light towards the hole so that I could see what's happening and I saw the screw turn alright.
I was really gentle while tuning, too. Can't believe that I damaged something by force, or similar.
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I've just learned that A3000 came with a plastic screwdriver for the VGA tuner. I was tuning mine with a standard metal one. Perhaps that's why the display always gets corrupted as something non-conductive must touch it. Should I try looking for a plastic one?
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You shouldn't really worry about using a non-metallic screwdriver as the brass screw of the pot should be electrically isolated from the circuit.
There's no high frequency RF circuits in that region which will be effected by a metallic tuning tool either.
They probably suppled a plastic tool becuase they're cheaper, and to minimise electrostatic discharge risks inside the computer when you go poking a metal screwdriver in there.
@Crusher
No idea what you mean by "coalface". Maybe carbon??
The modern multi turn pots are usually made of stuff called cermet which is kind of like conductive ceramic.
As for tuning it, normally you find a "peak" in the tuning range.
So you keep turning it one way until you start to see the image getting better. Now keep turning while counting the number of turns until it starts to degrade, then go back by approx half the number of turns you counted to put you roughly in the center of the tuning range.
If the picture remains crappy, make sure the monitor cable is securely plugged in.
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Sorry, I forgot to mention that it is possible the pot is damaged.
If so just replace it. Any decent electronics supplier will have them.
Any generic 1k ohm, 25 turn horizontal mount pot should be fine.
Take the original part with you to get the physical dimensions and footprint.
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I figured that it could be that the pot was simply too old and once I moveid it from the default position, it died. Upon closer inspection, the screw looks rusty too. Good thing that it can be replaced though. ;-)
I guess I'll play around with it a little more and if I fail to get any results I'll simply buy a new pot and replace it.
Thanks for your help. Much appreciated.
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x56h34 wrote:
I've just learned that A3000 came with a plastic screwdriver for the VGA tuner. I was tuning mine with a standard metal one. Perhaps that's why the display always gets corrupted as something non-conductive must touch it. Should I try looking for a plastic one?
IIRC, the original tool is plastic with a metal tip, so I don't think that is the problem. (I have mine under my keyboard at home so I'll look when I get home).
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Thanks dude. I am though 90% sure that the pot itself is damaged. The display is simply terrible. There is no picture at all, just interference. If I am lucky, I can tune it back to 40% usable picture, but it's still garbage. The display looks like those locked out cable TV channels.
I'll try and look for a pot which fits nicely and is of higher quality.
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x56h34
What part of the City do you live in. I can recommend a few good places to buy electronic components.
Active Surplus - at Queen St E. and John St. (1 block west of Spadina)
Stay away from Active Components and Radio Shack - they charge way too much for substandard components.
Cheers
Shaf
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@Castellen
Carbon, coal... same thing to me :-)
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OK, I am now positive on what is wrong. The pot is damaged. If I try to move the whole pot box as right as it can go without breaking it off from the motherboard, the picture magically comes back. As soon as I touch the screw with a screwdriver, the pot moves and loses contact with the motherboard and I get crappy picture again. In conclusion, it seems that I need to either get a new pot or possibly fix the solder points on the old one. It's possibly best to try and get a replacement which would work but would sit in there firmly.
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@shaf:
Thanks for your suggestions. I usually shop for these kind of parts in the Steeles/Victoria Park area. There are a bunch of stores there that sell components, and it's closer to where I live.
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@x56h34
Hello, small world. I work at Victoria Park and McNichol just south of Steeles.
I would be interested to know if you have any local sources of Amiga stuff. I am tired of ordering from the U.S. and would prefer to buy locally.
Since Comspec changed to PC only I don't know of any other sources. Btwe if you ever had an Amiga repaired at Comspec between 1988 and 1993 I probably did it.
Cheers
Shaf
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Shaf,
The only source of Amiga hardware left in Canada are individuals like you and me. There is one active dealer that I know of, which is LiveWireSystems, however their stock is rather limited (only Amiga One and Elbox offerings...nothing like accelerators, or rare/fancy stuff).
BTW, are you still able to repair Amigas? Maybe you could fix this scan-doubler pot trouble for me? I have a friend who usually does these kind of repairs for me, but he's been rather busy lately and hard to get ahold of. I know how to solder and all, but only basic connections. I wouldn't dare touch a motherboard yet with my Walmart Soldering Iron (if you get my drift). :-)
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x56h34
Yes I still do minor Amiga repair for the few people I know that still have Amigas, but due to time and resources I am not quick. (you try getting any work done with a 3 year old running around the house constantly wanting you attention!)
I have fairly good soldering skills but would prefer to wait a few week before attempting to take on the task. My only current Soldering Iron is a Weller 25 Watt. I am trying to replace my old 15 Watt unit that a friend borrowed and then broke.
Cheers
Shaf
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shaf,
This can certainly wait. I'm in no rush. Let me know when you are ready, if you're interested that is.
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Hi :-)
Sounds like a cold solder join as the image comes up when you move the pot, have you looked at the pot solders ?
Many A3000 had that problem and making tree new good solders had cured it :-)
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oups! double post!
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Hi
> by Castellen on 2004/2/24 19:35:25
> You shouldn't really worry about using a non-metallic screwdriver as the brass
> screw of the pot should be electrically isolated from the circuit.
> There's no high frequency RF circuits in that region which will be effected
> by a metallic tuning tool either.
No, you should really use a non-metallic screwdriver (or one with a really small metal plate at the top), if I try to tune my flickerfixer of the A2000 with a normal metal screwdriver [Ok, I've not tested with the A3000T, but there is no need for tuning and I don't start testing it now :-)] I could bring the picture to stand, but in the moment I remove the screwdriver, the picture begins to run.
With the plastic screwdriver that is shipped with the A3000, it works.
So even if you have to resolder the pot, you should also use such a plastic screwdriver for tuning.
Noster
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@voxel:
Thanks for your suggestion. There's no doubt that this A3000's pot has bad solder connections underneath it, but I was thinking since I will be taking the whole unit apart maybe it would be smart to look for a better pot altogether. One with a knob which would stick out through the screw hole and be reachable outside of the case. Worst case scenario, I'll fix the solder connections and keep the original pot. :-)
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@ Noster:
Thanks for the info.
Haven't personally had that problem with any of the A3000s I've worked on.
As a radio communications engineer by trade, I usually have plenty of ceramic tuning tools about, but I have used metallic tools before on the A3000 and haven't noticed anything obvious.
Looks like the video circuit in that section is quite sensitive after all. It's possible that the stray internal capacitance between the pot's isolated screw and the rest of the circuit is enough to cause problems in your case.
In which case a ceramic or plastic tool would be a far better option for other A3000 users as well.
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@x56h34 :-)
quote:
to look for a better pot altogether. One with a knob which would stick out through the screw hole and be reachable outside of the case
endquote
I don't think you will find that kind of pot ;-) and even if you can I don't recommend such a modification.
As the circuit is made, you don't have to touch that pot more that one or two times in the whole life of your amiga.
And the pot used on the design is an endless multiturns model (so you can't break it by rotating it's knob too much, you will only ear a "clac clac" sound when you reach either sides and just turn the other direction to try to make a good tuning :-)
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Thanks Voxel! I will try and fix the solder joints and see what happens.