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I don't like how jumpy it is just wiggling the control without actually turning it.
Sounds like you could use a replacement, are there no modern parts that would fit? I expect I know the answer but felt I should ask.=)
There's a wholesale electronics place around here. I think I'll desolder one and take it down there and try to match it up.
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Looks like Piher or Omron
What is the resistance range (measured between the outer pins, i.e. #1 and #3)?
It says hdk 5v b502 on it. I'm out driving atm.
5k ohms.
As long as you have it out, it's fairly easy to test. Set your DMM to resistance mode, attach one DMM lead to the middle terminal and the other DMM lead to either terminal next to it (not the very outside ones attached to the case). Then observe the reading as you slowly turn the knob over the whole range. You should see a fairly smooth increase or decrease (depending on which way you are turning and which outer terminal you connected to) in resistance. If it's real jumpy or goes to infinity at points while you turn, it's either dirty or bad.
I went to a surplus store today and looked around but they had a bin of about 10000 of them and I found nothing close to this on in the 5k bin. I spent about 30 mins looking. This particular one is very twitchy when you touch it so id like to replace it, but sourcing it seems like a nightmare.
Id like to replace all three cause they were very scratchy when I first messed with them. But the one would be a boon at least.
"B502" could mean linear, 5000 ohms.
A potentiometer's "taper" is the exponent of the relationship between the position of the wiper and its resistance value. "Audio taper" (or "A") is a logarithmic relationship, because the perception of loudness is logarithmic by the Fletcher-Munson curve. Non-audio applications usually use an exponent of 1, in other words a linear relationship, which is notated "taper B".
The shape of the pot looks like 9mm width, snap-in, horizontal, with a 15mm long plastic keyed shaft, which you can use to narrow down a parametric search.
But if it's too hard to find a replacement, it may be possible to flush the inside with contact cleaner (or specifically "Tuner Wash") to remove any dirt fouling the track and wiper. The products for cleaning out potentiometers (such as 401B Nutrol Control Cleaner) have a small amount of light oil added so that the track will stay lubricated. You need to find an aperture in the case where the red plastic straw can be inserted, otherwise it's too hard to introduce the cleaner into the pot.
I used deoxit fader lube on it, which made it less scratchy but it's still jumpy when you touch it.
I might try some qd followed by more fader lube.
problem with deoxit brand is, even on low it comes out like a fire hydrant.
That's actually a good thing. If it didn't have some pressure behind it, it couldn't flush out the dirt and debris causing issues. These cleaners work both chemically and mechanically.
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Ok I shot fader lube into the black hole and the indentation on the pics above. I tested the linearity on the pot and it goes up smoothly. I'll solder it back in tomorrow and see if it does anything.
https://www.mouser.com/ProductDetail/Alpha-Taiwan/RV120DF-20-15A-B5K?qs=VmiElb0AgQOS%252BsasPBt0HA%3D%3D
i think this is the right one.
Welp i desoldered all three pots. I cleaned them with fader lube, and then checked them for linearity, and they're all smooth. I resoldered them in. Same problem. So the pots are eliminated. I think I may have placed a cap In a via on one spot instead of the positive trace. There were three pins in a row and I think I went with the closest instead of the outside. So I need to hunt that down and change it. Also when I reattached the neck board, one of the pins in the gun was bent down, but I straightened it and it didn't fix the prob. So I'll hunt down that one cap then after that I'm gonna have to manually go through each I guess. I'm out of ideas.
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Considering you don’t have the schematics or an oscilloscope, but you do have a second identical capacitor kit, there is a trial and error approach you can take that will hopefully lead you to the capacitor that is causing this problem.
What you need to do is use the second kit and solder the second identical capacitor to the back of the board effectively doubling the capacitance. You really need to make sure the polarity is correct. Basically you solder one, turn on the monitor, check the picture, turn it off, remove it and solder another. You have to do this one by one or in small groups, like 3 or 4. You can locate the capacitor causing this by observing the size of the distortion – it will either get slightly bigger or slightly smaller when you double the capacitance of the capacitor causing it.
Once you have identified it, we can go from there. Also make sure you don’t reverse the polarity of any of the capacitors you solder to the back and make sure you don’t solder them somewhere else. Ideally you should use a multimeter to check each one before turning on the monitor.
If however the problem is caused by a capacitor being soldered with reversed polarity due to incorrect marking on the board, this approach will not help, but Jeff already told you how to check that in Post #32 on the previous page.
On my opinion a better solution would be to ask the seller of these capacitor kits that mainly motivate buyers to harm their devices to give you the exact location of each capacitor and its orientation. A second option would be to buy a working identical monitor and not mess with replacing anything in it, just use it visually to identify the capacitors' location and their polarity orientation. And that would only help if the real reason for the failure is a capacitor misplacement at all.
The easiest solution, of course, would be to buy another monitor and not mess again with something that is vintage and no longer produced until/if sufficient knowledge/experience is acquired.
Quite a few posts back, Nick sent me a diagram of all the correct polarities from when he recapped his an noted them. As I mentioned, I referenced that and all the polarities are correct. So between that chart and the board markings, plus the fact I checked the voltage on all of them. I'm about 95% sure this is not a polarity issue. However the tolerance of some of these caps is way out of wack, soo think I'm just gonna go through, check them all. Show what they should be compared to what they are and let you all decide.
Another possibility I'm not sure has been brought up would be a damaged via/pad that lost connection to the PCB trace. It tends to be more common if a cap was leaking and damaged the trace/pad. I'm not aware of these caps being typically leaky. But can occur on older boards just from heat damage when working on the cap.
Had a similar issue with an Atari Jaguar recently displaying intermittent behavior after a recap. Eventually figured out that one of the main filter caps that had been leaking a little had damaged the connection from the pad to the PCB trace. Sometimes it would make just enough contact, other times not. It also helped narrow down as when I scoped the main 5v rail, it looked horrible sometimes, and clean others.
In this case, you would want to check each solder joint you worked on (board removed from chassis and not plugged in) with a DMM for continuity to a different solder joint along the same trace. Maybe even while trying to gently wiggle the capacitor in question (would most likely require a helper/second set of hands).
Guess what, been manually checking the caps and I found a .22 in the place of a 22 and vice versa. I swapped them and I'm continuing to check them one by one. I feel that was probably the problem though cause any more the a couple caps would probably be causing a much worse problem than what we're seeing.
Another thing to consider when recapping is the fact that when you try to match both the capacitance and the rated voltage, you end up with much smaller modern caps, which naturally will have higher ESR.
Case and point: below I have an old ELNA from the 90s and a brand new Fujicon. They are both 47uF/16V, however the Fujucon is much smaller:
Caps.JPG
Let’s compare their ESR, shall we:
ELNAandFujicon.JPG
This is why I never use recap kits. Instead I choose them individually and I go with much higher voltages, to the point where the sizes are more or less the same. This results in the ESR being much closer to the original.
Fortunately most the caps in this kit are about double the voltage of the original.
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Boom. Fixed. it was those two caps I assume. Thanks for the help guys.
Looks awesome!
Looks great!! Which two caps was it that you had concerns about? I looked back but I couldn't easily find that. What exactly did you find was wrong just the .22 and 22uF in the other's spot? That's a big difference so would make sense.
Did you end up replacing the pots or are the originals still in place?
Yeah it was a .22 and 22 that i had reversed. I took them out and used a capacitance meter on them, and they read fine, so I put them back in the right places and everything works great. Im gonna bring the v center down a little cause as you can see its a little high, then call this one done. I also replaced a .47 that was a little low on the voltage rating with on slightly higher. The snow that was on the screen originally before the recap is gone as well so in the long run i belive it all worked out great.
i took all three pots out and thouroughly cleaned them and then checked the linear resistance and they all move smoothly now.
I also managed to get my hands on a mouse card and a nos mouse for the apple IIe/pla so I'm pretty excited.
But at which two caps positions on the PCB exactly were soldered these?
C212 and c305.
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