Hardware GBA Volume Potentiometer weird behaviour

[Kurai_496]

Hi guys, I am fixing a GBA which suffered some corrosive damage to the board. After cleaning and repairing I noticed no soud at all.
So as I did in any other Gameboy restore I replaced the speaker but instead of giving crispy "loud" sound it went with super low volume. I examined the volume pot (which felt only a bit grainy) and having some new spare I decided to replace it. When carefully removing it I noticed some pads lifting up when warming the solder.. after removing I've seen some corrosion under them, that's why they unglued from the board.

I've seen on the internet an image of how the traces should have been and and used some wires to get the volume pot pins right where they should be.

Problem is.. from schematics I see they use not all 5 pins but only 3. Lets call V2 the mid, V1 and V3 the outers. For reference lets call P1 P2 P3 P4 P5 the real pins on the potentiometer. (so V1 = P1 , V2 = P3 , V3 = P5)

My damaged pads where P2 P3 P4 so.. looking at the schematics and at a pic with exposed traces only P3 should have been connected.
I connected P3 to Amp IC pin10 as it should and it has some sound but super low as said and sound wheel does not work in increasing or decreasing it. BUT if I connect P3 to P4 it begin to sould loud and crispy, and the volume wheel works flawlessy! On the loudspeaker and headphones too.

Looking at schematics and board picture with traces exposed it makes no sense..Does someone knows why? Looking at schematics..that pin is not even connected anywhere. Can it be because of the new pot being different from oem as internal traces? I bought from ebay..it said compatible with gba and gbc. The pot is good . i tried two and do the same.
Oh, one last thing.. I checked with my multimeter and the weird thing is if I check with only P3 it gives me about 20 ohm at max and about 9 on minimum.. if I join those 2 pins.. it can get down to about 3 ohm.
It has sense because of ohm law but.. still does not have sense on that GBA board.. I tried to find correc voltage/resistence values but no luck anywhere.. only something about the color but it uses the pot in a different way..so its useless.

For who is wondering why I am asking such a complicated question if the console works ok.. well its because it work in a way it was not supposed to do.. so I'd like to understand why. It would be a shame if this "mod" will mess the little amp or something else..

Thanks

 

[FAST6191]

I have seen some replacement pots say you need to bridge pins depending upon the device you are installing it on, or use alt points; many replacements being more concerned with fitting and being more or less in the right range than being drop in replacements.

Without looking at your replacements/poking it with a multimeter I don't know what the internals will look like (5 pin stuff is often aimed at stereo audio type setups where you want the resistances to be able to changed coupled to each other such that one does not end up louder than the other but also spare cross talk between the channels).

I would not worry either; if you shorted something out then it would be max volume or none at all and the wheel would not work. You might have even got a bit of a volume boost.

https://www.petervis.com/walkmans/sanyo-mgr-80/stereo-potentiometer-repair.html might also be of interest.

 

[Kurai_496]

Hi, thank you for your useful answer!!!
Unfortunately the OEM one was in really bad shape so I had not the possibility to check it properly.
Btw, after reading your answer I had a look with my multimeter at one of the replacement pots I have as spares:
looking at the pot from the wheel to the contacts and on upper side.. I can say that first 3 pins are joined together, then I got 9to20 ohm from pin 3 to pin 1, and 0 to 9 ohm from pin2 to pin3. (the 2 pins I joined in the GBA are pin2 and pin3 (from schematics pin 2 seems to not being connected to anything.).

Oh ok, so you think my little jump does not involve any unhealty short right? Glad to know! I was worried about it!
I've read the linked article and I thank you for it! It packs a lot of useful info about those rotary pots! Now things are definitely clear than before!
Thanks a lot!