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How do I test an SX-1250 volume pot

bisonguy

bisonguy

Active Member
I picked up an SX-1250 to refurb as a winter project. It was dropping the right channel and I had hoped it was just a matter of using de-oxit. However, when I removed the lower cover I saw that the flat amplifier board, AWG-042 was cracked and someone had tried soldering wires across the traces (picture attached). I have been able to source another AWG-042 board but the volume pot has been removed. My problem is how do I confirm that the pot on the cracked board is still good. I have no idea how this board got in the sad state it is in now or what would cause the board to crack. I'm comfortable with removing the pot on the cracked board to put on the replacement board but don't know what I should be looking for in testing the old pot.
 

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It's easy, but a bit tedious. This is generic, for any pot.

Get an ohmmeter and first read the two outer connections of three (if four, the fourth is the loudness tap and doesn't come out of the pot at that location - so don't count it (yet))on one wafer for the correct resistance, leave the ohmmeter connected to the two and rotate the pot shaft slowly over it's whole range looking for any change in resistance. Shouldn't be any - but if the element were cracked the wiper's pressure might open it up.

Then check the loudness tap, it will come out of the pot in a completely separate position than the other three in a group, even though it may be brought around the side of the pot to form a group of four. Do this from alternately all 3 other connections if you're a bit obsessive.

Then do this next step twice, once from each of the two outer connections to the center wiper, connect the ohmmeter and slowly rotate the pot shaft slowly over it's whole range looking for any jumps in resistance, it should be smooth, even if a bit mysterious as to the logarithmic increase or decrease in the value.
After doing the one outer connection, switch to the other outer connection on the opposite side, leaving the center wiper connection alone.

THEN repeat it all for the other pot.

If you really want to go nuts - compare the resistances at the same positions, but don't expect better than 10 percent, if it's closer: great (probably is), if not, oh well....
 
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