Quote:
Originally Posted by indigo.girl
I've removed R8 from the circuit and also snipped one end of R11 so I can test everything independently.
R8: Total resistance is 1M ohm (higher than 700K spec). As I move the wiper resistance changes from 0 to 1M ohm on a log scale so most of the increase in the final quarter turn. But I don't see the peak resistance at the 3/4 position like I do when its in the circuit. This still doesn't explain the 240k ohms I measure across R8 in situ (would increase expected value up to 90 K)
R11: On spec at 108 K ohms (100K)
R9: On spec at 23 ohms (22)
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1Meg instead of 700k isn't that unusual for a pot of that age, at the moment I wouldn't think it's cause for concern, the fact that it measures high shouldn't affect the output from your MP3 player.
You won't see the peak resistance spot you saw earlier if R11 isn't connected across the pots track, to explain this then imagine the pot has a linear track of 1M ohm with a resistor of 100k ohm connected in parallel across its track and you were measuring the resistance between the pots wiper and the bottom end of the track, let's call the bottom end of the track A, the top of the track C and the wiper B and the parallel resistor R1, with the wiper B set to A there would be a resistance of 0 (zero) ohms between A and B.
Now we set the wiper to the half way point along the track, we now have a resistance of A--B which = 1M/2 which = 500k which is in parallel with a resistance, consisting of B--C = 1M/2 which = 500k in series with R1 (100k) which = 600k giving a final resistance between A and B of 270k (500k in parallel with 600k) Without R1 A--B would equal 500k.
Now we set the wiper (B) to the top of the track (C) we now have a resistance A--C which = 1M which is in parallel with R1 (100k) giving a final resistance of 90k (1M in parallel with 100k) Without R1 A--C would equal 1M.
So....R1 connected in parallel: zero resistance (0)---> peak resistance (270k)---> less than peak resistance (90k).
R1 removed: zero resistance (0)---> 500k resistance---> 1M resistance, no peak at mid travel.
Post might cross.
Lawrence.