Wierd ALPS motorised Pot

[jaycee]

I pulled this thing out of a Pioneer XR-P250 stereo system that the neighbour threw away when the CD player quit. I was planning to use it in my amp and figured it would just be an ALPS motorised pot (it looks similar to an RK168)

However when I measured the pins, it didn't correspond to any of the dual pots ALPS seem to have data on. It also had some wierd connections in the original Pioneer. Here is a rough idea of how one channel was connected:



I measured the following:

Between 7 and 3 = 84K
Between 8 and 3 = 42K
Between 8 and 7 = 42K
Between 2 and 8 = 42K->0->42K (turning CW)
Between 2 and 7 = 84K->0 (turning CW)
Between 2 and 3 = 0->84K (turning CW)

Obviously I could just use the normal connections (2, 3 and 7) but 84K is a little high for a volume pot. I'm also intrigued as to how this arrangement is supposed to work. The pot was mounted on an individual board containing the pot, H-bridge IC, preamp based around a 4558, and muting transistors.

edit: The code sticker on the pot says
ACX7003-A
506t

[Sonusthree]

Does it have some kind of 'loudness' control built in for low volumes?
Just a guess

[jaycee]

That was what I was thinking.. but 42K-0-42K thing coupled with what appears to be a filter makes me think its some kind of trick to keep the impedance constant across the whole range of the control.

[gmphadte]

It has seven connections...
then it is a dual stereo pot with loudness connection.
What confuses is the ground connection and is shorted internally for both the pots.

Gajanan Phadte

[jaycee]

Cool, I get it now.

Just wondering how best to use this. Obviously, I don't want a "loudness" circuit in my hi-fi amp altering the sound!

looking at this helps. Hm, according to tests, this is a linear pot.