Folks,
The Maplin potentiometer arrived yesterday afternoon and I got a chance
to fit it in the evening. It's not the "drop-in" replacement I hoped
for, basically because it's slightly smaller than the TT original.
Here's a list of the problems and what I did:
1) The bushing was10mm diameter compared to the TT original 9.4mm
diameter, so it wouldn't push through the hole in the sub-panel. I
didn't want to drill the Corsair panel, so I gently filed down the
bushing thread until it fitted.
2) For the pot to line up with the sub-panel hole, the centre of the
spindle must be 17.7mm above the PCB to surface. At that height the lugs
on the Maplin pot don't protrude through the PCB and can't be soldered.
I ended up inserting short terminal pins into the PCB holes and then
soldering the pot lugs to the pins. Actually that also helped because
the Maplin side-to-side lug spacing is slightly narrower than the original.
3) Getting the pot spindle to the right height, square with the front
panel, and horizontal, is not easy. I soldered only the centre lug of
each gang, put the board in position, and made adjustments several times
until I was happy; then I soldered the other 4 lugs.
4) The Maplin spindle needed shortening - that was easily done with a
small hacksaw. The critical dimension was 26.6mm from the front-gang
lugs to the tip of the spindle.
5) The Maplin spindle is 6mm diameter compared to 6.3mm on the TT
original, but the knob tightens OK and there is no sign of "wobble".
6) Once assembled it operates well - nice deep notches. But here's the
thing - it has the opposite taper to the TT original !!! So things are
very "cramped" at the high audio-frequency end. I can only assume the TT
original was a Reverse Log. Here's a very rough comparison I did between
the two, just guessing the rotation angles:
http://www.karinya.net/g3txq/temp/notch_pot.png
If I were doing it again, I think I'd use a modern miniature pot
attached only to the front panel, and wire from there to pins on the
PCB. That would give much more freedom in choosing the pot and avoid the
mechanical alignment problems.
73,
Steve G3TXQ
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