Bummer. I had hoped for a better fit because the description said a
quarter inch shaft and the drawing showed a 3/8" bushing with the
quarter inch shaft. So I guess I won't order from Maplin.
My scheme of shunt loading a linear pot probably isn't a lot better than
the log pot, needing to be reverse log. Maybe using a series resistor on
the wiper of would band spread the HF end better.
Mouser offers a compact dual section linear pot that might make wiring
easier.
73, Jerry, K0CQ
On 2/9/2011 3:40 AM, Steve Hunt wrote:
> 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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