Sonic Purity
New Member
Looks like Moving Ahead has addressed most of the remaining questions (yay!). Having only one DMM, when i use that method, i power up, adjust one channel, power down, move test leads to the other channel, power back up and adjust the second channel.
Unless i’m missing something, the un-addressed question remaining is what replacement trimpots to buy. Opinions may vary, and it may depend where in the world you are and what’s available to you. I’m in the U.S. and i usually order from Digi-Key, when they have something suitable. The original trimpots are so unreliable, almost anything else you might put in will be an improvement.
I went to https://www.digikey.com/products/en/potentiometers-variable-resistors/trimmer-potentiometers/80 (actually i went to the home page and drilled down the categories to trimmer potentiometers), then started entering the fewest necessary parameters, being sure to check the In Stock checkbox. I came up with zero 30kΩ trimpots—more on that in a moment. Next i tried 500Ω. Specifically my parameters were:
In Stock
Mounting Type: Through Hole
Resistance: 500Ω (this round)
Number of Turns: 1
I received 44 results. None exactly match what’s in there, which in this case i’d say is a good thing. My parents’ TK-66 is where it belongs in the console custom-made for it across town, so i can’t go look at exactly what was in there and measure the lead spacing. When possible, my next step would be to find one whose lead spacing is suitable. The ones with somewhat long thin wires are nice, because those can be bent to fit if they’re close in dimensions and spacing to the originals.
Actually, it would be a good idea to include power rating. Not inclined to do a bunch of math, for this circuit i seriously doubt that any of the trimmers need to be more than 1/4W. Less might be OK, but since i don’t know, i’ll set a lower limit of 1/4W. So in the Power parameter box on the Digi-Key page, i select all options 1/4W and higher. While i’m at it, i’ll add Part Status: Active. This narrows things down to 30 options. This is where i start looking at the individual photos and lead spacing. Personally, i’m OK using any of the many very different styles of trimpot which show up in the list, so i’d likely go with best lead spacing match, and would be more inclined with something top-side adjustable. If you have a preference for tool-free hand adjustment (i don’t), that would further limit the choices.
So what about the 30kΩ pots? Digi-Key doesn’t have any. One can go look elsewhere, or pick “close enough”. If we retain the 500Ω results and cancel the 500Ω parameter, we can see that the closest offered options are 25kΩ and 47kΩ. The Kenwood engineers may have originally picked 30kΩ because back in the late 1960s, it may have been a common, inexpensive standard value close enough. Or they may have made the choice based upon their overall circuit design showing an optimum value of 15kΩ there, and then picking a trimmer of double that value so that normally the trimpot would be set at or near the middle of its range. I’d be inclined to pick one of the 25kΩ options, expecting that the optimum setting might not be at all near the mechanical center of the trimpot’s range.
That’s what i do, when picking a replacement for this sort of part. If i can’t get anything with leads which fit, i solder on bits of solid wire to make wire “stilts” and bend to fit. Other people have other methods/philosophies.
That’s it (i think).
))Sonic((
Unless i’m missing something, the un-addressed question remaining is what replacement trimpots to buy. Opinions may vary, and it may depend where in the world you are and what’s available to you. I’m in the U.S. and i usually order from Digi-Key, when they have something suitable. The original trimpots are so unreliable, almost anything else you might put in will be an improvement.
I went to https://www.digikey.com/products/en/potentiometers-variable-resistors/trimmer-potentiometers/80 (actually i went to the home page and drilled down the categories to trimmer potentiometers), then started entering the fewest necessary parameters, being sure to check the In Stock checkbox. I came up with zero 30kΩ trimpots—more on that in a moment. Next i tried 500Ω. Specifically my parameters were:
In Stock
Mounting Type: Through Hole
Resistance: 500Ω (this round)
Number of Turns: 1
I received 44 results. None exactly match what’s in there, which in this case i’d say is a good thing. My parents’ TK-66 is where it belongs in the console custom-made for it across town, so i can’t go look at exactly what was in there and measure the lead spacing. When possible, my next step would be to find one whose lead spacing is suitable. The ones with somewhat long thin wires are nice, because those can be bent to fit if they’re close in dimensions and spacing to the originals.
Actually, it would be a good idea to include power rating. Not inclined to do a bunch of math, for this circuit i seriously doubt that any of the trimmers need to be more than 1/4W. Less might be OK, but since i don’t know, i’ll set a lower limit of 1/4W. So in the Power parameter box on the Digi-Key page, i select all options 1/4W and higher. While i’m at it, i’ll add Part Status: Active. This narrows things down to 30 options. This is where i start looking at the individual photos and lead spacing. Personally, i’m OK using any of the many very different styles of trimpot which show up in the list, so i’d likely go with best lead spacing match, and would be more inclined with something top-side adjustable. If you have a preference for tool-free hand adjustment (i don’t), that would further limit the choices.
So what about the 30kΩ pots? Digi-Key doesn’t have any. One can go look elsewhere, or pick “close enough”. If we retain the 500Ω results and cancel the 500Ω parameter, we can see that the closest offered options are 25kΩ and 47kΩ. The Kenwood engineers may have originally picked 30kΩ because back in the late 1960s, it may have been a common, inexpensive standard value close enough. Or they may have made the choice based upon their overall circuit design showing an optimum value of 15kΩ there, and then picking a trimmer of double that value so that normally the trimpot would be set at or near the middle of its range. I’d be inclined to pick one of the 25kΩ options, expecting that the optimum setting might not be at all near the mechanical center of the trimpot’s range.
That’s what i do, when picking a replacement for this sort of part. If i can’t get anything with leads which fit, i solder on bits of solid wire to make wire “stilts” and bend to fit. Other people have other methods/philosophies.
That’s it (i think).
))Sonic((