Volume Control For MXT

[Sven1]

I don't have a MXT schematic. Do have a couple pictures of the PCB, snapshotted the area front and back. because there is no trimpot. It might be as easy as replacing R28 with an external pot. Carl Moreland from Geotech could give you a better answer.

 

[locator]

@Sven1Hello Sven. The resistor R28 you mention is the base resistor for the transistor Q7, which is part of the VCO. There are a lot of circuits on the MXT that mesh together, so I wouldn't make any major changes. But you're right, Carl will definitely know more.

@OneAcreYou are welcome, Sir.

[OneAcre]

Looks like we're waiting for Geotech...

[Noah (FL)]

A piece of tape may work just fine too? All of my Tesoro's have tape over the speaker to tame the volume for days when I don't feel like wearing headphones. 

[OneAcre]

Noah... It's taped up now, mostly to keep out dust. Still pretty loud though.

[Geotech]

Sven's first suggestion will work. Schematically, it looks like this:

On the PCB there is a 2-transistor amp that drives the speaker, and on the ground return there is a 220-ohm resistor. For the volume control a range of values will work. A lower value will improve the perceived linearity, especially if you can't get an audio taper pot. This version allows you to reduce the volume to absolute zero.

You can also do this:

This further improves perceived linearity, but now the volume can't be reduced to absolute zero. Probably a 5k audio taper would work best here.

If you get an audio taper pot then there is a right way and a wrong way to connect to the outer lugs of the pot. Make a guess, and if all the adjustment seems to be at one end of the rotation, reverse the connections. A linear pot can be wired either way.

 

[OneAcre]

Geotech...

It seems the first example would work the amp more than the second... I'm still learning here, but is the second example easier on the amp than the first? Reducing the volume to zero is probably not needed in this case.

[Geotech]

They both load the amp about the same, and both reduce the amp load as the volume is reduced.

[OneAcre]

All...

Many thanks! Time to get busy.