The Apple IIc appears to experience frequent volume control failures. After all, it’s over 40 years old so we can perhaps forgive it. My Apple IIc had incredibly wobbly volume control, likely due to being jostled while stored. Opening the machine for a full test and service revealed the volume control was broken and eventually detached. While I worked on a replacement, I simply soldered two resistors in its place to mimic a 500 ohm potentiometer set at around 60%.
My repair
I discovered that readily available RV09 7.5mm vertical potentiometers would likely fit mechanically with minor modifications to the pins and mounts. The body would fit inside the original knob body after some rework and would be the same height. Unfortunately, these aren’t available in 500 ohm values so a resistor would need to be connected between the two outer pins (the track). Using a 1k potentiometer and a 1k resistor achieves the necessary 500 ohms.
The next challenge is fitting the original knob. We need to modify it to resemble a traditional volume control. Carefully removing the core creates an open-ended cylinder, allowing us to fit a 3D-printed insert with a 6mm serrated hole. This insert can then be glued into the top of the modified knob.
Removing the core is simple. I used a pillar drill and gradually increased the drill bit size, finally reaching a 16mm bit. The hose clip in the photos offers grip and minimises the risk of the knob bursting. Applying masking tape first protects the original surface from the hose clip.
Fitting the pot simply involves fabricating the outer pins inward and the middle pin outward. The mounting strengthening tabs also need a slight expansion and will fit very tightly into the PCB. Holes could be widened if necessary but mine were fine although quite tight.
The resistor easily fits on the board’s underside. Unfortunately, some solder mask burnt off when I removed the old potentiometers remains the reason for the colour change in my photo as I had to apply liquid UV mask.
My volume control now works perfectly. It’s just audible at about 4mm of rotation and reaches full volume when turned fully clockwise.
I’ve included my 3D design measurements for reference. It fits snugly in use. While resin printing is preferable, it also works well with FDM printers. The STL file is provided but results may vary depending on filament and printer. Therefore, designing your own is recommended for easy alterations.
I hope this helps someone else. Thanks for reading.
My repair
I discovered that readily available RV09 7.5mm vertical potentiometers would likely fit mechanically with minor modifications to the pins and mounts. The body would fit inside the original knob body after some rework and would be the same height. Unfortunately, these aren’t available in 500 ohm values so a resistor would need to be connected between the two outer pins (the track). Using a 1k potentiometer and a 1k resistor achieves the necessary 500 ohms.
The next challenge is fitting the original knob. We need to modify it to resemble a traditional volume control. Carefully removing the core creates an open-ended cylinder, allowing us to fit a 3D-printed insert with a 6mm serrated hole. This insert can then be glued into the top of the modified knob.
Removing the core is simple. I used a pillar drill and gradually increased the drill bit size, finally reaching a 16mm bit. The hose clip in the photos offers grip and minimises the risk of the knob bursting. Applying masking tape first protects the original surface from the hose clip.
Fitting the pot simply involves fabricating the outer pins inward and the middle pin outward. The mounting strengthening tabs also need a slight expansion and will fit very tightly into the PCB. Holes could be widened if necessary but mine were fine although quite tight.
The resistor easily fits on the board’s underside. Unfortunately, some solder mask burnt off when I removed the old potentiometers remains the reason for the colour change in my photo as I had to apply liquid UV mask.
My volume control now works perfectly. It’s just audible at about 4mm of rotation and reaches full volume when turned fully clockwise.
I’ve included my 3D design measurements for reference. It fits snugly in use. While resin printing is preferable, it also works well with FDM printers. The STL file is provided but results may vary depending on filament and printer. Therefore, designing your own is recommended for easy alterations.
I hope this helps someone else. Thanks for reading.
