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  4. v2.0a – How to install the Drop-In VR1 replacement LCD contrast control for the Radio Shack / Tandy TRS-80 Model 100 & NEC PC-8201a & PC-8201

v2.0a – How to install the Drop-In VR1 replacement LCD contrast control for the Radio Shack / Tandy TRS-80 Model 100 & NEC PC-8201a & PC-8201

Each Drop-In VR1 kit is assembled and mailed from Canada — and costs $20.00 CAD plus postage. See ordering information.


These instructions are a work in progress for version 2.0a of the Drop-In VR1 replacement contrast control.

This is the permanent URL for these instructions.

Each unit is hand assembled from available components as well as a custom 3d printed tray and Wheel Lid / dial designed to bring it all together. I developed this component to solve the problem of 40 year old hardware not having modern replacement components. My NEC PC-8201 had a twitchy or bad contrast control and I didn’t want to harvest a good one from another computer — so after a little design work, this is the result. Questions and feedback is welcome — email bradblog@gmail.com

Unpacking [ currently obsolete – to be updated]

Carefully remove the Drop-In VR1 from the cardboard backer.

Gently straighten and separate the wires so they will easily slide into the holes on the PCB, as illustrated in the top diagram.

The Wheel Lid may be separate from contrast control unit — see Attaching the Wheel Lid

Where’s it go?

First off, become familiar with the parts of the Drop-In VR1 and where it goes on your Radio Shack/Tandy TRS-80 Model 100 or NEC PC-8201 or NEC PC-8201a.

Radio Shack / Tandy TRS-80 Model 100
NEC PC-8201

Attaching the Wheel Lid

If the Wheel Lid / dial is not yet attached, firmly press the Wheel Lid
 / dial on to the plastic wheel of
 the potentiometer.
: A small amount of adhesive 
 reinforce the fit of the
 Wheel Lid.

Position on the board

This step assumes you’ve already removed the original part, removed the solder, and have cleaned the holes on the PCB. The clearance of the signal wires is pretty tight and removing all solder from the signal holes makes mounting easier.

Orient the Drop-In VR1

The longer/flatter end of the Drop-In VR1 (with the 3 solder points) will be positioned to the left, and the smaller slightly rounded bump positioned to the right. This is for both the TRS-80 Model 100 and the NEC PC-8201 boards.

Orientation for NEC – Memory Battery to the left.
Orientation for Radio Shack / Tandy – Two resistors to the left.

Slide on to the board

Testing the fit of v2.0a on a NEC PC-8201a


Ensure the Drop-In VR1 Tray lies flat along the top of the circuit board. You may need to check for old solder bumps or lumps of solder flux/resin.

Solder to the board

Prototype soldered to a NEC PC-8201 board.

After inserting the Drop-In VR1 wires into the appropriate holes on the board, flip the board, apply solder resin, and solder to the bottom.

The VR1 is designed to fit snugly to the board so ensure you are pressing it to the board while soldering. You can also pull firmly (but not too hard) on the GROUND wires while soldering.

Finishing up

Prototype Drop-In VR1 soldered to TRS-80 Model 100 board.

Clip the excess wire as close to the board as possible. Then remove excess solder resin using Isopropyl alcohol or other printed circuit board cleaner.


v2.0a VR1 installed in a NEC PC-8201a

Carefully position the board back into the lower case and complete the reassembly.



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