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Welcome! In this post I talk about the reverse engineering of the GM328A transistor tester. I have drawn the schematics of the board and compiled new firmware for it. As a bonus, I also programmed Tetris for it.

Some time ago I wrote an article where I programmed one T3 LCR meter / transistor tester to run a clone of the cute t-rex game from the chrome browser. That LCR meter was one of the most useful instruments I ever had, specially because I have the same digital multimeter since I was 15 and that does not even measure caps. Unfortunately, the former did not endure the constant abuse (I often used it as a badge when going to conferences and such) and the LCD ribbon broke.

This LCR meter has a color LCD, which is great in case I continue using these as a made up badge. It also packs frequency and voltage measurement, a power jack, and PWM output. Additionally, the one I found was a mere 10 USD.

That said, these LCR meters make great development boards, in this post I detail the process I used to figure out the schematics for this model and also show a version of Tetris I coded for it. The reverse engineer efforts here are mostly related to the printed circuit board and not to the firmware.

Before we proceed, I have to mention that all these transistor testers are based on the same project, originally created by Markus Frejek and continued by Karl-Heinz. You can find information and the code at Still, there are tons of different variations of the circuit sold on eBay and the like, in this post I reverse engineered the schematics for this board only.

So, to sum up. In this project I:

Reversed engineered the printed circuit board;
Fixed a mistake on the frequency measurement circuit;
Adjusted and compiled the AVR transistor tester code to fit this board;
Programmed Tetris for it.
Before anything, I thought it was good idea to backup the original firmware, but this is where I hit the first obstacle. Although the board seems hacker-friendly, even having a socket for the microcontroller, its firmware was locked. So instead of copying the firmware to the computer, I replaced the microcontroller. This way I could always have the original firmware and still use the board if I messed up.”

Link to article

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