Latest T3 / T4 schematics (Rev. 1d) courtesy of Roland Elmiger.
Note about flashing alternate software:
The latest pre-compiled version of Karl-Heinz software provided on "
http://www.mikrocontroller.net/svnbrowser/transistortester/Software/trunk/mega328_T3_T4_st7565/" , v1.12k build 683, does not work (blank LCD screen) with the
unmodified version of this tester.
This is due to a change in the Makefile made in March 2016:
line "#CFLAGS += -DPULLUP_DISABLE" was un-commented in build 661, making the LCD SPI interface open-drain with no pull-ups.
Hence, display data is not received by the LCD.
To restore proper operation on unmodified testers, make sure these options exist in the Makefile, and re-compile:
#CFLAGS += -DPULLUP_DISABLE
CFLAGS += -DLCD_SPI_OPEN_COL
CFLAGS += -DSTRIP_GRID_BOARD #(if it's a strip grid board - you made need to try both settings)
The LCD in these units requires +3V +/-10% VDD, while the Atmega runs on +5V VDD. The PCB design includes two 1N4148 diodes in series from +5V VDD to the LCD VDD to attempt to drop the 5V down to something more reasonable for the LCD: ~3.6V, assuming 0.7V drop per diode. In reality, the LCD voltage is higher because of the small current drawn by the LCD, hence smaller diode drops.
Karl-Heinz suggests removing these two diodes and replacing them with a proper +3.3V LDO regulator, such as the MCP1703-3302.
He also recommends installing two additional pull-up resistors (10K - 30K) on the SCL and RST lines of the SPI bus between the Atmega and LCD.
They should pull-up to the +3.3V LCD supply.
If these hardware modifications are made, the compile Makefile options would be the same as the 683 build:
CFLAGS += -DPULLUP_DISABLE
CFLAGS += -DLCD_SPI_OPEN_COL
CFLAGS += -DSTRIP_GRID_BOARD #(if it's a strip grid board - you made need to try both settings)
Please see attached picture of suggested hardware mods from Karl-Heinz.
For owners of this tester which is offered on ebay under the designation as "LCR-T3" or "LCR-T4 Transistor Tester" I offer a way to update firmware. Devices have identical G-LCD (ST7565), but slightly different PCB. Update works on both versions of this tester. However, attention should be given to the fact that the ISP connector from the parts is mirrored. For this purpose I have pieced together a simple detachable adapter enabling the MCU to program without arranging the auxiliary ISP connector (see attached pictures). Many thanks to Roland Elmiger (hb9gaa) from the official discussion forum for providing the schematic diagram.