HP 8757C LCD RETROFIT |
A common problem in old measuring instruments is the worn-out cathode ray tube (CRT). Replacing it with an LCD is possible at a low cost if the instrument has an RGB video output.
However, adapting an LCD is much more difficult when XYZ vector display is required (such as the HP856x spectrum analyzer series). In this case, the only solution would be a kit from SIMMCON LABS.
In the following, I successfully modified the HP8757C scalar network analyzer using "CBS-8200 Arcade Game Converter" module and 8" VGA/LCD display, both purchased from AliExpress.
Because the CBS-8200 could not properly synchronize, Sync signal was obtained by combining 8757's H & V signals. A suitable power supply turned out to be Meanwell SCP-35-12 (or similar model), which can still operates at 65V (voltage used by the old CRT).
Since the LCD dimensions are slightly larger than the CRT, I added a front mask (PCB) to better align the buttons with their corresponding labels on the display.
LCD display controller was modified to support backlight dimming using the original brightness DAC signal. Control voltage, proportional to the menu brightness setting, is injected into the feedback circuit of AP3032 IC LED step-up converter. Series resistor 4K7 was added between AP3032 pin 3 and sensing resistor. There are many variations of the LCD Driver Board, so minor changes may be needed. See AP3032 datasheet regarding dimming option.
CBS8200 and LCD Driver Board used:
- CGA/EGA/YUV TO VGA / HCKJ 8200 V5.2 (2023/04/27)
- 2AV-1VGA-1HDMI-TTL-50PIN-LVDS-ACC (PCB800099-V9)
SETTINGS:
CBS8200: H position 51; V position 39; H size 52; V size 45, Clamp st 02; Clamp sp 04; 1360 x 768.
LCD Driver Board: VGA source; Mode SYS3, 1366x768/60Hz.
Note: all settings are automatically EEPROM stored.
Regarding the mechanical part, the pictures clearly ilustrate all details. I have not modified anything related to the HP8757C chassis, so the CRT display can be reinstalled at any time. I recommend this upgrade due to its excellent image quality and affordable price.
Schematic diagram
Front panel mask (gerber)
GSP IDC20 adapter board (gerber)
Internal view photos
