The internal construction of the HP55 programmable calculator, like other Classic-series machines, uses an internal plastic backbone that serves to hold the Printed Circuit Boards (PCB) and other parts. Photographs of the keyboard/display and the main PCB that are referenced below will be provided soon
The main PCB contains the all the ICs except the display drivers, along with the power converter that provides the proper voltages for the ICs. Both Mostek and AMI (American Microsystems Incorporated) made ICs for HP. Parts made by Mostek usually have a Mostek part number in addition to the HP part number. Starting at the left of the main PCB which itself is HP part # 00055-60002, is the large 28-pin IC, HP part # 1818-0078. The next row of three 16-pin ICs are, from top to bottom, HP part numbers 1818-0118, 1820-1393, and 1820-1393. The right row of three 16-pin ICs are HP part numbers 1818-0120, 1818-0119, and 1820-1393. The lower right 16-pin IC is HP # 1820-1169 (Mostek MK6020P) and the 16-pin IC above it is HP # 1820-1128. Note that three of the 16-pin ICs have the same part number, which makes me believe that they may be RAM ICs. An article in HP's Personal Calculator Digest said that the HP55 used three "quad" ROMs, which may be the three 16-pin ICs that start with 1818.
The picture of the keyboard/display PCB for the HP55 shows the arrangement of the LED display modules and driver ICs. Three 5-digit LED modules, HP Part # 1990-0335 (old part number, new part # is 5082-7405) are at the top of the PCB, with the driver ICs underneath. The 20-pin ICs are HP-manufactured devices. The one on the left is the anode driver, HP part # 1820-1029. The one on the right is the cathode driver, HP part # 1820-1226. The quartz crystal that provides enough accuracy for the the HP55 to be used as a timer is mounted on the back side of the keyboard/display PCB and can be seen in the upper right of the photograph of the HP55 main board. The inductor modules used to store energy for the LED display multiplexing are located above the LED modules, mounted to the back side of the PCB.