Heath/Zenith Z90

Speedup top
Speedup bottom
Speedup installed
Heath/Zenith Z90, ca. 1981

CPU: Z80 @ 2MHz (upgraded to Z80A @ 4MHz)
Floppy Drives: Teac FD-55F-030-U quad-density (720KB) and FD-55BV-06-U (320KB)
Display: 12" Green (25x80)
Ports: 3 RS-232
Model: Z-90-82
Serial No.: J143G060
OS: CP/M 2.2 or HDOS
Original Price: $3,195

This system has a double-density controller (Z-89-37), and an additional 16KB memory board (85-2359-1) to take it to a full 64KB. It echoes characters perfectly in offline mode, but the processor board doesn't appear to work at all.

Update (09/14/2014): On checking voltages, I found that the -12V line was down close to -1V. I traced that to a shorted 2.2uf tantalum capacitor (C511) and replaced it, along with several other tantalum caps on general principles. The -12V line now looks good, but the CPU board is still not initializing properly.

Update (08/13/2020): Found and fixed cold solder joints on the video board, and replaced several bad 4116 RAM chips. The system is up and running CP/M 2.2.04.

Update (08/23/2020): I obtained an original Micronics Technology 4MHz speedup PCB and installed the three ICS (74LS00, 74LS04 and 74LS92). The board plugs into socket U502 on the Z90, after you remove the 7492. The jumper wire connects to P508.16 for switchable 2MHz/4MHz, or P508.1 for locked 4MHz. The Z80 is replaced by a Z80A. My Z90 RAM passes the memory diagnostics at 4MHz so I left it locked at 4MHz.

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Last updated on Sunday, August 23, 2020