Repairing a defective Olivetti M21

Olivetti M21


Olivetti S.p.A. is an Italian manufacturer of computers, tablets, smartphones, printers and other business products such as calculators and fax machines. Headquartered in Ivrea, province of Turin, the company has been part of the Telecom Italia Group since 2003.

The primacy of the first PC can be assigned to Olivetti thanks to Programma 101, which was produced in 1964 and had a big success in the U.S. market.

Olivetti’s first modern personal computer, the Olivetti M20, featuring a Zilog Z8000 CPU, was released in 1982. In 1983 Olivetti introduced the M24 and in 1984 the M21 (the first and last of transportable of Olivetti), a clone of the IBM PC using DOS and the Intel 8086 processor (at 8 MHz) instead of the Intel 8088 used by IBM (at 4.77 MHz).

The M24 was sold in North America as the AT&T 6300. Olivetti also manufactured the AT&T 6300 Plus, which could run both DOS and Unix.

The comparison (before / after):

Repairing & things before of the Cleaning:


  • Mirror image.
  • Sometimes garbled image with using the ISA Bus.

Replaced parts:

  • Replaced a 74LS245N on the motherboard side.
  • Replaced a 75LS244N on ISA expansion Module.
  • Removed the battery for the Realtime Clock.


  • Bios v1.36 to v1.43 (latest).

source: wikipedia

  1. Peter Haartsen
    July 8th, 2015 at 14:04 | #1

    Very interesting to read about the repairing of the Olivetti M21. You appear to be very well informed about the M21, perhaps you can help me. I have an Olivetti M24 with a defective power supply. This power supply is, if I am not mistaken, identical to the M21 power supply. The thermistor is defect, i.e. the lead broke loose from the thermistor head and cannot be resoldered (I tried). I want to replace the thermistor, but I cannot find any specification. Do you happen to know the specifications of this thermistor?
    Kind regards,

  2. Peter Haartsen
    July 8th, 2015 at 15:18 | #2

    By the way, the thermistor is the component to the left in the back of the capacitors in this picture:

  3. August 23rd, 2015 at 18:10 | #3

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  4. Jannie van Zyl
    February 6th, 2017 at 16:35 | #4


    Do you, by any chance, have a service manual for the M-21? I want to repair one but am looking for a service manual specifically around opening and disassembling the M-21 case. Got a very clean model so don’t want to damage it when taking it apart.

    I do have all the electronic diagrams but can’t find a service manual for the hardware itself.

    Thanks in advance.