Intel Intelec MDS-800 Restoration


The +24VDC and -5VDC power supplies in the 8" disk drive chassis don't work. The two very large resistors in series with the pass transistors for the +24VDC supply gave off a cloud of smoke, so the pass transistors are likely shorted. It is also possible that the crowbar circuit on the +24VDC supply tripped, overloaded the supply, and smoked the resistors.


The system was missing the DB-37 to Centronics cable that connects the MDS-800 chassis to the diskette drive chassis. Lou made one from a parallel printer cable according to the Intel schematics. Unfortunately there are two possible diskette cables, one for single-density drives and one for double-density drives. Lou had to move a few wires to convert the single-density cable to a double-density cable. The system would still not boot ISIS-II after fixing the cable, so more debugging is necessary.


We used two laboratory power supplies to supply the +24VDC and -5VDC for the diskette drives. With the lab supplies connected to the disk chassis drive 0 is selected at boot, the head solenoid is activated, and the head is stepped to track 0. That is really good progress.

We can see the ready, track 0, index, separated data and separated clock signals on the diskette controller board, but it will not boot ISIS-II. This could be due to a hardware problem, or a bad boot diskette.

We have been getting lots of helpful suggestions from the experts on the intel-devsys Usenet group.

The Youtube video at has lots of ideas about debugging the diskette controller.


We toggled in the two instruction program in the screenshot below and ran it. With the diskette out Status Register A contains 18h. This means that the diskette controller is double-density and the controller is present. With the diskette in the drive Status Register A contains 1Dh, so the new bit is Drive 0 Ready.

The ISIS-II bootloader gets read into RAM starting at 3000h. We filled RAM addresses 3000h-3fffh with zeros, and tried to boot ISIS-II. After a reset we looked at the contents of 3000h-3fffh and it was still zeros. That means that the diskette controller did not read the diskette and didn't transfer anything to RAM.

We looked at the 74164 shift register A8. The input on pin 9 is 8 MHz, with the system tying to boot so the head is loaded, we saw 1 MHz on all of the output pins. This was a failure mode in the YouTube video, but not with ours.

In the example below from the video it shows that the low order bit of the diskette controller status register contains a 0 which means that the drive is not ready. Our's contained 18h, so that failure mode from the YouTube video also did not exist with ours.

After trying to boot ISIS-II with two different diskettes the Read Result byte contains 0Eh, so that means Address Error, Seek Error, and CRC error.

We created an IOPB for a Recalibrate and the head went to track 0. The Read Result byte contains 00h, so no errors.

We tried a seek