Bad DSSI RF72 Hard Disk

I have a number of RF72 hard disks. These are 1GB DSSI disks (DEC rather grandiosely called them Integrated Storage Elements, or ISEs). Most of them work well, but I have one that will not spin up. The console firmware in a VAX 4000-500 does not see the disk if I do SHOW DEVICES. I decided to see if I could work out why it won’t work.

Faulty RF72

I have removed the drive module to see if I can find a fault that I can fix, the picture of the board is below. The drive module uses a Motorola 68000. The blue connector on the right connects to a ribbon cable coming from the head-disk assembly (HDA) for the actuator and read/write, and the 8-way socket half way down the board and towards the left supplies power to the spindle motor. The power is driven by the large MPM3003 power transistors that can be seen at the top of the board. There are a lot of surface mount components, if one of these turns out to be faulty then I could be stuck as I don’t have the means to replace them reliably.

Faulty Drive Module for RF72 DSSI Disk

Clearly the disk has not been treated well because one of the locking clips on the DSSI connector on the left of the drive module is missing. There are some dents and scratches on the HDA itself too. The shock mount assembly is also distorted, so it must have taken a heavy blow.

RF72 With Physical Damage And Distorted Shock Mount Assembly

In order to be able to test the disk on the bench I needed to get power to it. These disks do not use the normal 4-pin Molex connector used in PC disks. Instead they use a 5-pin connector, a friend identified it as part of the Molex Mini-Fit range. The 5th pin is “Power OK” signal and appears to be a 5V signal. So I made the little cable below to connect the disk to a standard PC power supply. Note that the 5V pin on the PC side goes to both the 5V pin on the DSSI side and also to the 5th pin to provide the Power OK signal. I tested this cable successfully on a known good RF72 disk drive.

Male PC to Female DSSI Adapter Cable

Having got the drive module on the bench I did some tests and found the following:

  • One or two of the electrolytic capacitors have a fairly high ESR, but I found this to be the case on a known good drive module too.
  • When I apply power there is a click as if it is trying to spin up the disk but the fault light comes on immediately and nothing else happens.
  • The bad drive module will not work if I put it in a working RF72. It fails in the same way.
  • The drive module is not sending power to the motor. Looking at the power transistors (MPM3003), there is no signal on the gate pins.
  • There is clock activity on the MC 68000 on the drive module.

A possible issue with the obvious physical damage is that the ROM could have become unseated. So I re-seated it, and while I was at it I took the opportunity to dump the ROM, the image is here. There is a timestamp in the image of 17-MAY-1991 10:12:41 at address 0x6830. The re-seating did not make any difference.

I have not tried putting a known good drive module on the HDA of the bad disk just in case something in the HDA is damaging the drive module.

Clearly more investigation is required, I think the CPU could be working because the Ready LED does come on briefly and you can hear an attempt to start the disk. Possibly the DSSI interface is not working because the drive module is not being recognised by the VAX console firmware.

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