Research Machines RM380z

Known Issues
The power supply was blowing fuses and was outputting low voltage.

The bridge rectifiers were also getting very hot after a few seconds.

This unit also exhibited no signal on the video output and no life on the bus, this was due to all the corrosion this specimen was subjected to. This specimen is also missing a floppy drive as that one was very corroded and the bearings were seized that is why the photo has a large hole in the front left.

Causes

 * The large filter capacitors were bad.
 * The tantalums on the cards were bad.
 * Sockets and ICs were corroded

Solutions

 * Tried reforming the large filters however this was unsuccessful so these were replaced.
 * Replace the tantalums which lowered the current draw to what was expected cooling the rectifies down to normal operating conditions..
 * Replaced the sockets and replaced the ICs on the VDU which cured the video output however the machine still doesn't run..

Disassembly Notes
Simple disassembly is easy with two screws on the back of the lid to gain access to the cards however the power supply requires disordering cables and removing the whole case off the base.

Removal of the floppy drives in the vertical configuration is also complicated as they require the removal of the front panel and all the cards. This can be quite difficult especially if the nuts and bolts are corroded like mine, the tight space doesn't help as it can be difficult to get pliers and/ or spanners into the right place. When the lid is removed the cards and backplane are easily seen and the power supply is on the right underneath the CPU serial monitor board. the floppy drive mounts are on the left for vertical mount BASF 6106s. The serial number tag should be visible on the case above the PSU which this one is hand written SN 691

Disk drives
The early RM 380s use BASF 6106 floppy drives which are 2/3 height SSSD 5 1/4. The drives are Shugart interface compatible. Later on IBM PC full Hight drives were chosen and the mounts were changed from a vertical to horizontal. These are easy to replace as they are more common. The early BASF drives can badly corrode depending on storage conditions so be wary of this as 2/3 height floppy drives are not as common.

Keyboard
The keyboard is just standard parallel ASCII foam and foil unit external to the unit via a 15 pin cable. The foam pads in mine are still in good shape but are the same size as most replacement pads making replacement easy. My unit was made by alphametric in the UK.

Keylock
The keylock is a old off the shelf part from RS however the keys are unique to each machine so check to see if you have the keys. This is because the switch controls the power input and also lock function of the front. This one was also seized so was soaked in WD40 to unstick it and allow it to rotate freely.

Cards
The cards are pretty easy to service however the early RM380s use Texas Instruments chips which are know for getting corroded legs. The early ones also use cheap sockets which can corrode. For mine to be a reliable machine large scale replacements of chips and sockets was the only way as the IC legs were breaking in the sockets. Below are the first party Research machines cards in this case as originally configured via the original label inside.

Power supply
''Early supplies use the types below. Later revisions use different types.''

History
The Research machines 380Z is a British Z80 based microcomputer made in oxford England from 1977 to 1985. This machine found a popular use case in schools before the BBC micro was released and could be used in conjunction with Research Machines 480Zs to form a network with a 380Z running as a file sever over the proprietary RM 800kbps network. A kit version of the 380Z is believed to have been made called the 280Z however this cannot be confirmed. The RM380 can use multiple operating systems with COS (cassette operating system) been the default OS in ROM, the versions of COS changed over the lifespan adding and removing functionality. The main revisions known are.


 * COS 2.2 –
 * COS 2.3 – cassette-only and 40×24 video card support
 * .COS 3.0 – floppy disk support for external 8 inch drives.
 * COS 3.4 – main COS version supporting single density 5.25 disks and the 40×24 video card.
 * COS 4.0 – 80×24 VDU text display with no cassette support due to the VDU80/02 not having the cassette interface.
 * COS 4.2 – double density disk drives (380Z-D). This version is very rare due to the limited run of it.

The RM 380 could also run CP/M in both the 32K and 56K configurations. The two known versions of CP/M used on the 380 were CP/M 1.4 and later on it was CP/M 2.2. MP/M version 2 was used on 380s which formed the file server on a network of 480s using CP/NET

Related Links

 * https://www.computinghistory.org.uk/det/2226/RM-380Z-(Black-Case)/
 * https://www.theregister.com/2013/11/13/archaeologic_the_research_machines_380z_story/
 * https://vt100.net/rm/380z_review
 * https://archive.org/details/rm380zservicemanualpn13821a (This covers the later 380z but most of the schematics and information is still relevant.)
 * Index of /pdf/researchMachinesLtd (bitsavers.org)
 * Index of /bits/ResearchMachinesLtd (bitsavers.org)