Hardware Bugs

Known hardware bugs and limitations

• The limit of 128 colour palette (64 colours with 64 BRIGHT) as opposed to the desired 256 was allegedly due to not having enough spare physical pins on the ASIC’s die to accommodate it. As it stands there are two pins for each colour (Red, Green and Blue) and a BRIGHT output meaning that there are three bits for each colour, top two bits as per the colour pins output and the least significant bit set by the BRIGHT setting, also see the Palette.
• The inbound MIDI ports do not work due to a fault in the ASIC as explained by Steve Nutting in an interview on Fred 2 and also referenced by Your Sinclair in their article on MGT's demise.  Pressing keys on the attached keyboard (and thus generating interrupts) effects the palette interrupts on the screen, noticeable on the palette tile when running Flash!
• There is no buffering of the stereo audio output from the SAA1099 menaing that it is susceptible to damage when plugging audio into other devices, see Sound Issues.

Correctable hardware bugs and limitations

• There are two factory reworks on the Internal PCB, one to correct the swapping of A0/A1 on the first floppy drive connector and the other to add pull-up resistors to the keyboard input.
• The ASIC does not send an 8Mhz reset signal to the Sam Drive disc controllers when the Reset button is pressed which can cause corruption to a floppy disc left in the drive - hardware fix is the DPU by Edwin Blink
• The NMI button is not de-bounced causing multiple NMI’s to be generated - hardware fix is the NMI Debounce by Edwin Blink. The Messenger included a separate NMI board to get around this issue.
• There is a lot of reflection on the screen when using an RGB cable, there is a Reflection fix from Colin Piggot
• Background sound noise caused by voltage drops on the 5V line can be reduced with the Sound Fix from Edwin Blink
• The External PSU requires modification and it is recommended to swap diode D4 with a new IN5349B (12 V, 5.0 W Zener Diode Voltage Regulator) and remove diode D3 from the board and replace it with a new IN4001 (Rectifier, 50 V, 1.0A) plus replace R3 with a new 33R/4W resistor- this is covered in the article on Newsdisk 5 and extracted on the page External PSU Modification
• The serial port on the Comms Interface has a hardware bug as listed in Format Vol.5 No.8 where you get transmission errors after 100-200 characters regardless of baud rate. This is due to an electrical supply issue in the board which needs smoothing. To fix, solder a fat tantulum capacitor between the 0v and +5v across the supply lines near R3.
• The Sam Mouse Interface can also be erratic due to the same power issue - the HCT logic chips in it don’t like the SAM’s glitchy supply so shove a couple of the biggest electrolytic capacitors you can find and a tantalum capacitor inside your SAM across the +5v and 0v rails. from Simon Cooke on the Sam Users Mailing List October 1994.

See also ROM bugs and Search: “Troubleshooting”