Christmas 1979, Issue 5

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Having had my moans, Nascom are to be congratulated, Nascom 2 is a superb machine, and another satisfied customer bites the dust.

Other bits of info for Nascom 2 from various sources

The video fault mentioned above seems to be fairly common, and really comes back to the oscillator. The output of the 74S04 (IC56) seems to be almost sinusoidal in shape, and this makes the first divider (IC49) a bit unhappy. IC49, in turn feeds IC7la which generates the LD signal for IC65, and any jitteriness of IC49 is transferred to IC65. The best cure is the 100pF capacitor as mentioned above, but a 5pF capacitor between pins 12 and 7 of IC56 helps to square up. the clock a little. Severe clock jitter can cause some very weird timing problems giving rise to overall unreliability.

The memory supplied with N2’s at present is our old friend Nascom Series 1 Memory card, which was the subject of a note about ‘Memory Plague’ in an earlier news letter. Well as an N2 may well ask it to run at 4 MHz without any wait states, a little more investigation was considered desirable. The buss signals from an N2 are a lot cleaner than an N1 which helps, but ‘Plague’ still raises its head from time to time. First, check that you have used the 74157s in the memory not the 74LS157s. Both types are used on an N2, and are easily mixed up. Secondly, gridding the +5 volt and ground rails is a good idea, there is a sheet in the N2 kit about that. Thirdly, change all the 33R resistors to 68R, and change the links between P8 and P9, P12 and P13 for 68R as well.

These mods alone cured a board that suffered from severe ‘Plague’ at 2 MHz on an N1. All the other mods (the Rs and Cs from the earlier note, etc), were removed. The board worked perfectly at 2 or 4 MHz on both an unmodified ‘B’ issue N1 (worst case) and an N2. So it seems that proper gridding of the board, and curing any tendency for the address and control lines of the 4116s to under-shoot (the 68R resistors) will cure ‘Plague’ on its own.


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