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Based on the issue 4 schematic and photos of an Issue 4 board, it incorporates additional circuitry to correct the Cursor and Dot Enable signals.
Presets RV1 and RV2 are adjusted to get a 12MHz and 2MHz clock respectively (if the local oscillators are used).
Links LK7, 8, 9, 10 and 11 are normally fitted in position A to select addresses as used by Acorn System COS and DOS.
Links LK4, 5, 6, 12 and 13 depend on which CPU Board is being used:
For 6502 CPU Board clocked at 1MHz fit links LK4 and LK12 (uses both local oscillators)
For 6502A CPU Board clocked at 1MHz fit links LK6 and LK13 (uses no local oscillators)
For 6502A CPU Board clocked at 2MHz fit links LK6 and LK13 (uses no local oscillators)
The new links LK7, 8 and 9 are used as follows:
LK7 - is fitted when an Hitachi CRTC if fitted, the cursor delay is generated by the additional latches (IC19)
LK8 - is fitted when a Motorola CRTC if fitted, the cursor delay is generated by the IC12 latch and R12/13 and C6
LK9 - fitting this link adds sync to video out, it can be removed so that the RGB outputs, when used, don't have sync added
Links Summary:
| LK1 | Light Pen | Normally not fitted |
| LK2 | Invert Cursor | Normally not fitted |
| LK3 | VMA | Normally fitted |
| LK4 | 2MHz (LO) | Fit to use Local Oscillator derived 2MHz clock |
| LK5 | 2MHz (O2) | Fit to use O2 as 2MHz clock |
| LK6 | 2MHz (Bus) | Fit to use Bus derived 2MHz clock |
| LK7 | Cursor (HD) | Fit for Hitachi CRTC, the cursor delay is generated by the additional latches (IC19) |
| LK8 | Cursor (MC) | Fit for Motorola CRTC, if fitted, the cursor delay is generated by the IC12 latch and R12/13 and C6 |
| LK9 | Video Out Sync | Fit if using Video output to add sync, remove when using RGB outputs |
| LK7A,B | Address Select | Links normally fitted in 'A' positions for $1840-187F, fit in 'B' positions for E840-E87F |
| LK8A,B | ||
| LK9A,B | ||
| LK10A,B | ||
| LK11A,B | ||
| LK12 | 12MHz (LO) | Fit to use Local Oscillator derived 12MHz clock |
| LK13 | 12MHz (Bus) | Fit to use Bus derived 12MHz clock |
You may find you need to adjust the value of R8 to get good results, the original Acorn board used 150R but I have found that 100R works better. Also Acorn originally specified 200nS 2114 RAM chips, I have boards working with 200 and 300nS access times but have reports that 150nS are required so you may need a bit of experimenting to get best results.
If only using the RGB output then shorting the 3 clamping diodes (D2-4) will give a brighter picture.
Schematic
Bill Of Materials
Front Panel Drilling
Blank PCB
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