Amiga 1200 Rev 1D1 Noise on video signal (LM385)

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I’ve seen some 1200 Rev. 1D.1 boards missing the D215 (LM385), this will result in a higher voltage on the DAC VREF signal (U30) and noise will shown when you dragging workbench windows, accessing HD or floppy, etc.. When you place a LM385 then the picture will be much better.

If the LM385 is broke, you have the same result, then just replace the LM385.

Meassure the voltage between  D215A pin 4 (GND) & 8 (VREF), just should measuring 1.2 volt.

Without LM385 voltage reference diode

Reference voltage is 3.4 volt.

With LM385 voltage reference diode

Reference voltage is 1.2 volt.

Picture is much better, no interference.

Capacitor on D215D

If the Capacitor is missing on D215A, place that one aswell.

Use a 100nF (0.1uF), sometimes you need to place a higher range capacitor, for example a 220nF (0,22uF)

Note when replacing video DAC

If you fit an ADV101 then R215 and D215 should be in place. If you fit a VP101 in place of an ADV101 though you can simply leave the voltage reference installed it is permissible to use an external VREF on the VP101 and the external one is likely to be lower noise.

Note by: Peter Mulholland

Resources

LM385 – datasheet