Dave wrote:A lot of noise is induced into the cable connecting it to the QL, but it's small compared to the signal.
Hi Dave - were you referring in this line to the 2x 7-way cables connecting the MDV to the mainboard, or to the voltage reg 3-way connector?
The lesser amount of noise that is problematic to microdrives comes from the 7805 - the larger amount comes from the MC1377 thru the RGB port and also back to the J1 connector. 7805 noise can be reduced by two electrolytics just soldered to the 7805's legs.
For a comprehensive noise reduction program you would:
Remove the MC1377 and use other video output
Replace the 7805 with a buck regulator that has large electrolytic caps on the buck reg board
Add a large electrolytic cap across +5v and GND on the 8301, two places in DRAM, 8049, and near J1
Shield the ribbon cables that go from the QL to the microdrive boards, best way is replace the individual supplied gaskets with a single gasket that covers the back of both units, well grounded at both ends that covers the cables too.
Should any shielding be grounded, or would another foil-shield like that used around the ULA (and electrically isolated) by helpful fitted immediately behind the tape-head? How do you go about actually measuring the impact of shielding like this?
I just measure with an oscilloscope, which is a flawed method as attaching the probe can change the antenna properties of the part being monitored. However, it is useful for like for like comparisons and I can get a relative dB reduction level accurately.