Not often I have changed heads on MPG, but some knockers have been collecting
for too long so these need to opened up.

MPG3409-EF ( I have two identical 802-82C5 - B11X) SN varies by about 100.

One works perfectly, one knocks for about a min.
Both boards work on the worker.

Change heads and the knocker now calibrates,,, but
cannot read ROM.

hddl reads 02 02 00 01 in RAM, write correct to RAM only
and can read all modules, but of course none can be corrected
or updated to disk. (HS on DW is correct - no need to write)
3Dh on disk seems OK, but 3Dh in RAM is garbage.

So I read all there is about ROM and 3D.

ROM at 20h : 82 C5 FE 04 20 01 07 18 00 00 02 02 C7 4D FE 7B

3D at 20h : 82 C5 FE 04 20 01 07 18 00 00 02 02 76 B2 4D FF


Who can throw some light on 3Dh ?