We went through `Microdrive design' topic together
. Being no expert, please find my observations.
This is a summary of mandatory modifications.
1. Add resistor R104 (82 Ω) in TR1 collector circuit. It should be noted that this resistor is fitted in a non-standard way in the collector leg
between the transistor and the board.
2. Replace resistor R92 (was 220 Ω) with a resistor of value 390 Ω.
3. Add resistor R105 (1 KΩ) across capacitor C19.
4. Add resistor R106 (1 KΩ) across diode D17.
5. Add diode D22 (1N4148) in series with R100.
6. Add diode D23 (1N4148) in series with R101.
7. Add resistors R102, R103 (33 KΩ) between pins 19 and 21 of IC23 and VM12 (-12V rail). It is convenient to do this by laying the resistors on
top of IC23 and running a flying lead from their common connection to pin 1 (VM12) of IC25.
8. If EPROMs are fitted, upgrade firmware (IC33, IC34) to D11 standard and return salvaged EPROMs to Sinclair Research Limited. Details of
the methods used to upgrade to D11 standard are set out in sub-section 4.
9. Add MDV2 stand-off spacer.
The mod 5,6) we have discussed already, these add extra diodes on MDV head board. They should prevent drive from selecting when having no cartridge inserted. Your Iss#5 drives do not have them applied.
A visual indication whether the mods have been factory applied, is to look for flying resistors on top of ULA. See picture.
As already discussed, MDV in slot 1, should have screen foil on its ULA, to shield off noise coming from the PAL and TV modulator.
MDV in slot 2, may induce some noise from power-ground on copper side as its ULA is situated right above. The recommended mod is to divert the power-ground away from the ULA. See picture.
MDV in slot 2, may tend to format much fewer sectors, on occasions cannot complete format at all, while outside the slot 2 they can format alright. In such situations the mod can help.