martyn_hill wrote:Hi Bytesguy
Yes, those look about right and match the standard mods on my Iss5 board.
Whilst you're in touch with Rich, I'd very much recommend purchasing a spare 8302 - they are not expensive and it could well address the issue in one swell-foop
And if not, having the spare IC will come in handy eventually...
Aside from the 8302, the services offered by the motherboard itself to the MDVs are:
a) Power cct for the motor
b) Current for the LED
c) Current for the Erase head
- All the above are switched by COMMS OUT from the respective MDV ULA. And...
d) Switching for the Erase current (TR3) through diode D28 and resistor R25. Capacitor C46 filters the switching action a little.
Given that power is flowing and that multiple drive units exhibit the same format failure, item d above should be your next line of enquiry...
D28 simply jacks-up the voltage by c 1V at the emitter of TR3 as far as I can tell, reducing the overall voltage seen across the Erase head when TR3 is saturated ON - i.e. when the drive is active (COMMS OUT active), the WP switch activated _and_ the ERASE signal from the 8302 active. By 'WP switch activated', I mean to say that the WP tab or replacement sticky-tape is fully engaging the WP switch - actually DEactivating WP
TR3 could therefore be your culprit here - both Format and any Write activity will need TR3 to switch correctly.
It's the ubiquitous BC184 NPN type - easy and cheap enough to replace. There is only one Erase transistor for the entire MDV 'chain', so you might expect it to impact any and all units attached - as we are seeing here.
BTW - when using the ZX units with a QL, be mindful that the state of the WP tab/presence of sticky-tape will make NO difference to the QL's abiility to write to the cartridge! The QL will write and erase-away, blissfully unaware that it shouldn't
Hi Martyn, I think you might be spot on with the TR3 idea know that you have given that explanation of how it works! I'll see if I can probe it this evening to check it's actually working, but regardless I have ordered some BC184 which should be here by the weekend, so I will probably replace it as a matter of course. It does sound highly likely this is the case as reading is fine but writing/formatting seems to be the issue. Will update the thread with more info once I get a bit further along. Thank you!