Thanks for the info, but I'd like to keep my 2nd processors as unmodified as possible, while still using an internal PSU (I'm really not a fan of wall wart PSUs).Alternatively, you could just provide them with a +5V supply and ignore the PSU in the cheese wedge.
If you've now got a generously-specified PSU in the main BBC, then it's quite reasonable to run the 2nd processors off that: there's 5V power provided on the tube connector, and early versions of the 6502 2nd Processor used to have a jumper on the PCB to allow it to be powered that way - I ran mine like that for some years. Even if yours is more recent and doesn't have the jumper, it would be trivial to add as a wire from the pins on the connector round to what is currently the input from the separate PSU.
Or of course you could run the 2P off something like a USB "charger" 5V supply.
Note that the design of the cheese wedge PSU with the toroidal transformer also had provision for a 40V output for use in the teletext adapter - the basic design was common to all the units in that case which required power (Econet bridge for example), but normally built with the components for the extra output voltages not fitted; I'm not sure if they wound different transformers or if the transformer still has a redundant winding.
(in the "I don't recommend this but it worked" category, my system has both the 6502 and Z80 2nd processors fitted in the same cheese wedge case, ribbon cable in parallel across the tube connectors, and a switch to power up only one of them at once. Evidently a powered-down tube chip was sufficiently high impedance to not stuff up the bus in the BBC).
Statistics: Posted by rich — Sun Jul 06, 2025 3:33 am