// 26/09/2026 // by Elder0010
This machine belonged to the Raoul’s loot.
The Olivetti Prodest PC 128S is a disguised Acorn BBC Master Compact computer, rebranded with Olivetti’s branding and design elements. The system ROM is mostly unchanged from the Acorn original one, even tho some minor incompatibilities do exist. It’s possible to swap the ROM with the original Acorn one, restoring full compatibility with the original system.
The computer wasn’t working initially. It had no video signal and a continous beep sound.
The Caps/Shift Lock LEDs stuck on. On BBC hardware a continuous tone is documented as a RAM self-test failure, so the investigation started at the four 4464 DRAMs - and stalled there for a long time, because every RAM signal checked out: RAS and CAS pulsing on both banks, multiplexed address lines and data lines toggling with clean levels, no stuck bits anywhere. Fitting Tom Seddon’s beeb_test_os diagnostic ROM (the 64K build, into IC49 where the original AMD 27512 system ROM sits) changed absolutely nothing - same tone, same blank screen, and none of the alternating Caps/Shift LED flashing the test OS uses to signal it has reached its UI.
I started inspecting the data and address bus: no issues. Also the CPU was ok: correct reset procedure, sync and clock were ok. Using Tom Seddon’s beeb_test_os diagnostic ROM (the 64K build, into IC49 where the original AMD 27512 system ROM sits) changed absolutely nothing.
That turned out to be the real clue: a further inspection revealed that no one of the ROMS, 6522 or CRTC were selected for operation (hence the continuous sound, which is shut off by one of the 6522 and the absence of the video signal).
The Ferranti CF30050 I/O decoder ULA (IC22) had correct power and correct toggling address inputs, but left every one of its chip-select outputs sitting in high impedance. Replacing it fixed the machine.
