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Grundy Newbrain repaired

Grundy Newbrain repaired

Grundy Newbrain repaired

Defect:

  • Dead.

Analysis:

  • The acid of the capacitors has corroded some pcb tracks and pitches of the main pcb.
  • One of the two flat cable (main pcb -> keyboard/vfd pcb) previously repaired cause short circuits and doesn’t work.
  • Removing the Filter capacitor “RIFA” in the external PSU (removal made 7 months ago)

Repair # 1:

  • Replaced all the electrolytic capacitors.
  • Rebuild the PCB tracks corroded by the acid of the capacitors.
  • Replaced the flat between the main PCB and the keyboard/vfd pcb.

The computer now turns on but the VFD (Vacuum Fluorescent Display) displays weird characters and the composite video signal is dead.

Repair # 2:

I have used a Z80 NOP generator that i have made some years ago for repairing ZX Spectrum/Sinclair and that causes the Z80 to process only “NOP” instructions
and consequently on the address lines we can see with a oscilloscope a perfect square wave.

With this simple “trick” i have immediately found the faulty component that break one of the address lines of the CPU (A1). The exact same fault was found by Thomas Gutmeier.

  • Replaced 1 x 74LS257 (IC 412)

The computer now works perfectly.

Gallery of the repair:

Download: Grundy NewBrain Schematics (472)

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