Message #7 Posted by Tony Duell (UK) on 10 Mar 2003, 6:15 p.m.,
in response to message #5 by Larry B
... is easy if you know the trick. The trick is that there are 12 testpoints on the logic board of a 35, 45 or 55, going down the RH edge of the PCB (looking at the component side of the PCB). From the 'top' (display end of the calculator) they are :
Vss , Gnd, Phi_1, Phi_2, Vgg, IS, IA, Sync, WS, Init, Carry, Data
Start by checking the voltage on Vss (should be +6.2V) and Vgg (-12) wrt ground (or the middle pin of the AC adapter) with the adapter connected and the machine turned on. If both voltages are missing, check the AC adapter, power switch, PSU circuit on the logic board, etc. If one of them is missing/incorrect, check the PSU circuitry on the logic board.
When you've got that right, check the clocks Phi_1, and Phi_2 with a 'scope or logic analyser. If one or both is missing, check the clock buffer chip, the anode driver chip (on the keyboard/display PCB) and the oscillator 'tank'.
Got power and clocks? Now check the IA and IS lines with a 'scope. They should show irregular digital puse trains (IA is the instruction (ROM) address from the C&T chip, IS is the Instruction data from the ROMs)
If they're wrong, suspect the C&T or ROM chips.