HP-65 1333A18516 - [SOLVED] request for advice

[albertofenini]

Hello everyone

we are trying to restore a unit acquired as a non working unit,
when arrived it looked in very good conditions, so we thought it was a easy one ...

The main problem was that "it didn't turn on" .....

When this happens the first test we do is to move the logic board in a different known to be working unit,
and see what happens, this test proved that the logic board was indeed working

The unit had been previously restored as we found the gummy wheel replaced,
although the coupler between the motor and the worm gear appears to be original

Few cables were desoldered and the three led modules had been replaced,
will post more details about that soon

Anode and Cathode drivers appeared to be original although all of that area
had been reworked

We began resoldering the card reader and the motor cables, we removed the
led modules, the anode and the cathode drivers, cleaned everywhere and resoldered nicely
everything back in place

However, when turned on it remained with a totally black display

With the card reader now connected we tried to read a card, and even if the result was
unkonwn because of the non working display, the card went thru smoothless

A small step forward ...

We tested the inductances that are placed above the leds and those are just fine,
we checked the capacitor, the inductance and the resistance that are placed just
below the switches and those are fine as well

We checked the capacitor in the upper corner, and that it's also fine

Eventually we temporarily replaced the original keyboard pcb, which was in very good condition, with
a "correded but working" one and the display turned magically on

We tried then to read the card again and that also worked fine, so we now know that the card reader is at least reading cards properly

We decided then to transplant the led modules, the anode and the cathode drivers
from the "corroded but working" keyboard pub to the original one, but nevertheless
the display remains off ....

We came to the conclusion that it has to be a faulty trace in the keyboard pcb
unless someone can point to a possible different cause

We also checked all the joint that connect the two faces of the pcb and these are all good with no cold or bad connections

Any idea how could we find the broken trace(s) ???

Should we start from the logic board connector or should we investigate more in
the led modules area ?

Keep in mind that all the components have been proven be in a working condition

Thanks for any help !!!
Take care

Pictures can be found here :
HP-65 1333A18516