HP-65 more on the card reader

[albertofenini]

We have modified the keyboard PCB by adding two single hole pins for the power supply.
This way we don't need to de-solder and re-solder the power cables anytime that we want to test
the unit with a different card reader.
We also have soldered the scope wires on the logic board rather than on the card reader.
We have used thinner and better quality wires to avoid any possible side effect.

We have tested the unit with three different card readers.
Two are HP-65 style, fixed resistor, one is using the old sense amplifier, and the other
is using the same sense amplifier mounted on the HP67 version of the card reader.

They both behave in the same way, regardless of which battery pack is used.

A third car reader is an HP67 style, it has the trimmer and the new version of the sense amplifier.
This particular reader was also not reading, but once we have changed the 22microF capacitors
it started to read with no problems at all.

However, all of them have the same problem, a noise WB channel, thus not recording.

We also replaced the two transistors on the logic board, we thought that given the different
behavior based on the type of battery used, it also could have been an issue with the PSU
not performing well when writing a card.

However, even with two different transistors, all of the card readers are able to read, but
having a noisy WB channel they all fails to record a card.

Would it be possibile to add a capacitor and clean (filter) the noise ?
What do you guys think ?
And eventually, where and how should we connect it ?
Thanks in advance !!!