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Full Version: Re: recent HP 35 "Red Dot" on Ebay
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I'll check again, but I recall I went through them all and tried to check isolation and it all seemed okay.

But if anyone has an opinion that this HAS TO BE a board corrosion issue and CANNOT be a ROM, I'd love to hear.

I suggest taking an ohmmeter to the two traces indicated in red and green below, which correspond to keyboard rows 3 & 4, which seem to be shorted on your HP 35. (image of keyboard circuit board is from http://www.jacques-laporte.org/HP35%20keyboard.htm)

Note that the traces that I've indicated in the image below are on the bottom of the main board, not on the keyboard. You should be able to test them with the main board connected and disconnected to determine if the short is on the main board or on the keyboard (if that's indeed the problem).


I checked this and there is good isolation between R3 & 4 on my unit. However, I did notice that the resistance between R3 and Ground is the lowest of any of the 8 rows, about 50 Kohm. When the keys in that row are depressed, the resistance does not drop much.

Is this what I should be comparing?

I'm at the limits of my knowledge here - but logically if rows 3 & 4 are well isolated when the two boards are attached then it's not a problem of a short between those lines anywhere on either of the two circuit boards.

I guess you could check the resistance to ground on those two lines (R3 and R4) with the mainboard disconnected from the keyboard (check it on both the mainboard and the keyboard) to see on which board R3 has the low resistance to ground. If it's on the keyboard then maybe it is a problem with a trace on the keyboard, but more probably it's on the mainboard and it's in the chip, because 50Kohms is not a short unless it's going through a 50Kohm resistor on the circuit board.

Like I said, I'm at the limits of my knowledge here, in case anyone else wants to step in!

The soldering of the ROMs indicates that this is rather a preproduction unit - the beta ROMs have been replaced by (still faulty) release-ROMs. Which hints towards being a "lab beast". Nice one :)

which picture are you referring to? we have about 5 different units shown on this thread.