HP-67 with some odd good and bad things

[Paul Berger (Canada)]

I had earlier reported a ROM issue with a 67 in the old forum, in my case, as near as I could determine it performed functions ok but it would not display anything correctly, I did isolate it down to a single ROM, but I was only able to do that by desoldering the chips and swapping them. I have a 97, which is very similar hardware, that I have all the chips in sockets, that I used for that purpose. Since the 97 is similar the service guide for the 97, available on the museum DVD, can be helpful.

Your description of the damage sounds like liquid damage, many beverages are slightly corrosive and can get into almost every corner. The same 67 that I had the ROM issue with showed signs of liquid damage as well. Like your screws where rusty, there was also some corrosion damage on the main board and the aluminum name plate showed some corrosion damage. You may find that there is even some residue on the board depending on how far the liquid seeped in.

One of the first thing I would check is to make sure that Vss (+6.25) and Vgg (-12.0) are correct they don't have to be right on but should be within 10% of nominal value, I have found that faulty capacitors can cause the voltage to be low, and in one case on a 97 the capacitors measured the correct value but just had high serial resistance (ESR). Ground is pin 5 on the RAM/ROM chips Vss pin 1, and Vgg is pin 3.

Unfortunately the RAM is spread over all 4 of the RAM/ROM chips, the 8 pin chips on the board this is why it usually ends up with swapping chips to find the bad one, I am not aware of any documentation that states what portion of the memory is on what chip. Now you say that functions entered from the keyboard work fine, but they would not involve any of the RAM as the stack registers are on the ACT chip. What happens if you try to save a value in one of the storage registers? The 26 registers would take up all the RAM in about 1.5 of the RAM/ROM chips the remainder of the RAM would be the program storage memory. Did you go all the way through the program memory? If the RAM portion of one chip was defective I would not expect you to get an incorrect value stored throughout the whole of program memory. If you do get an incorrect value store throughout the whole of the program memory I would be more inclined to think it is a ROM issue or perhaps even an obscure ACT issue, same thing with the storage registers since they would span more than one chip. Is the card reader working? if so it might be of some value to compare what happens when you read in a program from a card as opposed to entering it form the keyboard. You could do the same thing for the data registers.