HP-33E Missing Pin on Memory Chip

[mikemorris]

Hello everyone. I have a new question for you all today. I have an HP-33E which visually looks to be in pretty good shape with just a few very small spots of blue from corrosion. However, it shows no life when energized. I've taken it all apart and found a little surprise. There seems to be a pin completely missing from one of the memory chips. Its a little hard to see from the outside of the PCB sandwhich, but when you take that apart it becomes very obvious. I think it is VCC for that chip. This may not be the only problem but it is probably a serious issue. Any comments or suggestions would be welcome:
- Have you seen this sort of thing before?
- Any suggestions on how to fix?    

[mikemorris]

Here is a view of the chip in question    

[Steve Simpkin]

I believe this is "normal" for these models.
https://www.hpmuseum.org/forum/thread-16465.html?

[Steve Simpkin]

Also see:
https://www.hpmuseum.org/forum/thread-4925.html

[mikemorris]

Thank you Steve! I've been here searching the forums for "HP-33" and "Spice" but did not find the thread you shared. Makes me feel more optimistic about this calculator in my hand!

[Steve Simpkin]

Yes, I have not seen a great explanation as to "why" these pins are sometimes missing.

[mikemorris]

And none of these posts seem to show the missing pin on the same chip. Mine is missing on the chip closest to the power supply PCB. The photo you just shared has it on the middle one. I suspect there is some code which correlates the specific model to the specific configuation of memory chips....

[teenix]

I have one of these chips with the silver paint faded a bit and can almost see what looks like a standard serial number on it like xMxx-x00x. I cannot make out the x's.

As the power pin is not connected, it seems a waste of an expensive IC if these are actual unused ROMs.

I tried measuring the pins with an ohmmeter, but all read open circuit, same as normal ROMs.

I thought maybe they balance the pressure on the pins of the other ROM IC's on the padded circuit film, but these IC's appear on soldered versions as well.

Weird.

cheers

Tony

[mikemorris]

In case its helpful to anyone, the attached photo shows the identifying codes on the three chips in my HP33E. My HP-33E is one of the earlier models with the pressure assembled PCB sandwhich. Serial Number 1920A96400, made in the USA. The "dummy" chip with the missing power lead is the one on the left which is closest to the power-supply PCB.    

[AndiGer]

I have a never working HP-37E that has 2 of those 00031-80008 chips and one 1MAx-0008 chip with x not readable, I suppose it is 1.
Both 00031-80008 have only 7 pins.
Andi

[brouhaha]

The older thread suggested "dummy" chips. None of them are dummies. There were two generations of NMOS ROM/RAM chips used in 3xE series, 1MA1 and 1MA3, which had differing capacities. In some cases the two were mixed, so every 3xE model had at least two different combinations of chips, often with different numbers of chips. I believe the 1MA1 has 1536x10 ROM and 4x56 RAM, and the 1MA3 has 1024x10 ROM and 16x56 RAM, but I have not yet verified these details. Some 3xE calculators in some or all configurations contain more memory than actually used, but none contain entirely superfluous chips. Unused memory might be disabled by mask options, but more likely it is just programmed to all-zeroes (ROM) or present but never accessed (RAM)

The 00031-80008 chip appears to be a RAM-only part. The 00031-80012 may be RAM-only at a different address. These are not exclusive to the 31E, and in fact the 00031-80012 should never be in a 31E.

The missing pin still seems quite mysterious. I believe that the fully working 3xE units with NMOS chips missing a pin are inadvertently powering themselves using phantom power from other pins. This phenomenon is commonly seen in CMOS, but in this case the NMOS chips were designed for low power consumption and high ESD protection, so they may do it also.

The CMOS RAM/ROM chip, of which exactly one is used in each 3xC model, is the 1LB9-nnnn. The 1LB9 is used in conjunction with NMOS parts.

[brouhaha]

(05-03-2024 01:22 PM)AndiGer Wrote:  I have a never working HP-37E that has 2 of those 00031-80008 chips and one 1MAx-0008 chip with x not readable, I suppose it is 1.

The unit was built or repaired incorrectly. None of the 8-pin parts should ever have duplicates.

[brouhaha]

(05-03-2024 02:20 AM)teenix Wrote:  I have one of these chips with the silver paint faded a bit and can almost see what looks like a standard serial number on it like xMxx-x00x. I cannot make out the x's.

I think the 33E (but not the 33C!) would normally have three memory chips (from my very old notes, not guaranteed):
* 1MA1-0004 _OR_ 1MA1-0204
* 1MA1-0003 _OR _ 1MA3-0002
* 00031-80008

For the 3xE series, the memory chip slots are fully interchangeable, so each memory chip can be in any position. For the 3xC series, the 1LB9 CMOS memory chip must be In the top position.

Note that the 40-pin chip includes the first 1Kx10 of ROM, which is why they're not fully interchangeable across all Spice models. There are three different ones, though each is known by both an 1818-nnnn and a 1MA4-nnnn part number.