My Battery Sucking HP-71 Memory



#8

I related elsewhere that I'd purchased an HP-71B that turned out to have a 96KB memory add-on. I told how the calculator would go through batteries at a very rapid clip with the module installed in the card reader bay. I also had verified that this behavior followed the module into a second 71B. At the time, Don W. suggested I post photos of the beast. So here they are. They were all taken through an illuminated magnifying lamp, so please forgive the image quality.

It's an interesting little monster. You can see that one of the screws is missing, because there's a corner of a chip filling the left-hand gap!

The right hand screw hole has a torx head screw too small for any of my tools. I still haven't gotten around to buying an approptiate sized one. But ...

You can do this to the module without unduly straining anything. Here we can see that PCBs have been stacked together, with chips soldered on both sides. The chips at the top in this photo are soldered in the same orientation as the chips on the other side of the board. The other assembly moving down and to the right has a single large chip on one side, mated to two smaller ones below.You can't see it in this photo, but there is another PCB with another five chips underneath the two boards visible here. There was some question in my mind if 96K was really the par RAM amount for this module. Seeing three CBs leads me to think that it is. So assuming it's 32K per board, we must have 4 4K chips and one 16K? How else could it break down? People used to dealing with RAM at the chip level in the same vintage can perhaps enlighten me on that.

Another feature you can sort of see in the above photo is the not-so-great soldering job. The following photo, taken with an additional magnifying glass inline with the first, shows this in more detail:

It is probable that one or more of these joints, or worse ones hidden from view, are responsible for the excessive current drain.

More when I get the right tool to remove the other screw.


Edited: 22 Sept 2005, 6:46 p.m.


#9

In the early '80s I used to test memory chips for quality and reliability.

If the memory module is fully functional then it is likely that the solder joints are ok, even though they do look messy, but high battery drain could be caused by leakage between joints, or by a damaged chip, possibly from electrostatic discharge causing internal oxide breakdown and high internal leakage currents. Some chips from the early 80s were very sensitive to static damage as the internal protection circuits were not good.

If you can remove the lid of the module try inspecting between all the joints with a 3x or 4x eye magnifier and look for slivers of solder and flux between the pins. Carefully remove any debris from between any joints with the point of a sewing pin. If any of the chips has been damaged by ESD it would need to be replaced. It may be difficult to find the right chip to replace it and it will be a very tricky soldering job, but I am sure one or two people in this forum could help!

Before you handle the module sit at a conductive, or non-static generating bench, wear only cotton clothes and if possible use a special anti-static wrist strap connected to ground.

Tell me the part codes on the chips and I may be able to tell you what memory size and configuration they are.

Good luck!


#10

Thanks for the advice, Gordon!

I guess I could peer at the labelling on some of the chips now, but I prefer to wait until I have the whole thing open. Got it regarding the static precautions. Good ol' CMOS!

#11

Each board has four industry-standard 8K*8 SRAMs, and an HP chip that acts as a Saturn-bus-to-bytewide-memory interface. I don't recall the part number of the HP chip, but rumor has it that a datasheet on that part was distributed by some of the user groups.

IIRC, there was some bug in the chip that prevented it from being used for more than 32Kbytes, hence the need for three of them to make a 96K module.

If the module is drawing too much current, yet seems to function correctly, there are several possible problems:

  1. dirty PCB - gunk on PCB can be slightly conductive, causing leakage current. This is a big problem with solder with water-soluble fluxes, but I doubt that they were used on these boards. Still, cleaning the boards could help.
  2. ESD damage to a RAM chip - fortunately they are common chips that can easily be replaced
  3. ESD damage to one of the HP memory interface chips - there is no source of replacements other than cannibalizing them from other similar memory modules, but if you can identify which of the three PCBs is causing the problem, you can remove it and have a better 64K memory module. Be careful to document the interboard connections so that you can rewire it properly; most signals should just be in parallel, but there is a daisy-chained signal used by the soft configuration process.

#12

Thanks for those details, Eric.

I'm not sure if this should be my first attempt at chip-level electronic repair or not. I might just confine myself to dropping boards out one at a time to see if the problem is associated with one of them, as you suggest. That would probably involve some soldering, but at least not whole chips.

Thanks again!

#13

The most infamous way for a CMOS chip to suck huge amounts of power is for an input pin to be left unconnected. This causes both complementary transistors to turn on and effectively short power to ground. Check all solder joints, etc. Clean all solder flux. Repent all sins.


#14

All of them??

Sorry, but there are one or two I'm quite happy with. 8)

Inspecting the joints is something I planned to do. I'm just not sure what I'll do if I do find a problem.


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