hp33e display fault


Hi All
I've recently got a hp33e but it has a display fault where parts of the display segments go out to whole digits and other various combinations a slight flexing/twisting of the case returns the dispaly to normal. How hard are these calcs to get apart has anyone seen this type of thing before and might it be a relatively easy fix.


I would guess (without seeing it) that you have one of the old-series 33Es that were built with minimal solder. The ICs, display pins, and flexiprint from the power converter (this module is soldered together) are clamped against a foam-backed flexible PCB by a plastic frame.

In time, the contacts oxidise or get dirty and you get faults like display digits, or segments, missing, some keys fail to work, the whole machine is dead, and so on.

You have to take it apart and clean the contacts. Dismantling a Spice (HP3xE or HP3xC) machine is not trivial until you've done a couple, and there's a lot of misinformation out there about separating the case halves. Here's how to do it :

Remove the battery cover, battery pack, and the 2 screws inside the battery compartment.

Ease the case apart at the battery pack end. Do NOT keep on separating the case here (as is often recomended) -- this is a good way to break something.

Now put your fingers in the battery compartment and your thuimbs on the front edge of the top case. Squeeze hard. The idea is to move the bottom case towards the front (0-key) end of the machine. It'll suddenly move with a sharp click.

Now carefully remove the bottom case, making sure you don't damage the battery contacts.

Free the battery terminal PCB from its mounting

Unclip the logic assembly from the hooks down each side. Take care here, if you break them off, you'll be looking for a new top case. At this point the keys, switch sliders/contacts are loose in the top case. Put it all aside so you don't scatter parts everywhere. If the switch contacts are stuck to the logic assembly, remove them and put them back in the sliders.

Remove the metal clips from the sides of the logic assembly

Now, with that assembly keyboard side up, lift off the keyboard/flexible PCB. You'll be left with a plastic frame containing the chips, the PSU module, and the display.

Clean the contacts on the flexible PCB, clean the display pins. Put it all back together.

This is a very common problem with early Spice machines


It is a VERY good idea to note where each chip is and its orientation before you open up the keyboard sandwich. It is very easy to get things messed up when the sandwich comes apart. Also be very careful handling the battery conenctor/cable. The conductors in the flex cable are VERY prone to cracking... then you will need to trace wthe wiring and replace them with pieces of standard starnded hookup wire.


About getting the ICs back in the right places (and the right way round). In all HP3xE and 3xC machines, all ICs have the pin 1 end (notched end) at the rear -- display end -- of the machine. And it's obvious where the 40 pin chip (CPU, keyboard interface, display interface) goes!

In an HP3xE, all the 8 pin locations are equivalent. You can put the chips in any order. In an HP3xC, the rearmost --nearest to the display -- location is different, it receives the backup supply to maintain continuous memory. So you must put the right chip there in a 3xC machine.

The difference between 3xE and 3xC machines (other than the 8 pin memory chips) is the power converter board. The +ve supply pin of the rearmost memory chip position is wired to a separate pin on the power coverter. On 3xE machines this is strapped to the main +ve supply on the power converter board/flexiprint. On 3xC machines there are a couple of diodes and a resistor on the power converter board to supply the backup supply.


Some of the "E" series machines seem to have "C" series keyboards where the RAM chip slot is wired differently. I always thought the backup pin was a no-connect on the normal RAM chips and it did not make any difference. Then I opened up a machine and dumped the chips without looking... had to open up another machine to get them right.


All the HP3xE and 3xC machines that I have worked on have the same keyboard PCB. The +ve supply pin (pin 8 IIRC) of the front 2 memory chip locations goes to the one pin of the PSU module (and also to the +ve supply pin of the CPU chip). The +ve supply pin of the rearmost memory chip goes to a different pin on the PSU.

On the 3xE PSU, these 2 pins are linked together, so all the chips get the same supply (and all memory chip locations are the same). On the 3xC PSU there are a couple of diodes and a resistor (10k IIRC) to provide the constant supply to the rearmost (RAM) memory chip

Or at least that's how I remember the schematics...


Hi, folks;

I didn't work with so many Spices, some solderless, some with soldered IC's (less than 20 units), but what I saw is closer to what Tony mentions. I remember looking closer at the PCB's layout and finding that, except for pin #8 of the first one, all 8-pin IC's were connected in paralell, like the common "piggy back" found in some HP41 fullnuts. Not too hard to figure out that that was the RAM position, C-type units, of course.

I did not notice any other missing connection. But when checking the ROM connections in an HP41 fullnut, there is a NC terminal I used to think was for WR (write) purposes, but later I found the HP41 schematics and it is labled "DATA" (I'll have to read about his later...). In this case, a RAM IC would not work in a ROM IC position. Based on what David wrote, I'll take extra care when dealing with Spices. David, did you see this difference in both solderless and regular (soldered) units as well?


Luiz (Brasil)

Edited: 24 Apr 2004, 10:11 a.m.


The machine that I was working on could easily of had the connection between the the two power supply pins open. I seem to remember it had a bad case of the green fuzzies/white powder/broken contacts/bad cable/etc that we all are so fond of in our Spice machines. I have only seen one or two machines with the very early keyboard that was not "C" compatible.

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