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HP31E, charging circuit and flex-cable versus discrete wires
Message #1 Posted by Vieira, Luiz C. (Brazil) on 30 Apr 2003, 3:44 a.m.


last year I post some messages about an HP31E (Spice) I was given as not working. After disassembling it all (soldered version), cleaning it, rebuilding posts and resoldering two IC's terminals it got back to life... except for charging batteries.

I remember many contributors sent me suggestions, in special Katie Wasserman (thanks, Katie) who suggested me a brief "check list" over some diodes and other components.

After all of these discussions about flex-cable I decided to carefully check connections in that HP31E and I found a broken copper trail right were the flex cable is soldered to the mainboard. While inspecting, I decided to check the flex itself and suddenly another cooper trails broke in front of my eyes (maybe I forced a little more than I should... OK, OK: guilty-as-charged). Well, soldering iron connected to the AC line and let's remove the flex.

After a couple of hours there it was, a fully operative HP31E... and charging batteries. No problems at all with the power circuit.

I used parts of an ordinary PC-IDE flat cable to re-make all connections. It fits fine in the small space between the power supply PCB and the structural frame. I also used small pieces of foam with adherent in both surfaces to keep the power supply PCB in place; it looks as a stable assy.

This is an experience of mine and by no means is intended to prove others are wrong. Even without seeing it, I'm 101% confident that Renato's solution is cosmetically better, no doubts about it.

Oh, yes: this is a soldered-type Spice and it shows the "2"-digit blinking effect, while I'm finishing services over another HP31E, "sandwich"-type, that does not suffer for this bug; an opposed situation from the one observed when comparing my HP34C with Norm's one.

Just a few more data.

Luiz C. Vieira - Brazil

Re: HP31E, charging circuit and flex-cable versus discrete wires
Message #2 Posted by David Smith on 30 Apr 2003, 11:23 a.m.,
in response to message #1 by Vieira, Luiz C. (Brazil)

Yes, I also use flat ribbon cable to do my Spice repairs. Flex-cable generally refers to that flat printed wiring on plastic (usually Kapton) cable that HP used in the machines. The material that HP used likes to delaminate ot develop cracks in conductors. It particularly likes to fail right near the place where the bare conductors become coated on both sides with insulating film. Apparently the film contracts with time and stresses rip the conductors apart.

The other flex-cable failure that HP's are famous for are the HP97 card reader cables where the bare conductors come unglued from the backing where they plug into the connectors. When fixing HP card readers it is best not to unplug the cable.

Reducing (production) costs
Message #3 Posted by Vieira, Luiz C. (Brazil) on 30 Apr 2003, 11:49 a.m.,
in response to message #2 by David Smith

Afterall, using flex conductors means a lot of mainboards being soldered on a producion line without men interference and at a lower cost. If all Spices' calbes were hand soldered, producion cost would surely increase.

Just a note.

Vieira, Luiz C. - Brazil

Re: HP31E, charging circuit and flex-cable versus discrete wires
Message #4 Posted by Ellis Easley on 30 Apr 2003, 12:09 p.m.,
in response to message #2 by David Smith

I'm not sure about the sockets HP used in the 97, but that kind of connector comes in a version that has barbs to aggresively prevent unplugging the flat cable. You can "unlock" it by sliding in a thin sheet of something else between the cable (conductor exposed side) and the contacts. If the material you use is too thick, it will end up locked in there - then you need to find a thinner material to unlock it! The best thing would be very thin steel (don't know what dimension, I think of the old feeler gages) which the barbs can't bite into much. It's important to have material with a square-cut end as wide as the cable to push on all the contacts at once, a single barb will hang things up. As David says, it's better not to unmate one of these if you can avoid it. I've just remembered a failure mode of these connectors which the barbs are intended to correct - we used cables about 2.5 or 3 inches wide (connecting the floppy disc controller and RS-232 boards to the Radio Shack TRS80 Model 3 motherboard) and when it is that wide, the plastic shell of the socket can't exert as much force out in the middle of the span as it can at the ends (due to the accumulation of the spring forces of the individual contacts) and so it "smiles"! (Think of Miss Swan on "Mad TV")

Re: HP31E, charging circuit and flex-cable versus discrete wires
Message #5 Posted by jose goncalves on 30 Apr 2003, 2:16 p.m.,
in response to message #4 by Ellis Easley

This piece of hard thin material you are decribing is exactly what the service manual for the 92 calls a "special tool" for releasing the flat cable. When I disassembled my 92 I found that even if I didnīt disconnected the flat cable I would face problems for it already was full of small cracks and interruptions. Iīm still thinking about the best approach to substitute that thing. Should I use common small gauge wire and solder ? I could install sort of a plug in the middle to allow me to remove the reader if I wanted to. Any suggestions ?

Re: HP31E, charging circuit and flex-cable versus discrete wires
Message #6 Posted by Ellis Easley on 30 Apr 2003, 10:06 p.m.,
in response to message #5 by jose goncalves

I have a small sample of some .1" center-center ribbon cable, two pieces about 5" long each, terminated with gold plated machined-pin connectors, pin on one cable and socket on the other. I use them to connect the keyboard to a TRS80 main logic board that I have mounted on a plank of wood, making the keyboard detachable. I think the samples are from Samtec. The pins are the size that mate with DIP IC sockets of the same kind. You could mount a SIP (single inline plug) socket strip at one end and then solder a ribbon cable terminated with the mating pin connector strip at the other end. This is probably overkill, since it doesn't need to be mated often, which is one reason for gold plated connectors, but it comes to mind because the connector is very compact in cross section (<1/8" thick), but it does require 3/8 or 1/2 inch above the socket before the wire can bend, so that might disqualify it - unless you can solder it on the tight end of the run and have plenty of clearance at the other end for the connector. Or if it is tight at both ends, but ther is a straight run in the middle of at least 1", you could do what I have done with the keyboard (which was an afterthought based on the possession of the samples!) There are rather bulky insulation displacement connectors for .1" ribbon cable, the cross section of the plug is about 3/8" by 1/2", you see these a lot in TVs, they mate with .025" square posts which are the same size used in dual row headers like thosed used in computers for disk drives, etc. only made in single row strips. Again, you can solder the cable at one end and use connectors at the other. Then there are the more compact single row connectors (usually red) with terminals crimped on individual wires on .1" centers like the harnesses that connect the speaker and the front panel controls on generic PC chassis, these also mate with the same .025" square pins. Thes are probably the easiest to find connectors but they don't come in a pin version to be attached to a cable - however you can crimp sockets on either side then join the sockets with a single row header, they are made for this purpose with the same length of pin on both sides of the plastic strip that holds the pins. There are some more compact connectors used by Asian manufacturers that are metric but if the number of circuits isn't too high, 2.5mm connectors fit in places meant for .1" connectors. They are generally used with individual wires rather than ribbon cables, the terminals are crimped to the wires and then pushed into the shells where they latch in place. One example you might have seen is the audio connectors on CD-ROM drives before they were standardized with the 4 pin latching Molex connector. Is the 92 built like a 97? I'll take a look at my 97 and see what kind of clearance there is, and see what ideas come up.

Re: HP31E, charging circuit and flex-cable versus discrete wires
Message #7 Posted by Ellis Easley on 1 May 2003, 3:32 a.m.,
in response to message #6 by Ellis Easley

I opened up my 97 and I see that there is not a lot of clearance above the PCB at either end, so a right angle connector would be needed. The opening in the middle plastic is 0.180" X 1.490". The red Molex connectors measure 0.196" thick so that's too much. The black ones, which are actually the ones used for PC chassis front panel harnesses now that I look at it, are exactly 0.1" thick - they need to be since common PC motherboards use a dual row header (0.1" centers both ways) and plug single row sockets on both rows - so they would fit through the slot in the middle plastic. If you had a bunch of PC chassis harnesses, enough to add up to 2 X 13 pins, you could cut the wires to length, tape the plugs together to make two 13 pin single row sockets, solder 13 wires into each PCB, and then use a double-header to join the two sockets. Page 4 of this .pdf from DigiKey shows single row headers with many combinations of lengths above and below the plastic strip (drawing calls them "A head" and "B head"):

The black connector is called "C-grid" by Molex, you can see it on this page:

The single row version only comes in up to 10 circuits and you have to buy the contacts separately and you have to have the right tool to crimp them right.

An idea that came to me when I looked in my junk box is to take a 28 pin machined pin open frame DIP IC socket, cut it into two strips, cut one pin from each (to make two 13 pin strips), bend the pins on one strip at a right angle and solder it into one of the PCBs. Then push 13 wires into the sockets of the other strip and solder them in place, and solder the other ends of the wires to the other PCB. Now you can plug the pins from the second strip into the sockets of the first strip. Page 5 of this .pdf has the kind of IC sockets I'm talking about:

The pin and the socket should be either both gold plated or both tin plated for reliability. Catalog number ED56283-ND or ED56286-ND are both all gold plated 28 pin sockets, one is 0.3" wide and the other is 0.6" wide, both cost $2.07. Catalog number ED3128-ND or ED3728-ND are both all tin plated 28 pin sockets, one is 0.3" wide and the other is 0.6" wide, both cost $1.13. The drawing for these parts, labeled "Solder Tail", shows the pin is .021" diameter but it doesn't say what range of pin sizes the sockets accept.

On page 3 of this .pdf are "single row sockets" which are strips made of machined pin sockets:

The drawing "Fig.1" shows that these parts have a pin .021 diameter and a socket that accepts pins from .015 to .025 diameter, which means you can plug one of these into another reliably (the contacts in the IC sockets are probably the same thing). The 7400 series has gold plating on both the pin and the socket. You can either order a 64 pin strip and cut it or order "custom" sizes - the price works out the same for a 64 pin ordered as custom. A 13 pin "custom" would be $1.04 ($0.26 base + $0.06 per pin).

I think I'm going to do this to my 97. To me, it makes sense to solder the wires to the card reader PCB and solder the socket to the main PCB, then you can unmate the connector before you remove the middle assembly to get to the card reader. Also, it looks like there is more room near the main PCB. It's probably a good idea to use a small screwdriver to separate the two connectors a little at a time to avoid bending the pins.

Flex x wires : a matter of taste ?
Message #8 Posted by Renato on 30 Apr 2003, 10:57 p.m.,
in response to message #2 by David Smith

Decision to use wire or flex circuit seems to be a matter of taste. I used flex on a very nice 34c, with box, manuals, qrg, cartons, ac adapter, case. I felt this one deserved it. If conditions were not like that, maybe I would take other route. Also, I am fascinated by the idea of recreating replacement parts - kind if makes me feel much more optimistic about the future life of these interesting artifacts. Challenge was also a factor in this case. Why do it ? Because it is there...

Re: HP31E, charging circuit and flex-cable versus discrete wires
Message #9 Posted by Howard on 30 Apr 2003, 5:45 p.m.,
in response to message #1 by Vieira, Luiz C. (Brazil)

I have to report that I have got my 34C fully functional (See other thread). It had been very badly corroded and the power flex strip was totally disintergrated. I traced it all out and made a new one. I just used three wires. It worked well. Thanks Luiz for giving me the confidence to give it a go. This is the HP34C that was for sale on ebay as being corrodd and non operational. I paid US$71 for it with a heap of manuals. Now with persistance I have a perfect 34C for what I believe is a great price. But more than that I have gotten great satisfaction in successfully repairing this 23yo machine.

Good news, indeed!
Message #10 Posted by Vieira, Luiz C. (Brazil) on 1 May 2003, 6:08 a.m.,
in response to message #9 by Howard

You're welcome.

Thank you for the feedback, Howard. That's what we're here for.

Luiz C. Vieira - Brazil

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