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Found these inside a non-working 71B
03-11-2018, 03:07 PM
Post: #16
RE: Found these inside a non-working 71B
(03-11-2018 01:45 PM)MikeSD Wrote:  I'm going to strip it all out and put new wires and reflow solded on the left chips, if my 72 year old eyes can still see that well Wink I think I can. One has movement (the chip, not my eye). Want to get the 71B working before reinstalling everything. If it doesn't work, I have another candidate.


1) I have a couple versions of the museum DVDs if I can find them but I don't recall a schematic. What doc was that in?

2) Looking at my photo above there are 6 mem chips. Is this how they are numbered?
left (Bottom) U6, left (Top) U7, middle (Top) U8, middle (B) U?, right (T) U9, right (B)U?. The right chip in my photo is different from left, I assumed it was the ROM.

3) Any idea what the buzzer mod is for?

Thanks again for the help.

The manual with the schematics is "HP-71B Hardware Internal Design Specification (more complete, 164 pages)" The schematics are in chapter 8 figures 8-3 and 8-5. Your board layout looks a little different than mine, so I am guessing the 71B you have is the later version that does not have the metal plate on the back. It is very likely that the schematics in that manual are drawn for the first version so there may be some minor differences between them and your board. On closer examination it would appear that the person that made the mods relocated a couple components to make space for the Ram modules.

The ground wire on the RAM is connected to the pad where the emitter of the transistor that drives the buzzer was. The transistor and inductor are now tucked in behind the battery compartment the red wire from from the emitter of the power transistor would be 5V. the emitter of the TO-92 transistor appears to be connected to the anode of diode CR7 which is ok that is ground. The base should be connected to the cathode of that diode but I cannot tell from the picture. So the buzzer mod is to relocate a couple components.

U6 is left bottom U7 is left top U8 middle top U9 is bottom right and top right would be a module that was added, probably ROM. looking closer the bottom right does not look like one of the original 4K module where U9 would normally be, so perhaps they sacrificed one of the original 4K modules to put in another ROM. You will note that the left and middle modules all have the same circuit board but the bottom middle has different solder blob jumpers I am guessing because it is ROM.

There is a better spot where they could have located the two RAM modules it requires slightly longer wires but get them out of the way, I will post a picture later. I am not sure where they might have placed the module you labelled as FORTH.
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Messages In This Thread
RE: Found these inside a non-working 71B - Paul Berger (Canada) - 03-11-2018 03:07 PM
What's U10? - Dave Frederickson - 03-15-2018, 03:43 PM

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