1. I have two working HP-27's and a non-working HP-22 and a non-working HP-25. I have opened the HP-22 and HP-25 and have found that the top has popped off the module which is supposed to be damaged if the charger is applied without the battery pack installed. Does it make sense to remove the module from one of the HP-27's and install it in one of the other machines or is the risk too great?
2. I am in my summer home in western North Carolina. In past years there has always been a number of slide rules for sale in antique stores. One store in particular typically had eight or ten slide rules priced in the twenty dollar range -- typically out of my price range but once in a while I would buy one I REALLY wanted; e.g., when I bought one of the slide rules with an Addiator on the back. This year the store which typically had several slide rules doesn't have a single one. The only slide rule I have seen is a Post Versalog 1460 with a leather case but the plastic somewhat yellowed offered for 52 dollars. Does anyone know if the slide rule market has somehow heated up for some reason? If my memory is correct there was a jump in prices back in the 1990's when the New York Times published an article declaring that slide rules were collectibles.
I'm no expert but I'd expect that the market for slide rules has cooled down, not heated up. When the economy is bad and people are reluctant to spend money on non-essentials, it makes sense to hold on to your collectibles and wait for the market to recover, when you'll probably be able to get better prices as a seller.
Regarding swapping parts from an HP-27 to an HP-22: I think I read somewhere that the electronics in the HP-27 are not compatible with those in the other Woodstock models because they use NMOS technology, as opposed to PMOS in the earlier models and CMOS in the 25C and 29C. I googled a bit and found this link to an old HP Forum thread that may help shed some light on the issue.
Apart from the PMOS vs. NMOS question, I don't think you should gut a working HP-27 to repair an HP-22 or 25; the HP-27 is much rarer, you should treasure it, or sell it and get an HP-25 to gut, there are plenty of those -- although I personally have great nostalgia for the HP-25 and would not do this myself. :-)
- Thomas
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1. I have two working HP-27's and a non-working HP-22 and a non-working HP-25. I have opened the HP-22 and HP-25 and have found that the top has popped off the module which is supposed to be damaged if the charger is applied without the battery pack installed. Does it make sense to remove the module from one of the HP-27's and install it in one of the other machines or is the risk too great?
I don't know what you mean by 'module' here, but you need to be aware that the HP27 is very different internally to all other Woodstocks. All other Woodstocks (and for that matter classics, topcats, the HP19C, hp67, etc) use PMOS ics. The HP27 uses NMOS. For this reason the supply voltages are totally different. None of the ICs can be swapped between a 27 and any other machine AFAIK.
In general it's only the -C (continuous memory) machines that are damaged by being connected to the charger without a battery pack fitted. I wouldn't risk it, though.