HP 27


I have an HP 27 that I bought on Ebay. I can not get it to light up, even with a new battery pack. If the previous owner tried to run the calculator with no or bad batteries, what component(s) could he have burned out? Any ideas how to repair it? I am a former EE so I have tools and skills to attempt a repair if I know what to replace.


Leave it on for about 30 minutes with charged battery pack after you are sure it is firmly contacting both battery post terminals (often a problem itself with weak pack springs, worn posts and rebuilt battery packs). See if you get a display following the "warm-up period" If so see the archives this site for some hints from Katie Wass, when she's not sniping ;+} on ebay or working she is very talented at fixing calculators and has posted much helpful information.


Thanks Frank, I think, sort of!

Steve, Actually, the warm-up problem would always show something on the display, like a zero, so that's probably not your problem. If you've got nothing on the display, the first check that you should make is to see how much current is being drawn by the batteries. You should hook up an external 2.5 volt power supply with an ammeter in series. If the power supply is working, you should see around a 100 ma (blank display) to 200 ma (most segments on) current draw. Assuming that you're getting that, you should next check the output voltages from the inverting supply. If you meassure them relative to the negative battery terminal you should find three of them: -3.3, +6.5 and +12.6. The easiest place to find these is on the non-component side of the main circuit board on 16-pin IC pattern that doesn't have a chip installed in it (labeled U5). Pin 9 = 6.5v,
Pin 12 = -3.3v, pin 15 = +12.6v.

If you're getting all that, the most likely problem is a bad chip and it's probably U1 (HP 5061 0430). They seem to have the highest failure rate of the chips used in this calculator. And if it has failed, the display will likely be blank. The really bad news is that this is the only HP calculator that uses this chip (that I know of) so you'll need to find another 27 to use as an organ donor.

I hope that helps.



The one I had and another collectors a couple of years ago were both blank and then, presto, in 5-10 minutes after being left on, display was on.


I've gathered up some of the discussions about this common Woodstock problem and how to repair it and made it into an article that I just posted. (Dave had suggested that I do this long ago.) I've tried to condense the discussion into a practical "how to" repair article and haven't covered everything that's been mentioned in these discussions. Please let me know what I can do to improve this write up as far as including important facts and other repair tips.



Thanks for the help, but I think I need to look for an HP 27 to remove chips from. If any one has one with poor cosmetics but works, I am now in the market for one.


I must say that you still *are* an EE indeed, since there is no thing as a "former" EE!



(Ola, Andrés; como estás? And Argentina? Nice reading your posts again. Best regards!)

I agreee 101% with your words. EE status forever. As long as your mind travels with you...

About the HP27... Hey, where are those fellows? Doctor Mike Meyers? Prevelakis? Glynn? Korensky? among many others? I've been running out of time somewhat, but I miss you here, guys. Get back...



You can also try hooking the calculator up to a charger for a couple of hours; this heats up the circuit board and seems to make it work.

I have a feeling it is a design fault in the main IC's (See previous postings in "Woodstock Design: Why so many problem 27's?" posted August 12, 2000).


If your power supply checks out, I have found the cause of a blank display or a 0000000000 display to almost always be the ACT chip. This is the 22 pin 0.4 inch wide chip near the bottom row of connector pins. Most of the Woodstock machines have their ACT chips numbered differently but... they are all the same internally!!! I have succesfully transplanted ACT chips amongst most of the Woodstock series machines, but have never had to work on a '27.

I do have a '27 that you occasionally have to turn on and off and then on to get it to turn on.


Can you replace the ACT chip of an HP-27 with the ACT chip of another Woodstock calculator (e.g. an HP-25)? The ACT chip of an HP-25 is black plastic, and the ACT chip of an HP-27 is ceramic with a gold top. I think this is because the HP-25 uses P-MOS technology, while the HP-27 uses N-MOS technology. Can you replace an N-MOS chip with its equivalent P-MOS chip without making any other changes and have it work?


I just read through Katies old posts about the HP27 and its bastard NMOS ACT heritage. With the different voltages that the '27 uses, I doubt that it can be replaced with one of the other Woodstock ACT chips.

I have gone as far as to place HP21 (and HP25) ACT chips into HP22, HP25, HP25C, HP29C, HP67, and HP97 units without any problems. I counted my dead ACT chip pile and it exceeds the number of digits on my various appendages (including the "carry" flag)

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