restoring HP29C - help needed


Dear All,

I have just received an HP29C from Australia.

I'll post few pictures of the inside box, since it's quite interesting to see the differences in the interior packaging because of the different power supply shape.

However, I'm now working on the board itself.

The calculator was somehow working, I have charged one of my battery pack for Woodstock and it was properly turning on.

Few keys however, where not responding, since it had been sold for spare or repairs, I was not expecting anything better.

So far, I have used Alchol, and DeoxIT D5. Washed and rinsed, but when it dries up, this is what I have :

What can I try ?

I would like to avoid re-engineering the board,
but I want (I have ...) to get rid of the oxidation.

oddity I don't open 25C very often, so it may just be normal, but isn't odd the transistor leg (the right one, looking at the picture) ?

Is desoldered and is inserted in a pin like a point of measurement.
Thanks in advance for any advice, take care all of you, Alberto


Dear Alberto,

maybe not the reply you asked for, but if the main pcb of the 25C is identical to the one of your 29C, I'd desolder the ICs and implant them into a acid-free 25C board.

The 25C is much easier to get than the 29C, so it may be worth a try.

Interesting: The battery pole buttons look very good; Much better than usual in Woodstocks, and much better than the pcb;-)




Sorry guys,

I have changed the subject of topic, this is actually an HP25C and not an HP29C



The cleaning I did last night has worked out, now all the keys work properly, and are responsive.

I have waxed the faceplate which shows some wear (upper row lettering) and is missing the "Continuous Memory" label.

Power Supply has been opened, cleaned and polished, by the way differently from other power supply for the Woodstock, this one is only a transformer ...

Now I have three major things to work on :

Corrosion : still on the PCB, I have to find a way to get rid of it... would you suggest the vinegar solution ?

Feets : two out of four were not present, and the other two had sort of melted ... has anyone ever re-built them ?

Front Label: is partially detached, would you detached at completely and the re-glue it, or just add some glue for the detached part ?

Thanks in advance for any suggestion, Alberto


I would remove the label and re-attach using original type 3M 467MP adhesive. Several listings on eBay right now for it.

I like the vinegar process, but I still don't know how much of the board should be under the solution. I know it is not good to submerge the LEDs.


The LED stick pops right off the connector pins. no bother at all. Never get it near vinegar, the wire bonds will not survive. Only the bottom half of the board needs the treatment. FWIW, pulling the IC's off the board first (before the vinegar immersion) will yield far better results.


Thank you Randy,

I'll remove the display, but for as many times I did the vinegar process, still I usually don't get results as good as those I see on the forum.

Clearly I'm doing something wrong, probably the percentage of the ingredients ?

Can I ask what is the exact recipe ?

50% vinegar and 50% water ? Soda ?

Thanks in advance, Alberto


Thank you aj04062,

I'll try to remove it, heating will help as with most of the labels ?
I did a quick search, I'm not related with this seller, but do you mean this type of adhesive ? item 170935523347

Thanks again for help, and if anyone has an idea on how to make new feet ...


Yes that is the stuff I use. It works very nicely. Make sure you have brand new razor blades when trimming it.

I heat the old label with a lamp or hair dryer. Be careful with the lamp as it is much hotter than the dryer and can melt the plastic. Hold the unit near the lamp for a few seconds and monitor the heat by touch. You don't need to get it that hot.

I then try to "rub off" as much of the old adhesive. Sometimes it will l come completely off leaving a nice aluminum surface. If it doesn't alcohol will dissolve the adhesive.


Edited: 24 Nov 2012, 10:32 a.m.


Here we are,

I have used this vinegar solution : 50% vinegar, 50% distilled water and some salt.

I left the pcb in a glass, in vertical position submerged for two minutes, then rinsed with distilled water.

I did the treatment twice, but there are two things that I don't understand so i'm asking for your help.

The leftover from battery leakage has been almost completely removed, but still some micro spots are there as can been seen in this macro :

Should I do it again one more time ?

I have noticed that the treated parts gets kind of matt, if you look at the vertical pin or the soldered chips these are not shiny as other metallic components.

Is this because I have used too much vinegar ?

Or may be is because I have left the PCB too much time in the solution ?

Following Adam instruction, I have removed the label, cleaned all the old glue and using a similar adhesive re-positioned, now the label is back in place like new, thanks Adam !

And this is, for now the result :

Now, I'd like to spend more efforts on the faceplate, till now I have used a gentle soap and then a wax that contains carnauba, I had used this in the past with success on an HP45 faceplate.

I know however that better results can be obtained using the Tamiya Flat Clear spray, which I have used once also on a Classic.
Shoud I want to use it on this Woodstock, how can I protect the keys ???

Thanks for help and all the tips.

One last thing, since I was curious about the fact the most of the lettering was in good condition, but the "Continous Memory" writing was gone, I dropped a line to the seller, and this is waht he wrote me this morning :

You are right I remember the italic or script continuous memory, I thik I removed it as it looked added on. The manuals were an instruction book and book of programs, They were lost in a house fire. Cheers John

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