HP-25C Repairing



#2

Hi,
Just picked up a non-working 25C. When unit is switched on and off, different LED's light up with a 0. Occasionally 2 O's will appear.

Fearing fried memory & ACT, I opened and cleaned a fair bit of corrosion. I reassembled to test and no change.

While apart, I noticed the large resistor looks to have severely overheated in the past. Does all of this evidence point towards this thing getting fried from plugging in without a battery?

Edited: 28 Oct 2011, 10:09 p.m.


#3

No. The resistor limits the charging current to the NiCd pack, and does tend to heat up quite a bit, but use of the charger without the batteries won't damage the resistor, only the chips.


#4

Still sound like fried chips? The AM radio "test" indicates some acknowledgement to the pushing of some keys, but no key combination makes the display change.


#5

AFAIK this looks like the usual fried ACT. Sorry.

#6

Assuming the power supply is okay, remove the rom (1818-0154) and the ram (different numbers, depends on version). Power it up. If you get a 0.00 display, the ACT is okay, basic math should work. If not, check for oscillations at the tank circuit, pins 13/14. If it is dead there, the ACT is dead. If it good, have a look at the display driver components. Not very likely they are at fault but it happens, especially with the 27.

If the ACT is okay, re-install the rom and check transcendental functions. Add the ram and check program storage.

While the ACT is a common failure, so is the rom in the 25. With ram or rom blown, it usually kills the clock and makes it look like a bad ACT.


#7

Awesome,

Thanks for the nice step by step.

Could I follow similar steps on a malfunctioning 67 (minus RAM of course)?


#8

The 67 isn't so cooperative. I isolate to the logic board, as the display/keyboard can cause non-intuitive problems.

Once I know it is the logic board, I strip all roms and test in a good board equipped with sockets. I've found it too painful to approach any other way as rom failures are more common on the 67/97 than ACT's since they don't get over-voltaged the way Woodstocks do.

#9

While we are on this topic of malfunctioning woodstocks: may I kindly ask for your jugdement regarding the 29C I acquired a few months ago? It worked fine for several days with a set of two NiMH batteries that were used in the original battery holder (and charged externally of course - I do not have the AC adapter). Then suddenly, while running a program, the device crashed and since then it will display nothing but twelve zeroes: 000000000000 is all it says.

Do you think it can be brought back to life? It's such a cutie... *sniff*...

Dieter


#10

Randy probably is the definitive word on this, but I saw that as a symptom of a bad ACT chip when researching my 25C issues.

#11

I would first test with a 3 volt bench supply using clip connections to the battery terminals - as consumer AA cells (non-flat top design) do not play well with the Woodstock contact design. Also, relying on the spring to make the connection between cells can be problematic since the plating on the spring ends has usually been destroyed by cell out-gassing. FWIW, low supply voltage will cause strange behaviors, many times similar to an ACT failure.

If still no joy with the above, the troubleshooting is the same but you have 5 IC's to remove, not 2. That leaves two IC's on the board, the ACT and the power on reset IC (1820-1983).


#12

Randy, thank you for your reply. Sorry if I'm asking, but I assume you are Randy from fixthatcalc, right? While I think I will manage to connect the 29C to a 3V power supply there are five left thumbs on each of my hands when it comes to replacing ICs and other delicate stuff. ;-) If nothing else helps, do you think there is a chance to get my little marvel back to life at fixthatcalc even if 30+ years old parts are required?

Dieter


#13

Yes, that's me... and FWIW, I gave up trying to repair Woodstocks years ago.

Over the years, I have purchased quite a few 20 series units, some working and most not working. What I have found was the number of units that yielded usable parts was so low, I could not possibly charge what it really cost to do repairs.

IMO, the economics of repair don't work when it comes to Woodstocks and Spices.


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