HP70 project - cosmetc and electronic issues



#2

Dear All,

I'm back on the calcs ...

I have been collecting watches for a while, mostly LED and LCD and kept an eye on the forum,
but I had stopped working on calcs.

If anyone is interested in watches, just email me and will gladly share some
of the things thst I have learned there.

By the way, working on small items like the LED and LCD modules makes our beloved calcs really HUGE stuff.

I have recently purchased this HP70, mostly because I needed the case for one onmy two units,
which has a thin crack in the body.
However, as always, when the unit arrived I couldn't believe it was gone for good...

I cleaned most of the battery leakage and washed the keys and body and cleaned the battery contacts.

I have put a battery pack and plugged the AC power supply, but nothing happened.

Since the unit was without the back cover, I could giggle with the logic board and by

moving it, the unit came back to life.

However, the behaviuor is erratic, it may lights up four times in a row, or may not light up for a while.

One thing I have notices is that when is on, the two transistors in the upper left corner, the coil and

the capacitor get really hot.


There are two issues where I would liketo receive your help and your ides :

on the cosmetic issues : the are thin scratches on the keyboard frame, what could I do to erase them ?

I was thinking about using some sandpapervery thin (like 2000) but I'm afraid I will loose the typical

aspect of an HP keyboard.

On the electronic side : would it be a good idea to re-solder all the connections ?

could this behaviour due to a bad connection ?

I was also thinking about replacing the power supply components, does anyone know

a good replacement for the transistors, the capacitor and anything else that could be needed ??

I'm attaching some pics, any comment and any idea will be, as always, greatly appreciated.














One last word about this forum. Entering the watch collector's world has been a nice experience.

Looking for resources I have come across different forums. Let me tell you that no other forum

by any mean is as friendly and as rich of nice people like this one, so let's keep up the good work !!!


#3

I have disassembled and cleaned some dozens of these "Classic" HP calculators, and I don't know any refinishing technique that would eliminate the scratches without radically changing the appearance of the keyboard. Frankly given the HP-70's well deserved status as the ugliest single calculator ever made in the entire history of the world (I gave my much nicer example away as a door prize at HHC 2006. Nelson got it.), I wouldn't sweat it.

I'll leave any electronics comments for someone else. After all, I don't even know a full wave rectifier from a bridge rectifier!

#4

Hi Alberto,

I think you're lucky that the 70 has no lettering on the keyboard surround. I have had some success with this kind of issue by light sanding fine emery paper, then spray painting with Tamiya flat lacquer. The good thing is that I can sand and respray multiple times without issues. Just may sure you use to appropriate lacquer type, I use the "TS" lacquer. I have done this with a 65 which does have lettering on keyboard surround.

On the electronics, I would try re-soldering, but there is always the risk they it will be worse after wards. I's start with by re-soldering all the pins on the bottom edge connector. Also make sure all the holes for the pins in the logic board are free from corrosion.

Cheers,
John

#5

Tony or Randy would probably be better sources of advice on the power supply circuitry, but if the transistors are getting hot to the touch, something is definitely very wrong. I doubt that it's just a bad solder joint. If it were my calculator, I wouldn't power it up again until I had come up with a theory explaining why the components are getting hot, and had attempted to repair it.

#6

Quote:
On the electronic side : would it be a good idea to re-solder all the connections ? could this behaviour due to a bad connection ?
I would not recommend re-soldering. You would be heat stressing almost forty year old components for no good reason. A good visual inspection should identify any possible bad solder connections and re-solder only those necessary.

Quote:
I was also thinking about replacing the power supply components, does anyone know
a good replacement for the transistors, the capacitor and anything else that could be needed ??
If they work, why would you replace them? You achieve nothing, other than to say you replaced perfectly good components.

As for the transistor and transformer getting hot - is this when the unit is functioning or when it is on the fritz?

The intermittent behavior - Corrosion on the board or on the pins - is obviously the most common cause of trouble but if you find none:

Remove the logic board and carefully inspect the 25 gold pins on the keyboard side - insure that the smaller, lower side extends outward enough to engage and make good contact with plate-through hole of the logic board. It only takes one compressed pin to cause major grief. When the pins look good, with magnification, inspect the plate-through holes of the logic board.


FWIW, I have seen boards (especially on 65's) with missing/worn plating that caused intermittent contact. I recently had a 65 that so much wear inside the holes that I ended up rotating most of the connector pins 90 degrees so it would not go crazy when the low end was lifted up three inches off the desk and dropped. This is a good test for all HP's as it was part of final repair QC.


#7

Thank you very much for all the replies I've got so far.

On the cosmetic side, I'll definetely try some sandpaper (the finest I have is 2000 grade and I'll go with that first).

On the electronic side, I have already fixed the obviuos, like the switch contacts and the battery contacts

PSU components may get hot because the unit was half opened and surely this is the worst way to test a unit.

Visual inspection of the gold pins revealed no corrosion but definitely not shining over there ....

I was thinking about it and I don't want to use sandpaper on the pins, neither in the corresponding holes in the logic board.

What could be a good idea to make them shine again ? Isopropolic Alcohol would do the trick ? Or should I go for the water and vinegar solution ?
Again, thanks everyone for help, take care Alberto


#8

Quote:
Or should I go for the water and vinegar solution ?
Yes, that is best for corrosion/battery leakage. Caig DeoxIT is also highly recommended for clean up and protection.

Quote:
I was thinking about it and I don't want to use sandpaper on the pins, neither in the corresponding holes in the logic board.
Sandpaper? One word: *NEVER*

A clean "pink pearl" pencil eraser is most abrasive thing I'd every take to gold plated contacts. This is the best way to restore "shine" if there is any shine left to restore...


#9

Thanks Randy,

any specific model of Caig DeoxIT ?

I have browsed a little and looks like there are different versions,
some of which available in italy as well.
thx for help, Alberto


#10

D5 Aerosol is good for cleaning corrosion and over-all use on the classics.

#11

Quote:
Thanks Randy,

any specific model of Caig DeoxIT ?

I have browsed a little and looks like there are different versions,
some of which available in italy as well.
thx for help, Alberto

Hello Alberto and compliments for your HP70 I read that it's not so easy to find that kind of calculator.......... expecially for a human price!

I'm curious to know where have you found the Caig DeoxIT here in Italy... If I've not well understood, may you suggest another equivalent product, I'll appreciate it, thank-you

#12

Alberto, I am not totally familiar with the DC-DC converter on the 70, but assuming it's the same as the 65 which I am familiar with, then the heating up of those components sounds like the calc is drawing too much current. which would indicate a short or near short on one of the three power lines, Vcc, Vss or Vgg. Unfortunately in the articles forum, Tony Duell's "inside the HPxx" doesn't cover the HP70, only the other classics, so I can't tell you where to measure Vcc, Vss and Vgg.

The only other possibility for a hot transistor would be the 2nF capacitor (large capacitor next to coil) is on the way out. If that's the case, then the oscillator might be oscillating too fast. However, that would really only apply if one transistor is hot, not both.

One transistor forms the oscillator, the other forms a kind of regulator via Vss line (I think !). On a HP65 the oscillator oscillates at around 670 Khz, do you may want to check that if you have access to an oscilloscope - try pin 1 of the "coil". The coil itself is extremely unlikely to be the problem.

Personally, I'd replace the 2nF capacitor, and both transistors, and the diode in the feedback (connected to the zener diode). All other diodes are for rectification of the three output voltages.

Cheers,
John


#13

Alberto,
you may want to start by checking the two transistors and the diodes in the DC-DC converter. With the board removed from the calculator (and no power), a simple ohmmeter check will let you know if these parts are OK. Simply check for "good" junctions in the transistors and the diodes (the ones with the black-black-orange-yellow colour bands).
I hope this helps!
Joel


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