Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old August 10th 18, 03:18 AM posted to sci.electronics.repair
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Default Vintage Hp Hewlett-Packard 25 Calculator with case and charger

On Thursday, August 9, 2018 at 4:09:37 PM UTC-4, wrote:
On Thursday, August 9, 2018 at 4:06:30 PM UTC-4, John-Del wrote:
On Thursday, August 9, 2018 at 3:34:57 PM UTC-4, John-Del wrote:
I'm sure the foam pad *looses a bit of rebound over the years.


*Loses...


Oh, I dunno - there is a certain elegance to "loosing" a rebound.

Peter Wieck
Melrose Park, PA



LOL. It was *you* Peter that I had in mind when I made that correction!!! I figured if anyone would catch that goof and make me pay for it it would be you.


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Old August 10th 18, 05:23 AM posted to sci.electronics.repair
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Default Vintage Hp Hewlett-Packard 25 Calculator with case and charger

On Thu, 9 Aug 2018 12:34:54 -0700 (PDT), John-Del
wrote:

I have an HP34C that I bought new back around 1980 or so to replace
my stolen TI SR-51A. It had always been a little flakey but a quick
thump brought it back around. A few years later I decided to have
a look inside when it required more physical persuasion to behave.
I took a picture of the front keys in case they decided to jump out
and get mixed up (they did), but I was surprised to find the half
dozen ICs *not* soldered down to the flexible circuit board.


Yep. It relies on the pressure provided by a foam pad to make the
connection. Bad bad bad idea:
http://www.hpmuseum.org/hp34.htm

They maintain connection by the foam cushion beneath the flex
circuit that maintains pressure between the flex circuit and
the ICs. I removed each IC, cleaned all the pins and the flex
circuit lands, put a tiny bit of dielectric grease on each IC
pin, and reassembled. It behaved itself for quite some time.
Right now it's been in storage for many years but every once
in a while I wonder if the correct solution would be to tack
solder them down. I'm sure the foam pad looses a bit of rebound
over the years.

What do you do when you run across flakey IC contacts in an HP?


If the foam pad was in good shape (springy), I would add a 2nd foam
pad to give it more pressure. This added pad is rather thin. I
should repalce both, but can't find a suitable sheet of foam. I now
have access to a laser cutter, so I might be able to cut some foam
that will work.

I think ordinary soldering is a bad idea and have never tried it.
However, if you do decide to try it, I suggest you use low temperature
180C bismuth solder paste, liquid flux, and a temperature controlled
toaster oven.
https://www.ebay.com/sch/i.html?_nkw=low+temperature+bismuth+solder+paste
Something like this:
https://www.youtube.com/watch?v=cClPqIJwhLs

On the toaster oven, this is how I do BGA reflow on HP JetDirect
cards, which use the same bismuth solder paste:
http://www.learnbydestroying.com/jeffl/pics/repair/BGA%20reflow/index.html



--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #13   Report Post  
Old August 10th 18, 12:55 PM posted to sci.electronics.repair
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Default Vintage Hp Hewlett-Packard 25 Calculator with case and charger

On Friday, August 10, 2018 at 12:23:46 AM UTC-4, Jeff Liebermann wrote:
On Thu, 9 Aug 2018 12:34:54 -0700 (PDT), John-Del
wrote:

I have an HP34C that I bought new back around 1980 or so to replace
my stolen TI SR-51A. It had always been a little flakey but a quick
thump brought it back around. A few years later I decided to have
a look inside when it required more physical persuasion to behave.
I took a picture of the front keys in case they decided to jump out
and get mixed up (they did), but I was surprised to find the half
dozen ICs *not* soldered down to the flexible circuit board.


Yep. It relies on the pressure provided by a foam pad to make the
connection. Bad bad bad idea:
http://www.hpmuseum.org/hp34.htm

They maintain connection by the foam cushion beneath the flex
circuit that maintains pressure between the flex circuit and
the ICs. I removed each IC, cleaned all the pins and the flex
circuit lands, put a tiny bit of dielectric grease on each IC
pin, and reassembled. It behaved itself for quite some time.
Right now it's been in storage for many years but every once
in a while I wonder if the correct solution would be to tack
solder them down. I'm sure the foam pad looses a bit of rebound
over the years.

What do you do when you run across flakey IC contacts in an HP?


If the foam pad was in good shape (springy), I would add a 2nd foam
pad to give it more pressure. This added pad is rather thin. I
should repalce both, but can't find a suitable sheet of foam. I now
have access to a laser cutter, so I might be able to cut some foam
that will work.

I think ordinary soldering is a bad idea and have never tried it.
However, if you do decide to try it, I suggest you use low temperature
180C bismuth solder paste, liquid flux, and a temperature controlled
toaster oven.
https://www.ebay.com/sch/i.html?_nkw=low+temperature+bismuth+solder+paste
Something like this:
https://www.youtube.com/watch?v=cClPqIJwhLs

On the toaster oven, this is how I do BGA reflow on HP JetDirect
cards, which use the same bismuth solder paste:
http://www.learnbydestroying.com/jeffl/pics/repair/BGA%20reflow/index.html



--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558



I'm going to dig it out of storage, charge the battery, and see how it behaves. If it behaves, I'll leave it as is. I didn't attempt a resolder back then because I didn't want to modify it and wasn't sure of the flex circuit's response to heat.

In later years, I've seen flex circuits (I think they're Kapton) that take soldering heat just fine.

I do keep solder paste in stock because I do a lot of smd IC replacement, and mostly use a hot-air station to keep the heat localized.

Thanks for the info.

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Old August 10th 18, 06:53 PM posted to sci.electronics.repair
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Default Vintage Hp Hewlett-Packard 25 Calculator with case and charger

On Friday, August 10, 2018 at 7:55:39 AM UTC-4, John-Del wrote:
On Friday, August 10, 2018 at 12:23:46 AM UTC-4, Jeff Liebermann wrote:
On Thu, 9 Aug 2018 12:34:54 -0700 (PDT), John-Del
wrote:

I have an HP34C that I bought new back around 1980 or so to replace
my stolen TI SR-51A. It had always been a little flakey but a quick
thump brought it back around. A few years later I decided to have
a look inside when it required more physical persuasion to behave.
I took a picture of the front keys in case they decided to jump out
and get mixed up (they did), but I was surprised to find the half
dozen ICs *not* soldered down to the flexible circuit board.


Yep. It relies on the pressure provided by a foam pad to make the
connection. Bad bad bad idea:
http://www.hpmuseum.org/hp34.htm

They maintain connection by the foam cushion beneath the flex
circuit that maintains pressure between the flex circuit and
the ICs. I removed each IC, cleaned all the pins and the flex
circuit lands, put a tiny bit of dielectric grease on each IC
pin, and reassembled. It behaved itself for quite some time.
Right now it's been in storage for many years but every once
in a while I wonder if the correct solution would be to tack
solder them down. I'm sure the foam pad looses a bit of rebound
over the years.

What do you do when you run across flakey IC contacts in an HP?


If the foam pad was in good shape (springy), I would add a 2nd foam
pad to give it more pressure. This added pad is rather thin. I
should repalce both, but can't find a suitable sheet of foam. I now
have access to a laser cutter, so I might be able to cut some foam
that will work.

I think ordinary soldering is a bad idea and have never tried it.
However, if you do decide to try it, I suggest you use low temperature
180C bismuth solder paste, liquid flux, and a temperature controlled
toaster oven.
https://www.ebay.com/sch/i.html?_nkw=low+temperature+bismuth+solder+paste
Something like this:
https://www.youtube.com/watch?v=cClPqIJwhLs

On the toaster oven, this is how I do BGA reflow on HP JetDirect
cards, which use the same bismuth solder paste:
http://www.learnbydestroying.com/jeffl/pics/repair/BGA%20reflow/index.html



--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558



I'm going to dig it out of storage, charge the battery, and see how it behaves. If it behaves, I'll leave it as is. I didn't attempt a resolder back then because I didn't want to modify it and wasn't sure of the flex circuit's response to heat.

In later years, I've seen flex circuits (I think they're Kapton) that take soldering heat just fine.

I do keep solder paste in stock because I do a lot of smd IC replacement, and mostly use a hot-air station to keep the heat localized.

Thanks for the info.




I plugged in the old boy and it works perfectly. Even the battery is holding a charge (I changed the cells about 25 years ago when I serviced the flex circuit). Tapping it has no adverse effect on any function, and the keys work perfectly with no lag and no bounce.

So the cleaning of the board and chips (and whatever I used for preservative is still holding.

There's nothing like the glow of red LEDs either.





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Old August 11th 18, 05:46 PM posted to sci.electronics.repair
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Default Vintage Hp Hewlett-Packard 25 Calculator with case and charger

On Tuesday, August 7, 2018 at 9:24:36 PM UTC-4, Peter Jason wrote:
Unit with charger & owners handbook. Working.
Best offer.
Peter


Hmm..My HP 28S still works fine since the late 80's. Just don't leave batteries in it to degrade.


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Old August 11th 18, 06:53 PM posted to sci.electronics.repair
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Default Vintage Hp Hewlett-Packard 25 Calculator with case and charger

Ya'll got me thinking.
I bought the HP 35 while I was working for TRW back around 1974.
Almost two weeks worth of pay and more than I paid for rent for
a month.
I also remember at the time, "Reverse Polish Notation? This is a
joke isn't it?"
http://www.hpmuseum.org/two35s.jpg
I bought it from the book store across the street from El Camino
College.
I have NO idea what ever happened to it.
I did by the HP10C when it came out.
http://www.hpmuseum.org/10c.jpg
I still have it, and a 12C I picked up last year in a thrift store
for $5.
http://www.hpmuseum.org/12c.jpg


--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com
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Old August 12th 18, 12:49 AM posted to sci.electronics.repair
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Default Vintage Hp Hewlett-Packard 25 Calculator with case and charger

On Saturday, August 11, 2018 at 1:54:08 PM UTC-4, Fox's Mercantile wrote:
Ya'll got me thinking.
I bought the HP 35 while I was working for TRW back around 1974.
Almost two weeks worth of pay and more than I paid for rent for
a month.
I also remember at the time, "Reverse Polish Notation? This is a
joke isn't it?"


Back when I was entering college, a scientific calculator was a requirement, and there were several on the recommended list. I narrowed them down to the Texas Instruments SR-51A and the HP (don't remember the model).

I bought the SR-51A because I never owned an RPN calculator and figured one less thing for me to learn would be a good thing. The TI was great until a redistributioner relieved me of it. I then bought the HP 34C and became used to the RPN method within minutes.

I still would love to have another SR-51A though. Aesthetically, he SR-51A was pretty where the 34C looked so damned professional. I guess it's off to ebay...



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