Calculator Repair

[TObject]

I am trying to repair a calculator embedded in the Radio/Clock/Calculator/Mousepad combo.
It works just fine, except the keys "1", "4", and "8". When you press any of the keys, nothing
happens.

I have a suspicion; it's a bad contact somewhere or maybe a short. When I went over
the calculator board with my tester looking for bad contacts for the first time, I cleaned up
a spot of solder splatter on the keys PCB board. I thought that what was causing the issue,
as the keys started working after that.

Shortly the calculator was returned to me again, with the same symptoms. I am having
problem locating the bad spot. Any ideas?

I could use help trying to find the schematics for this calculator. The chip is not marked.
The main PCB board with the chip is marked "94HB-1 0307 01-008178-00 TY/210602"

Here are a couple of pictures to help identifying the calculator:
http://www.jeepbbs.net/tobject/pictures/problems/calc/PCB.jpg
http://www.jeepbbs.net/tobject/pictures/problems/calc/Face.jpg

Thank you!

 

[Jerry G.]

I doubt very much that the manufacture will have any available replacement
parts, and or schematics published that they will sell. Considering the low
cost of these units, they would never have anyone spend the time to service
these, therefore they would not invest in having service support for them.
People don't realize the cost involved just to have stocking on the service
manuals and replacement parts, distribution, and keeping track of the sales
of them. It is even hard to get any service information on the medium to low
end electronics consumer products these days, never mind a calculator.

A common cause of keys failing is that the conduction coating on the bottoms
of the rubber key-buttons wears out, and thus there is not enough conduction
across the contacts. There are some remote control repair kits for these
types of buttons, where you can repaint this conduction coating back on to
the key-buttons. The kit to service one set of buttons probably costs more
than several of these calculators.

If you measure the contact resistance of the rubber key-button bottoms, you
will see that the ohms reading must be very high. You can compare it to the
other button readings that work, in order to have a reference to what it
should be. If you put a short across the contacts, and if the rest of the
calculator is working well, the contacts should be working.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"TObject" <NewsgroupReader@pronografics.com> wrote in message
news:EcOMb.59344$C87.3234@twister.socal.rr.com...
I am trying to repair a calculator embedded in the
Radio/Clock/Calculator/Mousepad combo.
It works just fine, except the keys "1", "4", and "8". When you press any of
the keys, nothing
happens.

I have a suspicion; it's a bad contact somewhere or maybe a short. When I
went over
the calculator board with my tester looking for bad contacts for the first
time, I cleaned up
a spot of solder splatter on the keys PCB board. I thought that what was
causing the issue,
as the keys started working after that.

Shortly the calculator was returned to me again, with the same symptoms. I
am having
problem locating the bad spot. Any ideas?

I could use help trying to find the schematics for this calculator. The chip
is not marked.
The main PCB board with the chip is marked "94HB-1 0307 01-008178-00
TY/210602"

Here are a couple of pictures to help identifying the calculator:
http://www.jeepbbs.net/tobject/pictures/problems/calc/PCB.jpg
http://www.jeepbbs.net/tobject/pictures/problems/calc/Face.jpg

Thank you!

 

[Sam Goldwasser]

It may also be that the 1, 4, and 8 keys share a common strobe line to the
micro. So tracing back to the black blob may reveal if there is an open
or short on that line.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.



"Jerry G." <jerryg50@hotmail.com> writes:

I doubt very much that the manufacture will have any available replacement
parts, and or schematics published that they will sell. Considering the low
cost of these units, they would never have anyone spend the time to service
these, therefore they would not invest in having service support for them.
People don't realize the cost involved just to have stocking on the service
manuals and replacement parts, distribution, and keeping track of the sales
of them. It is even hard to get any service information on the medium to low
end electronics consumer products these days, never mind a calculator.

A common cause of keys failing is that the conduction coating on the bottoms
of the rubber key-buttons wears out, and thus there is not enough conduction
across the contacts. There are some remote control repair kits for these
types of buttons, where you can repaint this conduction coating back on to
the key-buttons. The kit to service one set of buttons probably costs more
than several of these calculators.

If you measure the contact resistance of the rubber key-button bottoms, you
will see that the ohms reading must be very high. You can compare it to the
other button readings that work, in order to have a reference to what it
should be. If you put a short across the contacts, and if the rest of the
calculator is working well, the contacts should be working.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"TObject" <NewsgroupReader@pronografics.com> wrote in message
news:EcOMb.59344$C87.3234@twister.socal.rr.com...
I am trying to repair a calculator embedded in the
Radio/Clock/Calculator/Mousepad combo.
It works just fine, except the keys "1", "4", and "8". When you press any of
the keys, nothing
happens.

I have a suspicion; it's a bad contact somewhere or maybe a short. When I
went over
the calculator board with my tester looking for bad contacts for the first
time, I cleaned up
a spot of solder splatter on the keys PCB board. I thought that what was
causing the issue,
as the keys started working after that.

Shortly the calculator was returned to me again, with the same symptoms. I
am having
problem locating the bad spot. Any ideas?

I could use help trying to find the schematics for this calculator. The chip
is not marked.
The main PCB board with the chip is marked "94HB-1 0307 01-008178-00
TY/210602"

Here are a couple of pictures to help identifying the calculator:
http://www.jeepbbs.net/tobject/pictures/problems/calc/PCB.jpg
http://www.jeepbbs.net/tobject/pictures/problems/calc/Face.jpg

Thank you!

 

[TObject]

To everyone who answered, thanks for your help!

I fixed the calculator, but as you have guessed it did not worth the time spent.
The main problem with these cheap calculators is the quality of circuit boards.
The buttons board on this particular calculator is double layered - one layer
per side. Undoubdly in pursuit to make these things as cheap as possible,
the whole circuitry on the side of the buttons made out of the button contact
material.

The button contact material introduces two things. First, it's conductivity is much
worse (in comparison to copper). That makes it harder to locate the problem.
You don't simply have contact or not have contact. The longer or thinner the run
of that button contact material, the more resistance you get.

Second bad thing - it is hard to make contact with that material. You can't just
solder to it.

I started by reverse engineering the schematics from the offending buttons.
Then I checked resistance in the lines that go to the main chip. When I found
the line where resistance was measured in Mega Ohms (where other lines were
usually in single kiloOhms), I made a bridge over the offending part straight to
the board with the main chip. The button "8" started working, but "1", and
"4" were still not operational.

Then I found that one of the interlayer connections on the buttons board was bad.
It was a connection between your normal copper PCB layer, and that cost
saving button contact material layer. I repaired the connection, by taking a piece
of stranded speaker wire, unstranding it, taking one strand, tieing a knot in it,
putting it through the board so the knot grabs the button contact material, and
then soldering it on the other "regular" side.

This project was not fun. It is definitely not worth time to fix the cheaply built
electronics.