Post Reply 
Component on case bottom of 48S
10-31-2019, 05:52 PM
Post: #1
Component on case bottom of 48S
What is that large (~ 0.75 in dia) round component on the inside bottom half of the case of the HP48? A picture of the component can be seen in the photo bottom.jpg from here: https://www.hpcalc.org/details/4701?

I am curious because my 48S has/is suffering from a power-on problem which requires me to apply pressure on the case just above the 'B' key. Most of the comments I have seen assert that this issue is due to poor contact between the keyboard connector and the main PCB, so I disassembled my 48, cleaned the contacts, and reassembled and the problem was reduced in that I now need to apply less pressure but must still apply some pressure.

But, I did notice that there is a spring contact which reaches down from the PCB to this 'button' and that the pressure for turning the calc on must be applied in an area that happens to be right above the location of the spring contact between the PCB and this button. So, is the pressure being applied to aid in turning the calculator on helping to aid in contact between the keyboard and the PCB, or to aid in contact between the 'button' and the PCB?

Thanks

Ken
Find all posts by this user
Quote this message in a reply
10-31-2019, 07:04 PM
Post: #2
RE: Component on case bottom of 48S
That component is the piezoelectric 'buzzer'.

It is likely mounted to the case to provide increased acoustic output.

The spring completes the electrical connection to the buzzer.

The pressure phenomenon you mentioned is well understood and is due to the lack of contact between the keyboard assembly and the calculator 'main'. There is a diagram on my googlesheets page showing the connection of the 'ON' signal path that is faulty.

TomC

(10-31-2019 05:52 PM)KenW Wrote:  What is that large (~ 0.75 in dia) round component on the inside bottom half of the case of the HP48? A picture of the component can be seen in the photo bottom.jpg from here: https://www.hpcalc.org/details/4701?

I am curious because my 48S has/is suffering from a power-on problem which requires me to apply pressure on the case just above the 'B' key. Most of the comments I have seen assert that this issue is due to poor contact between the keyboard connector and the main PCB, so I disassembled my 48, cleaned the contacts, and reassembled and the problem was reduced in that I now need to apply less pressure but must still apply some pressure.

But, I did notice that there is a spring contact which reaches down from the PCB to this 'button' and that the pressure for turning the calc on must be applied in an area that happens to be right above the location of the spring contact between the PCB and this button. So, is the pressure being applied to aid in turning the calculator on helping to aid in contact between the keyboard and the PCB, or to aid in contact between the 'button' and the PCB?

Thanks

Ken
Find all posts by this user
Quote this message in a reply
10-31-2019, 07:33 PM
Post: #3
RE: Component on case bottom of 48S
Thanks

Ken
Find all posts by this user
Quote this message in a reply
10-31-2019, 08:28 PM
Post: #4
RE: Component on case bottom of 48S
Here's the link to my page:

https://sites.google.com/site/tchrapkiewicz2/hp48

Click on the'KB Conn.' link (the URL would not fit here).

Hopefully this is insightful.

TomC


(10-31-2019 07:04 PM)TomC Wrote:  That component is the piezoelectric 'buzzer'.

It is likely mounted to the case to provide increased acoustic output.

The spring completes the electrical connection to the buzzer.

The pressure phenomenon you mentioned is well understood and is due to the lack of contact between the keyboard assembly and the calculator 'main'. There is a diagram on my googlesheets page showing the connection of the 'ON' signal path that is faulty.

TomC

(10-31-2019 05:52 PM)KenW Wrote:  What is that large (~ 0.75 in dia) round component on the inside bottom half of the case of the HP48? A picture of the component can be seen in the photo bottom.jpg from here: https://www.hpcalc.org/details/4701?

I am curious because my 48S has/is suffering from a power-on problem which requires me to apply pressure on the case just above the 'B' key. Most of the comments I have seen assert that this issue is due to poor contact between the keyboard connector and the main PCB, so I disassembled my 48, cleaned the contacts, and reassembled and the problem was reduced in that I now need to apply less pressure but must still apply some pressure.

But, I did notice that there is a spring contact which reaches down from the PCB to this 'button' and that the pressure for turning the calc on must be applied in an area that happens to be right above the location of the spring contact between the PCB and this button. So, is the pressure being applied to aid in turning the calculator on helping to aid in contact between the keyboard and the PCB, or to aid in contact between the 'button' and the PCB?

Thanks

Ken
Find all posts by this user
Quote this message in a reply
10-31-2019, 08:37 PM
Post: #5
RE: Component on case bottom of 48S
Extra thanks. I will probably live with it as it is for a while since the disassembly and re-assembly was not so easy (It was manufactured in Singapore and these are reported to be more difficult to open) but I will certainly remember this pointer for future reference.

By the way, nice X-ray photos.

Ken


(10-31-2019 08:28 PM)TomC Wrote:  Here's the link to my page:

https://sites.google.com/site/tchrapkiewicz2/hp48

Click on the'KB Conn.' link (the URL would not fit here).

Hopefully this is insightful.

TomC
Find all posts by this user
Quote this message in a reply
11-01-2019, 12:47 AM
Post: #6
RE: Component on case bottom of 48S
(10-31-2019 05:52 PM)KenW Wrote:  I am curious because my 48S has/is suffering from a power-on problem which requires me to apply pressure on the case just above the 'B' key. Most of the comments I have seen assert that this issue is due to poor contact between the keyboard connector and the main PCB, so I disassembled my 48, cleaned the contacts, and reassembled and the problem was reduced in that I now need to apply less pressure but must still apply some pressure.

You need to replace the springy material beneath the keyboard connector, or at least pull it upwards between thumb and forefinger to stretch it out again. The material presses the keyboard contact against contacts on the pcb, but it loses its springiness with age and contact is lost. If/when you open the case again, give that a try for a permanent fix.

Who decides?
Find all posts by this user
Quote this message in a reply
11-01-2019, 04:16 PM
Post: #7
RE: Component on case bottom of 48S
The keyboard to main board interface is certainly one of the weakest points of the HP48 hardware. Relying on simple mechanical pressure between the keyboard overlay and the plated 'fingers' of the main PCB is a design that is destined to fail over time - given temperature, humidity and hence oxidation at the interface point. (I also wonder about some liquid (?) that could be applied to this interface that would inhibit further oxidation and promote conduction over time. (?))

I've considered some type of more robust connection here, but robust connections to the keyboard overlay (which is a thin piece of plastic with carbon(?) deposited traces) is not possible. The beauty of a properly soldered connection is that it is much more robust - not subject to oxidation over time. Perhaps replacement (ultimately) with some type of thin PCB material (with appropriate resistance added) that has solder points is the ultimate solution.

The HP48 is such a great combination of utility (and (mostly) mechanical structure) that I hope it will see a long life (perhaps a 'rebirth' ala the HP42).

TomC
Find all posts by this user
Quote this message in a reply
Post Reply 




User(s) browsing this thread: 1 Guest(s)