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41 CX repair



#12

My CX shiwed strange signs. It s not reacting to the keyboard or on/off. None of the resetting routines described in forum is working. So i decided to take it apart. In the bottom of the CPU i found a glass fuse? ( i think) the fuse? is not shown in any picture i´ve seen. In the fuse? is two metal tongues lying over each other.( in a car fuse there is one metal tongue who burns off when overloaded) Is it a fuse or what?


#13

It sounds like a reed relay.

I expect that the calculator has been sped up.

If you place a magnet near the device, you can confirm if it is a reed relay, because this will cause the contats to close.

You might find that a wire has broken internally, or that it helps to put a magnet near the reed relay when you turn the calculator on.


#14

hi Steve thanks. I need more help. What does a reed relay do? Can i make a bypass? The wire is broken inside the reed. The reed is also craced a bit. I dont know if it has been seed up. I bought it in a secon hand market for aprox a GB £. stefan


#15

If it is a reed relay, it will be a clear glass cylyndrical envelope with leads coming out of the (probably rounded) ends. The inside is clearly hollow. Inside are 2 flat wires that overlap for a short distance. These devices are usually about 3 to 5 mm in diameter ans about 25 mm in length (although there are probably a plethora of sizes).

A diode, on the other hand will be somewhat smaller (about 2 to 3 mm in diameter and less than 10 mm in length. It will have a stripe (red, white or black normally) around one end. The insides are less easily discerned.

Which one does it look like?


#16

´Hi Steve. It is a reed relay no doubt. I noticed that it was broken in one end. I have to repair it. Do you know if the overlaping wires should be closed or open when the calculator is off? stefan


#17

The reed contacts are normally open.

Let me give you the basics on what it's doing and why it's there.

People wanted to make their HP41's faster.

The HP41 has an internal oscillator that determined the speed at which the calculator runs. By changing the value of one component (reducing the value of a capacitor) the oscillator will run at a higher frequency, and the calculator will go faster.

However, some peripherals (notably the card reader) may misbehave if the clock speed gets too high. This generally manifests itself in cards that cannot be read in other machines.

But some people still want the calculator to go as fast as possible, and want to slow it down to write cards. To do this, the calculator needs to run at 2 speeds, a higher one to run programs, and a lower oe to write (and possibly read) cards.

To have this 2 speed arrangment, one possibility is to replace the capacitor with one that allows the calculator to run as fast as possible, then to switch in another caacitor in parallel with it for slower speeds. (Notice that capacitors have a higher value when placed in parallel, which is exactly the reverse of what happens with resistors).

Without having seen your calculator, this sounds like what was done and the method that was employed.

Although there are exceptions, the reed realy is open without a magnet nearby. Thus a broken reed relay is either going to give you the fastest speed (if it remains open) or the slowest speed (if it remains cosed).

However, it is possible that the original value chosen for the smaller capacitor (for higher speed) could have been arginal (which is pretty much the idea :-) and the oscillator no longer oscillates, or that the calculator can no longer handle the speed. In this case, removing the broken reed relay and replacing it with a link will give you a slower (but working) calculator. And you'll probably be able to use the card reader on it.

It is also possible that the wiring of the additional components was a little dodgy and that wires have come adrift leading to problems.

It is also possible that the calculator was damaged during (or after) modification, and you have a faulty calculator beyond the broken reed relay.

If it were me, I'd try using a link, and if that worked, I'd get a new reed realy and play around with capacitor values until I got a calculator with the two speeds that it was originally modified to have.

WARNING: If you're going to use a soldering iron inside an HP41, please:

1) have experience soldering

2) use something with a fine tip (or wrap thick copper wire around the tip and use the end of the wire as a tip)

3) turn off the soldering iron before you start to work on the calculator (unless you know your iron doesn't leak current)

4) Know what you're doing and make sure it's reversible.


#18

Hi Steve.
Bad news
I made a bypass and removed the relay. I also made sure that there was no wires adrift. I also cleaned up the calculator. I removed the CX CPU and installd it in a CV. I was no differense whit or whitout the bypass. ie open or closed relay. The display only showed som "chinese" signs. In diffrense from before they where stabil. befor they where random. So it must be somthing else.

#19

Steve

In an article in hp forum by Stefan Vorkoetter i noticed that there should be tre rows of interconnect between the CPU and the keyboard in a Cx(not in C or CV)two long and one short. The short interconnect was missing in my calc. (Strange it seems to have been in hard work befor i got it). Anyway i will repace the missing connection tomorrow. I have got a connection taken from an old ericsson cellular phone.

stefan

#20

Hi Stefan,

Are you sure it's not a germanium diode ? - Cheers, John

#21

Hi Stefan!

Would be fun if you could get in contact with me. As you probably know there are not so many die hard HP calc enthusiasts here in Sweden.

/Per


#22

hej Per.
As you can see in my replay to Steve i had bad news. Have you any idea what it might the matter with my CX?.

stefan


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