Musings and comments about our common interest
We are finalizing the design of three different products. As they are not fully prepared, we'll show them in 3D printing form. We will present it during three blog installments.
Until now we have been using a test order of zebra PCB circuits ( Zebra PCB version 3)
We have just received the definitive version (v4) which has a slightly modified dimension to have a better fit (we're talking tenths of mm) and a more precise alignment of tracks.
All orders received this week end are already supplied with the new model
We have introduced a new product. This is only for those that want to use the HPCL circuit on an old HP41c (with serial number before 1954AXXXXX). The screen set requires that the processor have specific capacitance. This is provided by the processor that came with the machines, but not by later processors (in case the original became defective) or if you want to install a CL super-processor. So we have bought a pile of 10nF surface-soldered capacitors, to install between two parallel circuit lines. We will provide instructions if needed:
We have done more tests and now we have put it for sale: it works well. Now thinking about packaging to send them in the best way - so that the adhesive stays put until installation. Alternatives are mounting it on a lower post assembly, or another clipping method to maintain the pressure between up and down.
Further pictures on the first production run:
You can see now some more pictures of the new zebra model:
In continuation of a previous blog entry about a new series of educational videos on HP graphical calculators, here are chapters 2 and 3:
And here is chapter 3:
We have received the samples of a new design for zebras for the HP41 family of calculators. This design will solve the current issues that the elastomeric zebras have. (This part is referred to in the service manual as 1251-5731 )
Let's see it:
Initial tests seem very good - 100% connection on first try! Dimensions are perfect - what we need to find is the optimal fill for the zebra part: how thick and compliant does it need to be, and which material is the best for the application, since it has to be durable and cannot rot, corrode or dissolve. Initial tests show that it works even without filling - the compliance of the film is enough to ensure good connection. However, we will put something in, to ensure durability.
I have come across this video about HP graphical calculators, in hpmuseum.org. It is worth taking a look at it if you want to enter into these calculators -which were the workhorse of all engineering students in the nineties. It is prepared by Stephen Mendes, and he promises several other videos!!
Now we are locked down due to the Coronavirus global pandemics - and it is likely that we'll have to stay for at least a couple of weeks. Now is the time to start things that we didn't have time in the past to do.
One thing is to fix everything that doesn't work at home - and probably this includes your old calculator.
I have started to repair calculators only recently. The repairs performed were increasingly complicated, but at the end I got Ignacio Sanchez's help and we designed a series of parts that help us in these repairs. But the first you need to do is the diagnostic of the damage.
During the next days we will discuss how to repair yours. However, my goal for this instalment is to make you believe that your calculator can be repaired. In my experience , 90% of HP41c calculators can be repaired - and I'd say that 70% are really easy to repair. And this means that YOU can repair it.
Even very destroyed calculators are repairable. Curiously, the worst calculators to repair do not show any damage. In many cases users put the wrong battery model and burn the calculator with much higher voltage than needed (for example, put four a23 batteries (12v * 4 = 48v) instead of the 6v that are produced by four 1.5v LR1 batteries. Then the screen is burned and then you need to wait for a donor calculator - something that doesn't happen quite often.
But for the rest of the repairs, there is a solution!
We'll look into it in the following blog issues.
There are cases (black spot on the screen, screen only works for half the digits, broken screen) where the only solution is to change the screen. These are not produced anymore, so you will need to find a donor HP41c.
To replace the screen: First unsolder the two screens (from the donor and the damaged calculator) Use a small soldering gun and help lifting the pins with the tip of a plier, one by one. Important: unsolder the screen at the calculator side: it is much more resistant!!!
Then solder the donor screen back in place. Locate the screen in its hole first. then start soldering pins, with a very clean soldering gun. There is no need to add more solder. After melting the solder, leave the gun while maintaining pressure on the pin with a flat screwdriver tip, to ensure good, durable contact. Patience until finished. Then reassemble and try again.