Friden Model C10 Calculator Repair / Problems page
The series of photos below were used to diagnose a problem with the governor flywheel spinning continuously (which it shouldn't - the machine should remain quiet until a key is pressed). At first I assumed the was that the copper electrical contacts were shorted together, but that didn't turn out to be the case. With the help of Robert Long (Thanks Bob!), it seems that some kind of short had developed inside the brass cube (may be a transformer?) - see the wiring diagram below with the inferred short shown in red. In the end, I simply disconnected White2 and it seemed to work properly thereafter. I think the transformer/resistor were used to smooth out the power/current in the bad old days of early electrification, so this change shold be fine nowadays.
I matched up the wires shown in the diagram above with the labels I used in the photographs below.
| My Photo | Diagram | | |
| Brown | Yellow (HouseholdW to Motor) |
| White3 | Black (HouseholdB to CubeA) |
| Black3 | Black (HouseholdB to ContactT) |
| Black2 | Black (ContactB to TubeR) |
| Red | Red (Motor to TubeL) |
| Black1 | Black (Motor to TubeR) |
| White1 | Black (CubeB to TubeL) |
| White2 | Yellow (CubeC to TubeR) |
With no function keys pressed, I used an ohmeter to verify that the copper contacts are not touching, and that the wires connected to each contact are isolated from each other. The TopContact wire (Black3) connects directly to one of the male plug prongs (where the electrical cord plugs in) at the back. The BottomContact wire (Black2) connects to a screwterminal on a big capacitor (at least I assume this tube shaped thing is a capacitor) as shown in the photo; I disconnected this Black2 wire from the terminal to verify its continuity with the BottomContact and also its isolation from the TopContact and all that seems good.
As an experiment, I blocked the two copper contacts from touching (with a piece of electrical tape between them) and pressed the add function key with the result that this had no effect on the operation of the machine: as before the governor is spinning always and the 'add' operations proceeds with each button press.
Based on this, it seems the problem is coming from some place other than the copper contacts and their wires.
I isolated the White2 wire (which shares the same screwterminal as the BottomContact wire Black2) and oddly this has direct continuity with the TopContact wire (Black3). This would seem to make a short circuit (circumventing the copper contacts entirely) -- could that be the problem?
This White2 wire goes into a brass cube that I assume is a transformer?
Here's a poorly lit video showing a potential problem -- Should the motor's governor (flywheel) spin continously and spark like this? (The calculator seems to operate well otherwise.)
At the very end of the video (above), you can see the stepped drum make a turn as I press the 'Add' key one time.
Here's a photo taken from the rear of the machine with the cover off.
You can see the motor on the left and the wheel to the right
The follwoing photos show how the original plug and cord were replaced with a modern grounded 3-prong cord and plug.
The three photos above show how a standard modern 3 prong plug and cord were wired into the original 2 prong plug.
The bottom photo shows a standard detachable power cord (from any computer etc.) inserted into the new plug.
You can buy these 3 prong plugs, but I removed this one from an old computer power supply I had lying around.
As you can see,I attached the grounding (green) wire to the Friden chassis, which should offer a bit of safety.
(Note that the two exposed screws seen in the bottom photos are not 'hot' - they simply attach the original plug to the chassis.)