Repair of the Friden STW-10 [Part 1]

By smiba on Tuesday 15 May 2018 10:02 - Comments (1)
Categories: Friden STW-10, Retro/Vintage, Views: 934


I've acquired a Friden STW-10 Mechanical Calculator yesterday from a seller on Marktplaats. The device was sold as fully working, but knowing that this device is over 50 years old I was prepared for the device to malfunction.

The seller had a large collection of devices and kept a maintenance log on them, the system was last oiled in 1996. Which compared to the service manual's recommendation of oiling the device every 6 months means its way behind. Obviously the recommendation is a sellers recommendation and the device won't suddenly malfunction after 6 months, but 21-22 years is a whole different story.

Current state of the device

Physically the device is in decent shape, no dents or very obvious damage other then normal wear from usage.

With the naked eye the mechanics of the device look to be in good shape too. No bent cogs or pieces means its most likely just thirsty for oil and maybe some fine adjustments of certain parts of the mechanics.

As the title of this blog post suggests: The device doesn't work as expected.
It suffers from the following issues:
- Multiply function sometimes gets stuck, not processing the input but looping inside of its function
••• Requires manual reset of the function by pulling one of its levers inside

- Division most likely sometimes gets stuck as well (but this one might be my fault)
••• No idea what lever to pull to directly fix this one yet, but the Division stop lever on the operator keyboard seems to do the trick if you keep on trying.

- Decimal pont setting 8 can't be pressed (stuck?)

- Subtraction directly gets the device stuck, making an awful noise because the motor can't move
••• Right now the device is stuck in this position and this will be the first issue to work on

1. Finding why the device is stuck after subtraction

After some reading online I found out you can connect a handcrank to one of the cogs to manually rotate the system step by step. This is very useful to debug any of its operations because the motor is surprisingly fast!

I couldn't find any of the service cranks online for sale so I opted for a different option: The poor mans handcrank:
(A screw and a wrench)
If it works it ain't stupid ;)

So off we go! I rotated the system back slightly to undo the operation that got it stuck. I wasn't able to rotate it back far enough to undo the whole Subtract operation, but this is not surprising as the system wasn't meant to go in reverse.

If you look at at the most right cog it will attach and right after it does, the whole system jams. After rolling back again in reverse I still wasn't able to rotate the cog on its own, meaning this one is most likely the culprit or at least attached to something preventing it from moving further. However after a lot of pulling around on various parts of the system I wasn't able to find a stuck part (at least in the somewhat accessible areas).

In a desperate cry for help I tried to oil all the moving parts that should receive oil and that I was able to reach with either the bottle directly or a little guide made from a cocktail stick (that the oil ran over to eventually drop down into the part). As Oil I used fine machine oil (Singer Super Oil 100ml) because this was the finest oil I had at home, and it seems right for the job.

After having the machine well oiled all the (not stuck due the machine being halfway through its subtract routine) function keys and switches ran incredibly smooth, no trouble at all. But the machine was still jammed.

In the meanwhile I gave the decimal point 8 button a few light taps with a small rubber hammer and it popped right up and keeps on working right now. After pressing it in and out a few times it stopped being stiff. So that issue can be crossed off the list.

Anyways I messed around a little bit more with various parts but at some point the system was able to move a little bit further and then became even more jammed, There is no movement possible at all anymore after this

As we can see the part (which I have marked with the number 1) should've moved upwards with the moving of the cog right above it. The strong spring is there to always pull it inside of the cog, but even that spring is not able to bring it there.
The part is attached to a long metal bar, which in return is connected to a lot of other moving parts... which are also connected to other parts all the way to the other side of the machine.

Its unlikely this part not being connected to the cog is causing the jam, but the area which is jamming the system is most likely connected to it
So I think this would be my next clue to work on, as this doesn't look like the engineers ever intended for this to leave the base of its cog while in operation.

Stay tuned!

Writing tips, corrections and questions are appreciated. I don't write blogs often

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