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MG TD TF 1500 - Clock wiring inside - who knows?

I'd found the "transistor" layout for the circuit shown in the image.

Can anyone tell me the original wiring of contact, coil and shunt in the original clock?

I guess the shunt is parallel to the contact and both are serial to the coil. Am I right or wrong?



Bela

Bela - I tried a similar circuit in the clock for our TD, but while that circuit will reduce the current across the points, you still have mechanical wear from the points sliding across them, which eventually cuts a deep grove in the pin that serves as the contact on the balance wheel. I ultimately got the circuit that uses an optical trigger for the circuit used to run the clock. The clock in our TD now works perfectly. The Outfit that I got the kit from is Clock 4 Classics at: http://www.clocks4classics.com/ Cheers - Dave
DW DuBois

I can confirm that - I got the same fix and it works perfectly.
Dave H
Dave Hill

Would solve my clock problem but not sure if I could fit it.
G Mills

Looks good and seems really to be an improvement.

But can anyone answer my question?
Bela

To answer your original question...there are no electronics in the clock. The contact is in series with the coil, but I don't remember seeing any shunt resistor. (Sorry, it was a couple years ago when I looked at the clock's internal bits.) If so, it would be in parallel with the contacts.

The circuit you show should work fine for reducing the current in the contacts and eliminating sparking. It's just a transistor switch circuit, which allows the transistor to handle the coil current instead of the contacts.
S Maas

The shunt forms the (normally green) block which makes up the base of the clock supply terminal.

It contains a resistor which was designed as a primitive, and largely ineffective, form of spark bypass/suppression.

Tim
Tim Wilkinson

Tim wrote:

.... a resistor which was designed as a primitive, and largely ineffective, form of spark bypass/suppression.


Thanks Tim!

And it is parallel to the contact as mentioned by S Maas?
Bela

Sorry no I should have said the shunt is connected across the coil.

Note that this applies to the standard T-type clock. Later versions - I think from about 1961 - or early clock modifications replaced the shunt with a rectifier (diode) which provided a somewhat improved solution (but made the clock polarity sensitive).

The transistor circuit provides a much better solution, if the contacts (and clockwork) are in reasonable condition. Most of the wear would have been due to spark erosion, so the amount would depend on how long the clock had run before failing - often not very long. Extra life can be obtained by *very gently* adjusting the level of the spring contact.

I agree that the optical trigger provides the ultimate solution by eliminating entirely the contact mechanism.
Tim Wilkinson

Thanks again Tim!

So it should be in that manner.

Bela

Sorry, brain fart.

Tim's right, if it's a resistor, it should be across the coil.
S Maas

That's right.

To complete the diagram there's also the push contact to start the clock, which acts to ground the coil in parallel with the brush contact.
Tim Wilkinson

like this?

Bela

That's it, schematically.

Would you like to tell us where you're going with this?
Tim Wilkinson

Hello Tim

I'd given the clock to a retired watchmaker. He is a specialist for mechanical clocks and I hope he is able to implement the electronics after overhauling the mechanical part and after studying the infos. The schema is only for my own documentation to keep in mind the situation before those changes.
Bela

The watchmaker recognized a missing start contact.

Are pictures of this detail available?

Bela

Bela,

I used this document to repair my clock, it is the best that I could find at the time. It contains a diagram of the complete electric circuit.

http://www.sa.hillman.org.au/TT_SmithsClock.htm

There may be others, if you want to search for them, try using the key words: "smiths pin car clock repair".

John

52 TD
J Scragg

Thanks John - very good for my own eduction.
Bela

This thread was discussed between 08/02/2016 and 16/02/2016

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