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Old 17th Apr 2014, 6:32 pm   #21
zach1965
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Default Re: Smiths car clock repair

Over the last several days I have hooked two different transistors to the clock.

Please note that the transistors were checked both before and after installation using a multi-meter set for continuity testing to make sure voltage passed as expected. The transistors “checked out” both before and after installation.

I first fitted a PNP 2N2907 assuming that the original TI transistor and the new transistor were both an “E-B-C.” There was no effect. I then reversed the “E” and “C” terminals and tried again. There was again no effect.

When I say there was no effect, it means that the balance wheel oscillated for approximately the same amount of time (measured by a Smiths mechanical stop watch, no less) when powered with the transistor in each of the two configurations described above and when unpowered. The balance wheel is “started” by the “time adjuster/balance wheel starter” mechanism and thus the force applied is consistent from trial to trial.

I then refitted the NPN 2N3904 transistor that I was advised to try before I found this thread. The results were the same in both the “E-B-C” configuration and the “C-B-E” configuration.

And those results were as follows:

When 12V power was applied, the balance wheel (viewed from the rear of the clock) rotated a few degrees clockwise until one of the coils is partially covered by the magnet (see attached photos -- the first is without power, the second is with power).

When the time adjuster/balance wheel starter is engaged and released, the balance wheel oscillates only briefly before returning the the state set forth in the preceding sentence; this same behavior occurs when the clock is connected to the appropriate wires in the car; the balance wheel oscillates for a much longer period when the clock is unpowered.

Using a 12V 1/2A power adapter (which I know is not ideal but it is convenient), I observed the following voltages (in all cases, 12V negative was applied to the metal clock frame, positive to the insulated connector, and the transistor wiring assumed “E-B-C;” there are three hairsprings connected to the balance wheel shaft and I understand that they transmit power to the balance wheel coils):

input side of 5.6K Ω resistor: 11.9V
output side of 5.6K Ω resistor: 0.92V
emitter/upper hairspring: 0.28V (as high as 0.54V when balance wheel moving following engagement and release of adjuster/balance starter mechanism)
collector/lower hairspring: 0.26V (varies from 0.21 to 0.30V when balance wheel moving as per the preceding sentence)

After pondering this state of affairs for a while, I suspect that there is some problem with triggering the transistor -- either the triggering voltage of the transistor is so low that the transistor is always triggered and/or that there is some fault in the balance wheel coil such that the balance wheel coil is “leaking” (for lack of a better word) current at all times and thus cannot properly trigger the transistor.

I would very much appreciate any wisdom you would be kind enough to share with me.

Please be reminded that I do not have a technical background -- speak as you would to a furry woodland creature.

Thanks in advance.


-- Kevin
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Old 20th Apr 2014, 5:57 pm   #22
zach1965
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Default Re: Smiths car clock repair

Some additional details that may be helpful to technical people who are laboring to help this hapless soul (remember, I have the technical sophistication of a furry woodland creature):

From http://www.mridout.force9.co.uk/ecw/sectronic2.htm The Smiths Sectronic Mk II movement [which is the model of both the battery and the 12V clocks -- and which, FWIW, was also used by Smiths for a variety of stationary clocks] [has]...two separate coil assemblies [the variation of the 12V clock, the primary subject of this thread] each with a concentric trigger coil and impulse coil (the two trigger coils are in series with each other as are the impulse coils) and therefore there are two trigger signals and two impulses in each direction of the balance's swing. There are... three hair springs to pass current to and from the coils... The hands are driven by a rack lever escapement working in reverse.
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Old 21st Apr 2014, 9:40 am   #23
Mike Phelan
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Default Re: Smiths car clock repair

Ah - it's not the model I was referring to in my earlier posts, so please ignore them.
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Old 21st Apr 2014, 7:24 pm   #24
zach1965
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Default Re: Smiths car clock repair

Mr. Phelan --


I think there is a lot of confusion regarding the various models of Sectrionic clock. I believe the Mk I (see http://www.electric-clocks.co.uk/SMITHS/sm-sectro1.htm and http://www.mridout.force9.co.uk/ecw/sectronic1.htm) had the germanium transistor and was the model that immediately succeeded those with mechanical contacts.

The Mk II worked in the same basic way but the coils are in the same plane as the balance and not at right-angles to the magnet.

Sectronic-type movements continued through at least Mk IV and then became quartz-controlled sometime in the late '70's.

The clock described at the beginning of this post is almost certainly a Sectronic Mk II given the number on the transistor and the history of the host automobile (basically the same as my Jag).

I have purchased two (2) working Sectronic Mk II stationary clocks from the UK. I expect them to be here in a couple of weeks and, having a working reference, I will continue the thread with additional measurements and observations.

If you think a different forum would be better, please so advise -- I think my problem boils down to selecting the correct kind of transistor (although I'm beginning to think that the the coils may be wonky) and so a different forum may be more appropriate.

Regards (and thanks again),


-- Kevin
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Old 22nd Apr 2014, 2:51 am   #25
julie_m
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Default Re: Smiths car clock repair

You could carefully unsolder a transistor from one of your new clocks and test it with a multimeter. You won't be able to know which terminal is the collector and which the emitter; but you will know the polarity and which terminal is the base, giving only two possible wirings for your new transistor.

I honestly can't see it being too fussy about transistor types; almost anything of the right polarity and with some gain should work in that application.
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Old 22nd Apr 2014, 8:10 am   #26
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Default Re: Smiths car clock repair

The clock shouldn't take much power at all (even in a Jag!) so any light current PNP silicon transistor should do the job so long as the connections are the right way round. Seeing the coil move when power is applied tells you that the coil is intact and not burned out by its previous experiences. That would have been a major problem.

In 1969, people were still learning how to use transistors reliably and it's likely that there is no effective protection from voltage transients from the rest of the car system. So a good choice of transistor would not be a high current one, but would be a high-ish voltage one. so the 2N2907, a 60v part looks attractive.

Getting the legs connected in the right order may get it working, if not a change to the value of some bias resistors may be needed. Nothing terrible.

David
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Old 24th Apr 2014, 5:42 pm   #27
zach1965
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Default Re: Smiths car clock repair

While waiting for the stationary Sectronic clocks to arrive from the UK, I purchased and received another battery car clock (I am very determined and at least a little crazy <g>).

This is, apparently, a early Mk II as it has a GERMANIUM, metal-cased (TO-39, I think) transistor!! So, Mr. Phelan, you were right all along but Smiths seems to have, um, "updated" its movements fairly frequently (incidentally, each of the car clocks has a different caliber number).

Unfortunately, the balance wheel coil has come adrift. Fortunately, since that's a mechanical problem (and I have repaired and rewound relays in the past), I think I can fix it.

Once repaired, this clock requires a 1.35V mercury cell (which are, of course, NLA). An adapter for common, alkaline, button-cell batteries that steps down the voltage from 1.55V to 1.35V is available for £30 plus international shipping (http://www.smallbattery.company.org....50_adapter.htm) -- and I don't want to pay that much, especially for an experiment.

This particular caliber does not have a resistor (see Post 7 for typical resistor installation) but one can be easily fitted.

The question is, what value should it have in order to step down the voltage to 1.35V? (And, yes, the clocks are, apparently, very sensitive -- see http://www.jaguarmk10420gsymposium.com/t254-420g-clock.)

A formula would be great as I intend to go to Radio Shack and (ideally) get a zinc-air battery (with more stable voltage-decline-over-time characteristics, as I understand it) that has a similar diameter to the old PX1 and, hopefully, along with an appropriate spacer, have a permanent, inexpensive solution.

Could some kind soul please help this non-technical person?
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Old 3rd May 2014, 10:44 pm   #28
gjm120761
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Default Re: Smiths car clock repair

Dear poster, might I ask please (as I have one of these clocks too) how you dismantled the clock? I am unsure if one of the screws on the rear of the clock is a speed adjuster and since my clock keeps perfect time I am reluctant to turn it. However, the bezel is in poor condition and I would like to remove the glass in order to paint it.
Many thanks, in anticipation, Graham
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Old 1st Nov 2015, 12:41 pm   #29
jschaeffeler
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Default Re: Smiths car clock repair

I have the same clock and was wondering if anyone found out which replacement transistor would be good?
Many thanks, Joerg
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Old 1st Nov 2015, 11:38 pm   #30
Robsradio
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Default Re: Smiths car clock repair

For what it's worth, the movement was used in many Smiths Sectronic battery clocks of the late 60s, it was the MK2.
The last car clock I repaired lead me to think the transistor was shot, but it was the capacitor in the end.
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