clock schematic

[dinghyguy]

Hi All,
Time for another Bryan Challenge...

So i have the usual clocks that don't work. After carefully taking it apart i find that the mechanical parts seem fine but the motor does not run or even hum. Has anyone ever seen or does anyone have a schematic for the circuit board and motor portion of the clock? My suspicion is that something like a capacitor has failed or perhaps the motor itself but i don't have a schematic to use to actually try and figure it out.
for example does anyone know the actual voltage of the clock motor after the circuit board has done its magic?

thanks in advance.

dinghyguy

[18Fiatsandcounting]

Jeez, Dinghyguy, how can I resist a challenge like this??!!

Well, I have only taken apart 3 Fiat clocks, and on all of them, the issue was mechanical and not electrical. Once I got inside the clock case and applied a solid 12 volts to the inside terminals, the clocks started ticking away. Sometimes it took a moment or two, with some gentle nudging, and I think the issue there was that they hadn't run in decades. My guess (just a guess) is that the clock motor runs on 12 volts, and the electronics around it is just to ensure a stable 12 volts without fluctuations or spikes. To make sure the clock doesn't stop when you start the car, that sort of thing.

That being said, one of my other hobbies is 1920s and 1930s AM radios, and the golden rule there is: "First thing, replace all the capacitors." While resistors and diodes and transistors generally have almost indefinite lifetimes, capacitors start to fail after 20 years or so. Especially electrolytic capacitors (the ones with a "+" and "-" on either end). I wish I could give you a schematic or even a better answer, but this is all I can come up with at this point.

-Bryan

[dinghyguy]

Hi Bryan,

I assumed something similar, I currently have two clocks apart, and in both cases the motor no hum. On one of them i actually applied about 6V direct to the motor terminals (i was afraid to apply 12v in case the circuit dropped the power to something less than 12) .....no hum. I agree with you about caps, i replaced all the caps in my Marantz receiver from 1978 and it made a huge difference.

I am staring at the circuit board and it appears to my untrained eye to have:
a diode
2 round caps with values 16V 47uF and 40V 015FA
2 flat caps one with C63 uF, and the other i cant see any markings.
2 resistors properly color coded
1 IC with 4 lines of marking RCA, 514, H16 and 938 Only 5 of the 8 pins are used.
1 metal tin can object with ITT 3932.160 B--7518 on it but no values.
So assuming you are somewhat right, and i replace the caps i think i will still need a motor or two. But i have no idea of what the specs are. Thus i was hoping a schematic might tell me more about the components, particularly the motor

cheers
dinghyguy

[18Fiatsandcounting]

Well, the first capacitor in your list is 16 volts and 47 microFarads, which is a very common capacitor. I'm surprised that they only used one rated to 16 volts, as the car's charging system can get the voltage up around 15 volts, which is very close to the limit. Unless that particular part of the circuit has a lower voltage across the capacitor.

I don't recognize the other one, but it appears to be 40 volts. 015FA sounds like a part number to me.

For the flat caps, those are probably ceramic disc capacitors, and they are typically way less than a microFarad. Sometimes down in the picoFarad range. The "C" designation is likely a tolerance band for the capacitance. Are you sure it doesn't say 63 pF (picoFarads)? That would be reasonable for a disc capacitor.

The IC stuff I can't help you with. But, this sounds like a voltage regulator circuit more than anything else. Keeps a stable 12 volts (or whatever the motor needs) and smooths out voltage spikes, transients, etc.

-Bryan the EE guy

[dinghyguy]

i did a bit of searching and found the IC is an obsolete Op-Amp and the metal tin can is a quartz oscillator, both of which i suspect are perhaps OK.
what i can not find is the motor. I took the motor off the gear train and the only writing on it are the numbers 246.
I looked at digikey and Mouser to see if i could find anything similar, but i had no luck.

The actual circuit board is pretty simple, i think i could actually trace the circuit if need be but if i cant find the parts there is no point.....

Suggestions?
dinghyguy

[18Fiatsandcounting]

Suggestions?
dinghyguy

Well, you could always just get a new (used) clock. Send me an email if you're interested in one of the ones that I have. I don't use them.

And now that you mention it, yes, that circuit is likely an op amp timing circuit, putting out an AC signal (sine wave or square wave) that drives the motor at the right speed no matter the voltage to the clock. Pretty common; see: https://en.wikipedia.org/wiki/Electronic_oscillator

But, at this point, we still don't know if it's the motor or the circuit that is bad. Know anybody with an oscilloscope?

-Bryan

[spider2081]

Has anyone tried measuring an AC voltage at the motor terminals ? I'm thinking a Quartz crystal oscillator would produce either a Square or sine wave output. I don't think it would produce a DC output.

[18Fiatsandcounting]

Has anyone tried measuring an AC voltage at the motor terminals ? I'm thinking a Quartz crystal oscillator would produce either a Square or sine wave output. I don't think it would produce a DC output.

Agreed, and my guess would be a square wave. Time to hook a 'scope up to that bad boy and see what you actually have!

-Bryan

[dinghyguy]

Update on the clock circuits.

One of the circuit board produces 12VAC in a square wave when viewed on the Oscilliscope (the other produces nothing). So that is one bit of information.

Unfortunately both of the motors i have are dead, no action no how. so now he question is where might i find a 12vac motor small enough for the clock. Plus i have no idea if it is a oscillating stepper motor (impersonating a balance wheel) or a rotary device spinning at a constant but unknown RPM.

thoughts and comments welcome.

cheers
Dinghyguy with the ever enlarging hole for the new garage