Restoring the Mustang Clock

by Sam Goldwasser
Copyright © 2007-2008, all rights reserved
Photos Courtesy of Mike Goodnight

Introduction

This note describes the modifications to retrofit an original Mustang center console clock with an electronic controller. This preserves authenticity while eliminating issues of contact point wear and failure.

The original Mustang clock uses a balance wheel movement that is wound by an electromagnetic "kicker" every minute or so. Basically, the spring is wound by a ratcheting mechanism (I have no idea of the technical term!) when a set of points closes. The points carry the entire electromagnet current (several amps) and eventually become worn, burnt, and pitted. Should for some reason the "kick" not be forceful enough or if the points stick, it's possible for the points to remain closed, burning up the coils and draining the battery (or at least blowing a fuse).

Many Mustang clocks found on eBay show signs of this having taken place. Some have the points totally blown off but most are at least severely damaged. A few have clearly overheated and smoked coils.

The modifications described below can be used with any Mustang clock where the timing mechanism itself is working even if the points are in terrible shape. All that's needed is some metal where the points normally are located to close the circuit. But with the modifications, the maximum current through the points is less than 1 mA so the contact resistance is almost irrelevant and electrical wear is essentially eliminated.

Modified Mustang Clock Showing Pulse Circuit and Start Button

Features

Authentic original external appearance is maintained (face, hands, etc.).
Authentic original Mustang clock sound including tick-tock and winding clunk.
Can be retrofitted to any Mustang clock regardless of condition of points as long as there is any sort of metal present. (The prototype had one of the points missing.) There is negligible point wear since maximum current when the points close is now about 1 mA instead of 4+ amps. Even a clock with burnt coils might work (though rewinding the coils would be highly recommended!).
Virtually impossible to result in coils smoking, blown fuse, or rundown battery from any plausible failure. An energy storage capacitor bank provides the current impulse rather than the battery, which charges the caps slowly between winding pulses. The circuit is inherently current limited by resistors at its input.
Pushbutton starting to get it going if it doesn't start after being hooked up. Automatic starting could be added but this seems to be of little importance since the start button is only needed if the battery is disconnected or the fuse removed.

The Circuit

The main change is that a 10,000 uF capacitor bank provides the kick current instead of the battery directly. It is charged over about 10 seconds and the maximum current from the battery is under 50 mA. This is reduced to near 0 mA once the capacitor charges. When the contacts (or what's left of them) close, an AC-coupled transistor turns on a MOSFET to provide the actual current to the electromagnet. For starting, the pushbutton switch substitutes for the contacts.

The input to the circuit is via current limiting resistors. So, even if the capacitor bank or MOSFET failed shorted, current drain on the battery would be under 50 mA and nothing would overheat. Since the drive circuit is AC-coupled, there is no current drain should the clock contacts remain closed (as in forgetting to restart the clock after replacing the battery).

Closeup of Pulsing Circuit Prototype

Wiring Changes

The connection between the coil return and contacts inside the clock must be unsoldered or cut, and wires attached to these to go to the pulsing circuit. The pulsing circuit will not fit inside the clock housing so it must be located elsewhere, but as close as possible (no more than a foot or so). To minimize resistance, #16 or #14 AWG wire should be used between the clock and the circuit board for the two coil wires (see below). Tie or glue down the two new wires that run inside the clock so that they won't break loose and can't interfere with the clock mechanism. (Since routing and tying down fat stiff wire may be difficult in the confined space inside the clock housing, I used a #22 AWG wire for the coil return simply to exit the housing and soldered it to a fat wire externally.)

Five wires need to run from the clock/wiring harness to the pulse circuit:

Ground. Tap off of harness ground (black) and run wire to pulsing circuit.
Pulse circuit +12 V. Cut green wire, splice a wire to the battery-end, and run to pulsing circuit.
Coil +12 V. Splice a fat (#16-#14 AWG) wire to the clock-end of the green wire and run to pulsing circuit.
Coil return. Run a fat (#16-#14 AWG) wire to pulsing circuit.
Contact sense. Run wire to pulsing circuit.

Update as of March, 2008

According to Mike, the clock has been running for several months now continuously and keeps nearly perfect time.