Re: Datsun clock repair


"R Oxley" <tychounderNO@xxxxxxxxxxxxx> wrote in message
news:vhmzf.9364$ZA5.4452@xxxxxxxxxxxxx
>
> "Honest John" <honestjohn@xxxxxxxxxxxxxx> wrote in message
> news:VYmdnbZhTbq0o1DeRVn-iA@xxxxxxxxxxxxxxxxx
>>
>>
>> There's usually a reason that the resistor fried in the first place.
>> Burned
>> resistors mean to me the circuit is drawing too much current, >
>> H.J. ( electronic tech for 15 yrs )
>>


I finished reverse engineering the circuit. The diode is mounted reverse
biased, which makes it a zener regulator. It is in series (making them a
voltage divider) with the burned resistor, which out of circuit measured as
670 ohms, close to the standard value of 680 ohms, the six also matching the
surviving blue band. So, I know what to do about the resistor, but now have
to figure out the value of the zener diode. Someone mentioned these circuits
running on 1.5 volts, so that is a good value to start with. I might try a
68 ohm resistor in series with the 680 as a ten to one voltage divider, and
increase the value of the smaller resistor until the circuit runs. The
voltage drop should be close to a standard zener voltage rating. But with
the pulsing of the transistor, a simple voltage divider might not be stable
enough. I might have to just solder in a series of zeners, one at a time, to
see what works. We'll see.

Once I get this clock running properly, I'll also measure it's milliampre
current draw, and properly fuse it, to help prevent burnouts in the future.
A 1/8 amp fuse should be ample, I should think. That can also be determined
by experiment.


Bob


.

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