Repairing a faulty clock in a Westinghouse oven

[RODALCO]

A while ago our 9 year Westinghouse oven clock packed the sad.

After many requests from the misses I decided to have a look at it.
Sliding the oven out was easier then I expected and the top cover came off with 4 screws.
The clock was of course fixed to the front plate and probably a major task to remove.

Ok, Yesterday I tackled the problem by measuring the two resistors which were put on stand offs, obviously to dissipate excess heat.
One of the two R's was open circuit. The other R was ok and measured 4700Ω
I did a temp rewire and put a 5100Ω in the place of the open circuit 4700Ω resistor.
The clock worked, hoorah !

The faulty R looked like a 3 Watt resistor.
By measurement I revealed that 105 Volts was dissipated across each Resistor
from: U = I*R the current is 22mA.
from P= I²*R Power is 2.3 Watts
Double that by two makes 4.6 Watts, not very efficient.
In one year makes 40 kWh wasted in those two resistors.

I purchased 4 x 5 Watt ceramic resistors and fitted those on a tagboard away from the clock circuit board, and connected them via jumper leads.
I used a combination of 2200 and 3300 ohms in series twice.
Mainly to reduce power wastage and the clock still works fine.
New current is now 19 mA and and power is 2 Watts, still 4 Watts in total and 35 kWh per year.

Now I see why you hear the old story about the microwave clock that the clock often uses more power over the year than the actual magnetron does in short bursts of time.

Surprising that a small transformer is not fitted instead.

Photo's will be attached in about 30 minutes.

 

[cowana]

I purchased 4 x 5 Watt ceramic resistors and fitted those on a tagboard away from the clock circuit board, and connected them via jumper leads.
I used a combination of 2200 and 3300 ohms in series twice.
Mainly to reduce power wastage and the clock still works fine.

Higher wattage resistors will still dissipate the same amount of power - they'll just stay cooler as they have a larger surface area. So power consumption remains the same. (Unless each resistor is now replaced by 11000 ohms instead of 4700).

 

[RODALCO]

Westinghouse clock photo's

Four photo's are attached to show the clock set up as it was and the new set up with the external tag board.
A Diehl clock IC is used in this oven clock.

I put this topic on for other people to learn from that things can be repaired in most cases.

I fitted it all together today after the oven was used and there was still a lot of heat trapped under the top cover of the oven.
That poor old 3 Watt resistor worked hard in it's previous life.

 

[ben7]

Rodalco, you could have used a capacitor using capacitive reactance, and a smaller resistor to limit in-rush current instead of the large bulky resistors. What about changing that old-looking capacitor that is on the clock board?

That chip sure looks old!

 

[RODALCO]

Hi Ben, I can't get the clock board out so this was the easiest option.
the board elco looked alright and is not bulged.
To use capacitors crossed my mind, but the inrush current may still destroy the clock IC, despite of the inrush limiting R. so I kept the waste full resistive option.

cowana - that is correct and that is why I choose 4 x 5 Watt resistors to spread the heat over a larger surface area instead of 2 x 3 Watt resistors as originally done by the manufacturer.