In the hey day of Accutrons, servicemen would have simply swapped out the coil assembly if the watch would not hum or drew too much no load current. Well those days are gone. Now a theoretical and practical knowledge of electronics is essential. If coils are intact but the assembly won't work properly then components must be exchanged because there are not enough good coils to go round. There are a number of things that I am aware of that go wrong with the components but I have still a lot to learn, so I hope to get some input from people more experienced than myself.
1.Transistor fails. I check transistors by seeing if there is conductivity between the base and emitter and base and collector. However I wonder if there is a mode of failure in which voltage applied to the base does not cause the transistor to switch off.
2. Capacitor. Nominally 220 microfarads I often see measurements of up to 260 which might suggest leakage since I can't understand how capacitance could increase.
3. Resistors. Both the single and twin resistor coils have a nominally 2.2 megohm resistor. Those used were indicated to have had a 10% tolerance.
However the resistance has increased over time. I have gone through my collection of dud coils and measured all these resistors.
1.9 to 2.4 megohms 9
2.4 to 2.5 1
2.5 to 2.6 16
2.6 to 2.7 27
2.7 to 2.8 29
2.8 to 2.9 17
2.9 to 3.1 1
Note that the 9 resistors below 2.5 megohms were all from Swiss watches and the rest US. Obviously the Swiss used higher quality components.
Has anyone got a good idea on what the tolerances for good Accutron performance should be?
The last two 218s I serviced had intact coils but would not hum, so in each case I swapped the electronics with those from other coils.
After the swap each promptly hummed with no load currents below 6 microamps.
Measurements on the four sets of components were:
Bad 2.64 megohms Good 2.55 megohms
240 nanofarads 218 nanofarads
2.62 megohms 2.57 megohms
193 nanofarads 222 nanofarads
Please note that all my experience with Accutrons is with 218s. You might think that after having serviced about 200 I would know a thing or two. Unfortunately the more I know the more I realize I don't know.
Regards to all,