Dave
The NTE386 transistor is direct fit for the BP Boss machines. If you are in Arizona you are lucky. Boss machines do not like cold shops and will blow transistors like crazy when cold. Most of the time only one transistor out of 4 in a drive block will be bad. These transistors are NPN type and are all the same. To test use a multimeter and check resistance on ohm setting. The two catwhisker wires entering the case are leads. The base is on left, emitter on right. They are off center in the case to prevent accidental mounting the wrong way. A good transistor will conduct only in one direction. Typically you will have resistance of .400 to .800 ohms in one direction. When you switch probes you will have message from multimeter of open line. This should be a good transistor. A failed transistor will show resistance of less than .100 ohms and will usually conduct in both directions when probes are reversed.
I think you are correct about keeping machine as factory rather than converting to updated drive system. If you ran it daily update would make more sense.
jh  |