Resistors have voltage ratings, and power ratings in addition to
their resistance values. It is probably not the voltage rating that
is at issue here, but possibly the power rating.
More likely, however is sometimes a value is needed that cannot be
bought due to temporary shortages, in those cases, you can sit and
wait for the part, or you can make your own out of two parts.
How do you know tektronix did this? Most of the stuff we have hasn't
been at the factory for 20 or 30 years.
-Chuck Harris
Jeff Dutky wrote:
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Colin,
Yes, this thread is a response to my thread "Fix or Part Out a 475A" and specifically is a discussion of the perplexing arrangement of resistors R1354 and R1356 across the collector-emitter of the Q1354 transistor in the beam intensity amplifier circuit. In the actual circuit we have two fat (1/2 W) resistors mounted vertically with their upward pointing leads soldered together.
In the schematic this looks exactly as you would expect a voltage divider to look, except that there isn't anything tapping the voltage between the two resistors, so the question was why Tek put them in rather than using a single resistor of twice the resistance, and why they mounted them in this odd way rather than flat to the PCB.
The two theories on offer (from Roger Evans and Milan Trcka) are 1) that the two parts were used to get better thermal properties in a small space, and 2) that the two resistors were put together to get a total resistance that was not available in a single part.
-- Jeff Dutky
