Re: Tek 2215

Craig Sawyers <c.sawyers@...>
 

The symptom of attenuated and nonlinear vertical deflection pointed directly at the collector load
resistors and ohms measurement from the bottom of the printed circuit board conclusively showed
they were bad.
They were the first thing I checked because it was easy to do and visual inspection showed that
the
printed circuit board itself was discolored from high temperature in the area under and around
them.

The value for the 51 ohm series resistors is not critical but I wonder how hot they run and about
higher
parasitic inductance of spiral cut film resistors. Why did Tektronix use carbon composition
resistors
there?


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Posted by: David <@DWH>
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Tek had a bit of a blind spot when it came to power dissipation in resistors on circuit boards.
They invariably mounted them tight to the board. So even if the part was operating inside its
dissipation limit, that power specification is always quoted by the resistor manufacturer "in free
air". Resistors which are dissipating a decent percentage of their power limit should be mounted on
spacers (which can be just a few mm) to ensure good air flow. If you put them tight to the board,
the contact is a hot spot which (a) discolours, or in some instances burns the board and (b)
compromises the lifetime of the resistor. Seen the same thing is some classic era Fluke gear too.

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