Re: Tek 2215


I think reliability was the reason Tektronix used carbon composition
and then special film resistors for the focus resistor chain; normal
thin film parts would be been questionable unless significantly
voltage derated. Other oscilloscopes made during this time used a
custom thick film network which was only inexpensive because the same
network was used to regulate the cathode voltage which was not
necessary in the 2 channel 22xx oscilloscopes. I have never heard of
these thick film networks failing.

Carbon composition (and thick film) resistors are bulk devices so they
do not suffer from surface effects like in a thin film resistor. They
also tend to fail open rather than closed although at high voltages, I
suspect that is no advantage.

As far as I have been able to tell, the film resistors Tektronix used
later were specifically designed for high voltage operation and had
some type of special processing done after being trimmed to their
proper value to prevent the surface effects like electromigration
which would normally be a limitation.

On Mon, 21 Mar 2016 00:14:28 -0400, you wrote:

Ha. They probably didn't think someone would be using it 30+ years later. High voltage does do strange things over the long term.


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