Zenith H2445R Tennyson Porthole Resistor Question

[Penthode]

This is a good topic as to determine when to or not to change a resistor. The old carbon resistors are notorious not only for out of tolerance values but also drift with temperature.

An absolute match is essential in a discriminator circuit. However most tube circuits will run without problem with a tolerance of +/-100%. Resistors in bias dropping circuits are perhaps more critical but for example the grid return resistor on an audio amplifier can vary by 1000% and you would be hard to notice any difference in performance.

For example I find that sets that rely in the vertical sweep circuit on capacitive-resistive feedback instead of a blocking transformer for the vertical oscillator will generally benefit from a "shot gun" change of resistors in the entire sweep circuit to offset any value drift which will cause the vertical height/linearity to change.

It is important to determine where the resistors are in a circuit and their application to determine if it may cause a problem. With a 70 year old TV set, I will do a general visual inspection for charred or stressed resistors. I will also do random spot checks to ensure my chassis run in production did not get a bad batch of resistors.

I consider a resistor's stability to temperature generally more important than its absolute value in many cases.

Terry