Quote:
Originally Posted by Spencervs
They are in tolerance but I worry they may keep decreasing until perhaps a bang or busted valve or two. At least the other way they will eventually just starve the circuit.
Is this possible that they decrease and should I replace them on sight? Or is it the rest of the circuit causing the readings to be off?
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You are worrying FAR TOO MUCH! Resistors can and do decrease in value BUT usually because they are overrun by a fault condition or underrated in the first place. As has been said previously, to check resistors accurately you need to test out of circuit by unsoldering one end. You can often check resistors in-circuit (in valve radio's at least) without disconnecting but you need to study the circuit to see if any possible parallel paths might reduce the apparent resistance. This is certainly true of transistor circuits and with televisions. Always best to unsolder one end of a resistor to check its value if there is any doubt.
As a general rule, resistors will increase in value and you tend to notice it far more with high value types. It's not uncommon for (say) a 220k anode load resistor to creep up to several megohms or even go open-circuit. As previously stated, lower value types (in the ohms range) can often go lower in value but because they are overrun.
As for causing a bang and 'busting a valve'..........very unlikely! A faulty valve is more likely to cause a low value resistor (like cathode bias) to fail rather than the other way around.
I have many sets with original resistors that are still within tolerance and even if they have drifted slightly above or below, that is no reason to change them. Most radio's will work quite happily with resistors way out of spec.
Stop worrying and enjoy the radio's!