11-16-2012, 03:16 PM
Resistors oft cannot be checked in-circuit.
I usually see first if there is any DC path in parallel to a resistor, if not (it connects to some capacitors or to a tube's grid) then it is OK to check in-circuit, if there is - you have to use your judgement.
For example, say I have 10K resistor and there are some branches off of it where you see 250K or some 1M resistors creating DC path, it still can be checked, but if the resistors are comparable then they will shunt the resistor you are checking.
A good indication of a bad resistor, if the DC path is present and it is still above the nominal, it is clearly bad as if anything it should be lower.
An electrolytic cap in parallel or through small resistance may give you headache when checking a resistor as it will take time to charge and during that time your value will be all over the place if using DMM and the analog meter will show low slowly rising value.
But as bad as resistors are, the caps are the worst as almost no cap can be adequately checked by a C-meter in-circuit, the resistors will distort the heck out of it.
I try not to snip the resistors off but unwrap the lead with a hot iron, but this is a bit tedious.
In my experience (which mind you is not too extensive when it comes to old radios) with the 1940 and older type radios almost 100% of the carbon resistors (dog-bones) have to go, none of them is below 20 to 80% deviation, most being 50% up which is unacceptable though the radio may still play and that dupes folks into thinking the resistors are OK.
Wire-wounds on the other hand, unless open, do not age at all value-wise, as the wire does not go anywhere; all wire-wounds I saw are right where they are supposed to be. Those I replaced were opened, probably due to overheating.
I usually see first if there is any DC path in parallel to a resistor, if not (it connects to some capacitors or to a tube's grid) then it is OK to check in-circuit, if there is - you have to use your judgement.
For example, say I have 10K resistor and there are some branches off of it where you see 250K or some 1M resistors creating DC path, it still can be checked, but if the resistors are comparable then they will shunt the resistor you are checking.
A good indication of a bad resistor, if the DC path is present and it is still above the nominal, it is clearly bad as if anything it should be lower.
An electrolytic cap in parallel or through small resistance may give you headache when checking a resistor as it will take time to charge and during that time your value will be all over the place if using DMM and the analog meter will show low slowly rising value.
But as bad as resistors are, the caps are the worst as almost no cap can be adequately checked by a C-meter in-circuit, the resistors will distort the heck out of it.
I try not to snip the resistors off but unwrap the lead with a hot iron, but this is a bit tedious.
In my experience (which mind you is not too extensive when it comes to old radios) with the 1940 and older type radios almost 100% of the carbon resistors (dog-bones) have to go, none of them is below 20 to 80% deviation, most being 50% up which is unacceptable though the radio may still play and that dupes folks into thinking the resistors are OK.
Wire-wounds on the other hand, unless open, do not age at all value-wise, as the wire does not go anywhere; all wire-wounds I saw are right where they are supposed to be. Those I replaced were opened, probably due to overheating.
