Checking Resistor values - UK Vintage Radio Repair and Restoration Discussion Forum

PaulR · 26th Feb 2006, 1:37 pm

Hi All,

A really simple question, this, in fact I hesitate to ask!

When reading posts about repairs, success stories etc I often see references to resistor values and their being way off. How exactly do you test these? I always assumed that one end of the resistor would need to be disconnected so that the rest of the set wouldn't interfere with the reading and so I rarely check the values.

Is this right or can I check with the resistor in situ?

There - I said it was simple!

Thanks

Paul

paulsherwin · 26th Feb 2006, 2:44 pm

In most cases you can check the value with the resistor in place. It does depend on the circuit though.

Paul

XTC · 26th Feb 2006, 3:36 pm

Make sure there are no residual voltages with the volts range ot the meter. There are several reasons for this; safety, they would upset a resistance reading, they might damage your meter on the ohms range.

Look at the circuit diagram and get an idea of the likely loading effect of any other components in parallel. Very often it's either infinite e.g. with anode load resistors, or is so high in comparison with the resistor you are interested in that it can be ignored.

If the resistor does appear to be critical and there are other components which interfere with the reading substantially, either desolder one end or find another connection you can break to get a better reading.

Generally, the tolerances of resistors in valve equipment were either +/- 10% or +/- 20% when they were new, and are not critical. Carbon composition resistors almost always increase in value with age.

Pete.

PaulR · 26th Feb 2006, 4:06 pm

Right, Thanks for your replies. I will look a bit more closely at the resistors in future.

Paul

Station X · 26th Feb 2006, 4:19 pm

One other thing to watch out for is that really leaky capacitors will sometimes shunt resistors and result in a low reading. I say "really" leaky because most leakage won't show up on a meter using just a 1.5V battery. Generally a much higher voltage is necessary to break down the paper insulation.

I change all paper capacitors anyway and while I've got them disconnected check the value of any associated resistors. If for example a screen decoupling capacitor leaks it will draw extra current through the dropper resistor, possibly overloading it and changing its value.

Carbon composition resistors are apt to change their value quite a lot and may change their value when current flows through them and heats them up. For this reason I tend to check their values after soak testing the radio for an hour or two.

Paul Stenning · 26th Feb 2006, 10:33 pm

Since the original tolerance is normally +/- 20%, I think most people replace resistors that are more than 25% away from the marked value.

PaulR · 26th Feb 2006, 11:23 pm

Thanks once again all for the information.

Paul

26th Feb 2006, 1:37 pm	#1
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Checking Resistor values Hi All, A really simple question, this, in fact I hesitate to ask! When reading posts about repairs, success stories etc I often see references to resistor values and their being way off. How exactly do you test these? I always assumed that one end of the resistor would need to be disconnected so that the rest of the set wouldn't interfere with the reading and so I rarely check the values. Is this right or can I check with the resistor in situ? There - I said it was simple! Thanks Paul

26th Feb 2006, 2:44 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Resistor values In most cases you can check the value with the resistor in place. It does depend on the circuit though. Paul

26th Feb 2006, 3:36 pm	#3
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: Resistor values Make sure there are no residual voltages with the volts range ot the meter. There are several reasons for this; safety, they would upset a resistance reading, they might damage your meter on the ohms range. Look at the circuit diagram and get an idea of the likely loading effect of any other components in parallel. Very often it's either infinite e.g. with anode load resistors, or is so high in comparison with the resistor you are interested in that it can be ignored. If the resistor does appear to be critical and there are other components which interfere with the reading substantially, either desolder one end or find another connection you can break to get a better reading. Generally, the tolerances of resistors in valve equipment were either +/- 10% or +/- 20% when they were new, and are not critical. Carbon composition resistors almost always increase in value with age. Pete.

26th Feb 2006, 4:06 pm	#4
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: Resistor values Right, Thanks for your replies. I will look a bit more closely at the resistors in future. Paul

26th Feb 2006, 4:19 pm	#5
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Resistor values One other thing to watch out for is that really leaky capacitors will sometimes shunt resistors and result in a low reading. I say "really" leaky because most leakage won't show up on a meter using just a 1.5V battery. Generally a much higher voltage is necessary to break down the paper insulation. I change all paper capacitors anyway and while I've got them disconnected check the value of any associated resistors. If for example a screen decoupling capacitor leaks it will draw extra current through the dropper resistor, possibly overloading it and changing its value. Carbon composition resistors are apt to change their value quite a lot and may change their value when current flows through them and heats them up. For this reason I tend to check their values after soak testing the radio for an hour or two. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. Last edited by Station X; 26th Feb 2006 at 9:24 pm.

26th Feb 2006, 10:33 pm	#6
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,073	Re: Resistor values Since the original tolerance is normally +/- 20%, I think most people replace resistors that are more than 25% away from the marked value. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

26th Feb 2006, 11:23 pm	#7
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: Resistor values Thanks once again all for the information. Paul