View Full Version : How to tell if a resistor is going bad


cuthien
05-08-2007, 12:39 AM
Can someone show me how to tell if a resistor is going bad (out of specs) in my tube amp. I have a volt meter but dont know how to put it to good use. Thanks.

silverHalo
05-08-2007, 01:16 AM
Set your multimeter to Ohms. Touch one of the probes to one leg of the resistor and the other probe to the other leg. Then check the reading on the meter and see if it is the same value that the resistor bands say. If you don't know how to read a resistor I suggest a quick "google", there are plenty of websites that have an explanation on resistor reading. The tolerance rating of the resistor will tell you how much math you will have to do to find out how far out of spec the resistor is. Older amps sometimes have resistors with a 10% tolerance, this means if it is a 100 Ohm resistor, it could give a reading on your meter anywhere in the range of 90-110 Ohms and still be technically in spec. But with a 1% resistor the range would only be 99-101 Ohms. It also helps to have a auto-ranging meter when testing if you don't know what the value of the resistor is. These meters automatically adjust the Ohm scale for measuring ohms, kilo-ohms, and mega-ohms which makes things a bit easier if you have a lot of resistors to test.

CUlater
05-08-2007, 02:35 AM
One word: magic smoke. Ok, that's two... :cool:

jeffn
05-08-2007, 03:15 AM
Remember you MAY have to unsolder one end of the resistor to get a sensible reading.......

cuthien
05-08-2007, 03:26 AM
Remember you MAY have to unsolder one end of the resistor to get a sensible reading ====> This would take years to accomplished. Is there an easier way than this.

Thanks guys.....

jrsh92
05-08-2007, 05:38 AM
Is it surface mount, as in the tiny boxes that sit on the surface?

jeffn
05-08-2007, 06:02 AM
Remember you MAY have to unsolder one end of the resistor to get a sensible reading ====> This would take years to accomplished. Is there an easier way than this.

Thanks guys.....

Sometimes a resistor will look burned if totally cactus. But I've found normal looking resistors wildly out of spec to the point that the amp wouldn't function. Without unsoldering you might be able to yell between a good one and an open circuit one, but I'd doubt you could detect one merely out of spec.......... I think.

Krautrawk
06-15-2007, 01:16 PM
Remember you MAY have to unsolder one end of the resistor to get a sensible reading ====> This would take years to accomplished. Is there an easier way than this.

Thanks guys.....

Just a tip...if you don't unsolder you're not measuring the resistor, you're measuring the equivalent resistance seen between the two nodes connected to the resistor, and if you have any active devices connected to those nodes, then it won't be the same thing at all.

jrsh92
06-15-2007, 01:23 PM
Yeah, if you have any part of a tube connected to the resistor, for example, the resistor is bridging two pins, then you are measuring that part of the tube in parallel with the resistor. Now, if you have something to compare it to in the same sitution, like two tubes that are set up the same way (one is known to be all good, something isn't right on one and you suspect it may be that resistor), then compare.

avionic
06-15-2007, 01:40 PM
Remember you MAY have to unsolder one end of the resistor to get a sensible reading ====> This would take years to accomplished. Is there an easier way than this.

No,the only way to accurately measure a resistor thats in a circuit is to isolate it from the circuit ie. lift one lead from the circuit....sorry :scratch2:

Dave

Family_Dog
06-15-2007, 01:45 PM
Can someone show me how to tell if a resistor is going bad (out of specs) in my tube amp. I have a volt meter but dont know how to put it to good use. Thanks.

Resistors in a tube amp seldom go bad, unless there is a fault condition causing a tube to conduct heavily thereby cooking the cathode resistor, which will appear distressed. Same could happen to any resistor in the B+ (high voltage) part of the circuit.

You could get by with simply measuring the resistance value with an ohmmeter most times, but there are times when you should disconnect one leg of the resistor to get a true reading. If it is difficult to do so because, for example, the leads go to tag points with multiple wires going in and you stand a chance of doing some serious damage to the rest of the components going to the same point, then simply find a good point not too close to the resistor body and snip the lead with a decent pair of side-cutters. Do the measurement, and if the resistor tests good (within 5-10% tolerance depending on the tolerance rating - gold for 5%, silver for 10%, no colour indicates 20%), then carefully resolder the leads together again.

Take the usual precautions regarding discharging filter capacitors to ground with a resistor of suitable size & rating to completely eliminate any danger of an electrical shock.

wa2ise
06-15-2007, 02:51 PM
Set your multimeter to Ohms. Touch one of the probes to one leg of the resistor and the other probe to the other leg. Then check the reading on the meter and see if it is the same value that the resistor bands say.

Be sure the power is turned off first.

If there's a capacitor in the immediate circuit, you may see an increasing resistance reading which should stablize at the value of the resistor.

Old resistors above half a megohm tend to drift higher in resistance.

john_w
06-15-2007, 03:02 PM
Now, if you have something to compare it to in the same sitution, like two tubes that are set up the same way (one is known to be all good, something isn't right on one and you suspect it may be that resistor), then compare.

If you've got a stereo set-up, this is a good technique to figure out if the problem is anywhere near the resistor. It won't necessarily give you the value of the resistor, but the readings between any two points (or any point and ground) should be roughly the same between the 2 channels.

To T-shoot a circuit this way it's best to start at the output and work your way back towards the source, measuring impedance values (ohms) between each point and ground, and comparing the good channel against the bad channel. As soon as you find a spot that's radically "off", look for something cooked around that spot. If nothing looks bad, then you may have to start pulling components.

Yes, it is a lot of work! But techs learn what's most likely to go bad and check those things 1st.

Tom Bavis
06-15-2007, 03:04 PM
Since bad resistors USUALLY are high, you can often tell it's bad without removing it. If it measures high in-circuit, it's bad. If it measures low... you will need to disconnect one end to know for sure.

aidynphoenix
03-07-2010, 01:50 AM
sorry for reviving this thread.. but when a resistor goes bad.. does it allow more or less current typically??

battradio
03-07-2010, 03:01 AM
The short ansewer YES . If the resistance goes up the current goes down ,
If the resistor has been overheated the resistance goes down and the current goes up.

aidynphoenix
03-07-2010, 03:59 AM
tyvm for your responce.. that is just what i was looking for!

Family_Dog
03-07-2010, 03:59 AM
A 'bad' resistor will usually go higher in value, therefore allow less current flow. Old Carbon resistors were usually 20% tolerance and it would be wise to replace them. They would also increase the noise level of an amplifier, particularly in the early stages of amplification. You can safely substitute 1W resistors for older type 1/2w resistors, they look better too!


-F_D

aidynphoenix
03-07-2010, 04:29 AM
my problem is with power indicator leds in a speaker..
the red overload led is the first one to come on.. however it is suppoded to be the last.. unless i am mistaken there is 2 resistors and a transistor for every led in there (approx-10 of em in there) so thats alot of components to be checking.. 20resistors, 10 transistors, theres also a diode in there.
and the worst part. they are all different values!
it gives me a headache thinking about checking them all :thumbsdn:

john_w
03-07-2010, 01:46 PM
A 'bad' resistor will usually go higher in value, therefore allow less current flow.
-F_D

Must caution though, the key word is "usually". A resistor CAN short through to some extent, which might produce a level of current that causes damage down the line.

I think this is rare, but it's a worst case scenario. So if you replace a bad resistor but it's still broken, and you find bad parts down-line, you might not be totally baffled.

Sorry to complicate things. :)

aidynphoenix
03-07-2010, 02:49 PM
i dont quite understand exactialy the signal path and what compoents are related to that particular led.. the transistor being involved. means once again i get to learn something new..
from what i have heard sofar. i think there is a chance there is more current going through one or more of the resistors..

im thinking because the board was cracked when i origionally took it out.. some leds were not functioning. therefore mostlikley the red led (that was working) was getting more current that it probably should of have.

bobrown14
03-08-2010, 06:33 AM
Remember you MAY have to unsolder one end of the resistor to get a sensible reading ====> This would take years to accomplished. Is there an easier way than this.

Thanks guys.....

You should be able to get a reading with the resistor in circuit. Resistors GENERALLY either pass current or they don't = open reading in circuit or not. The do not usually "drift" out of spec.

Cheers,

Bob

john_w
03-08-2010, 01:20 PM
You should be able to get a reading with the resistor in circuit.


That often works, but if the hot side of the meter is exposed to one or more relatively low impedance parallel paths to ground (including the path going "backwards" thru the circuit, away from the resistor), the reading can be way too low. I would try it in-circuit first though and see if the reading makes sense.