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FDP Forum / Fender Amps: Vintage (before 1985) / Resistor Testing in. Circuit, Truth or Myth?


USA / Sesame St.

Feb 14th, 2017 06:38 PM   Edit   Profile   Print Topic   Search Topic

Everyone knows that to get an accurate reading when testing a resistor in a circuit you must remove one of the resistors soldered leads from the circuit. I did some experimenting using a DMM on an original amp circuit from 1966, caps fully discharged.

I tested several resistors in the circuit and noticed those with the same value had different readings. Some were out of spec with higher readings while some were within. I took a new resistor with the same value out of the bag and tested it. It was a 220K metal film 1% tolerance and the reading with my DMM was 219K. I clipped my DMM probes onto the resistor and held the leads firmly against the same contacts that the original resistors were soldered to in the circuit. Metal to metal contact, continuity is the same as if it was permanently installed in the circuit. I tried this in several locations. I got the same reading as when it was not in contact with the circuit.

Now I realize that testing in a circuit could throw off the readings some but what I don't understand is how a new resistor could have the same reading whether it's loose or in the circuit. This tells me that the readings I'm getting on those old resistors aren't being effected by being installed in circuit, yes or no. What am I missing here? Do I still need to test them disconnected from the circuit.

(This message was last edited by roadhog96 at 06:20 PM, Feb 15th, 2017)

Contributing Member

So. Cal. USA

Feb 15th, 2017 12:16 PM   Edit   Profile   Print Topic   Search Topic

I don't think you are missing a thing. It depends on the circuit. If the voltage from your DVOM can come through the back side of the circuit to your probe(s) then the value will be affected. Path of least resistance is followed. No trace back as in most Fender preamp and power circuits and the reading is accurate. Plate/grids on preamp tubes for example test well in the circuit.

Its capacitors that don't test well in circuit unless you are testing for ESR in larger caps. Value you pretty much have to pull one end, same with leakage.

Hope this helps.


USA / Sesame St.

Feb 15th, 2017 04:18 PM   Edit   Profile   Print Topic   Search Topic

That makes perfect sense. I only had a problem with two resistors that were piggy backed together so the reading were the same on either one of them. As far as the caps go I need one of those ESR meters, it would make life easier. Just wish they weren't so darn expensive.

I found that most of the resistors were out of spec and some were at the max limit so they will be changed also.
By the time I get done there won't be many original parts left in it. I wonder if it will be a noticeable difference once they are changed.

(This message was last edited by roadhog96 at 06:23 PM, Feb 15th, 2017)

Contributing Member

So. Cal. USA

Feb 15th, 2017 05:36 PM   Edit   Profile   Print Topic   Search Topic

I own the ESR meter shown in the link below. You can buy a kit for a little less. $102 is pretty cheap for the time this unit saves.

Fantastic for finding out filter cap condition BEFORE you fire up something quite old. Nice for just checking up on you amps every 9-12 months once they get some age going.

As the old saying goes, are you going to play it or look at it. I like to play my amps so parts get changed. I'll use carbon comp resistors in a vintage amp for the "look" of the original. My builds I go for quiet and use metalics. Caps just have to look new I guess.

Resistors and caps in the signal chain can effect tone and introduce noise. In the power section they seem to take away punch and dynamics when aging and my cause hum. It should sound better after you service. The degrade so slowly you don't notice it unlit after tune up time.

Blue ESR


USA / Sesame St.

Feb 17th, 2017 08:15 AM   Edit   Profile   Print Topic   Search Topic

That's the same ESR meter I've been looking at, I guess I'll have to break down and get it.

As far as the resistor types go, do you believe you can hear a difference between the way an amp sounds with carbon composite or metal film resistors. I think the only difference you could hear is the amp would have less unwanted noise with metal film. I can't imagine it would actually sound better with CC. I think the only reason vintage electronics had CC resistors is because that was the cheapest way to manufacture and I don't believe there were many other options available back then, had nothing to do with tone.

I'm trying o decide whether to keep it as close to original with all CC 10% or upgrade to metal film. There's more or a price difference than anything else.

(This message was last edited by roadhog96 at 10:16 AM, Feb 17th, 2017)

Contributing Member

So. Cal. USA

Feb 17th, 2017 09:46 AM   Edit   Profile   Print Topic   Search Topic

I've never been able to hear a difference with CC unless they are on their way out. I think their noise level is decent too. I like to use the metal stuff in the high voltage spots, dropping resistors etc. They seem to last longer (as if I'm going to be around to tell).

The price difference couldn't be more than $3-5 for an entire amp build. I do have a couple of build around nearing 20 years old with CC and they are doing fine. Both have been re-capped however. ESR was rising. I still have a few vintage amps, 60's stuff. The CCs in high voltage spots have all needed replacement for the most part.

That ESR meter can be a good tool for other things like finding grounding/continuity issues. Very accurately reads very low ohms. My Fluke pukes down that low.

FDP Forum / Fender Amps: Vintage (before 1985) / Resistor Testing in. Circuit, Truth or Myth?

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