resistor measurement way off?

Hi everyone, I am just checking the resistors in my Crown D150A amp that went bad. I have removed all the outputs and drivers and now I am just spot checking some resistors. I have one that in the schematic/parts lists shows as 2M ohm (confirmed red/blk/grn gold) and I am measuring in place and I am getting about 48k ohm. I am new to electronics, but am I missing something or is this resistor compromised?

Thank you!
 
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Best to lift one end of the resistor before measuring, since the circuit can load the resistor.
If those are carbon comp types, just replace them with modern resistors of adequate wattage.

In rare cases you have to consider the inductance of the resistor. Don't use wire wound types
as replacements unless the original was also. And some wirewounds have noninductive winding.
 
Hi, thank you for this info. I will go ahead and lift one side or just remove it and test again. FYI, I read on the forum that if it is an audio path you want carbon film and power path you want metal film, does this still hold true?
 
You definitely are better off with metal film on any signal path. Carbon resistors are inferior for noise, stability, temperature coefficient, linearity and probably a few other things! However very high values of resistance are hard to do with metal film and metal oxide are the choice then. In surface mount world "thin film" = metal film, "thick film" = metal oxide.
 
So, if I am reading this correctly, Metal Film would be ideal, if you cannot source that, then Metal Oxide and the the carbon film? Sorry if I am making this more confusing!

The post I read said this:
Power = metal
Audio path = carbon film.

I am just confused because you mentioned Carbon are inferior for noise but that post said to use them for audio path?

Then someone also says, only use carbon if others are not available, like you spoke of.

Here is the post I was referencing:
Types of Resistors for Hi-Fi use... | Audiokarma Home Audio Stereo Discussion Forums

Just clarifying, thank you!
 
Opinions are sometimes wrong. Carbon comps are largely obsolete in audio and industry for very good reasons.
You do have to distinguish guitar amps from audio amps. Guitar amps are whatever the musician wants.

For audio, metal film, metal oxide, carbon film, wire wound all have their places, depending on the application and cost.
All of these types are readily available and widely used in the electronics industry in large numbers.
So just determine the most suitable part for each location in your particular audio amplifier.
Requirements for that part come first, then choose among the options left.
 
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Carbon film performs very well for signal and low power applications. Metal film is better and more robust. Metal oxide is used mostly for higher power resistors. Wire wound isn't used much for audio as it's a bit inductive, other than for power resistors like emitter resistors.

Nothing in the circuit can make a resistor read higher, assuming there's no residual voltage on the PS caps, only lower. If I really want to know I'll lift one end of the resistor. Usually I just compare the left and right channel. If the measurements are the same, there's likely no problem.
 
Very odd, first off, rookie mistake, to not measure the other side. Back to very odd, one side is 46k ohm and the other side is 48k ohm (supposed to be 2M ohm). But like you said I will verify by lifting one side and checking, but it is reassuring they are both within a reasonable amount.
 
Wire wound isn't used much for audio as it's a bit inductive
Not necessarily. There's a winding technique where the resistor is made from two coils wound in opposite direction and tightly coupled magnetically. It's known as Ayrton–Perry winding. That effectively cancels out the inductance.

I wouldn't use anything other than metal film in a modern circuit. I'll typically use metal oxide film for power resistors.

The only advantage of carbon film over metal film is cost. At QTY 1k a carbon film resistor will set you back $0.01/each whereas a metal film will be more like $0.014/each. That's a 40 % increase but in a typical DIY audio circuit that means a difference of a dollar or two for the entire circuit, which shouldn't break the budget for most.

Carbon composite resistors are to be avoided unless you're restoring very old amps and want them restored to their original spec. They're awful resistors. They have crappy tolerances, drift over time, have high excess noise, high 1/f noise, etc.

Tom
 
Hi everyone, I am just checking the resistors in my Crown D150A amp that went bad. I have removed all the outputs and drivers and now I am just spot checking some resistors.
With due respect, lifting parts at random hoping to find a "bad one" with no clue or systematic method is not the way to repair anything.
It becomes searching for a needle in a haystack, literally 🙁
What if you actually have a bad cap or transistor or Zeners or poor connection or anything else?
You will not find it.

You may replace all parts and yet not solve the problem, which maybe a cracked track or solder.

Plus every time you pull and replace a part, you stress it and definitely risk damaging or destroying the PCB itself, the only vital component you can NOT buy on its own.

You'll need to troubleshoot it, meaning turning it on (through Variac or bulb limiter), measure voltages, check for power transistor shorts, eventually inject signal and measure or scope it, etc.

You WILL pull and measure some resistor ... WHEN/IF troubleshooting points at it , not at random.
 
You won't hurt my feelings @JMFahey, that is why I am here to learn.

The transistors I took out to test them, so there is the reasoning behind that one, this is what I was told to do by some forum members.

Your comments are well received and if you have any other suggestions, I am open to hearing them. At this point I am looking for anything obvious and then also going slow, researching, reading and watching and hopefully learning.
 
I have the schematics; I am reading the service manual. As I spot check resistors, I am also seeing where they fall into the schematic, what they may do, things like this...so this is all a learning process, not necessarily looking for a smoking gun but if I happen upon one, bonus!!

I have also noticed some people saying to look visually, see if there are resistors that are overheated, or showing any other obvious signs. As I have watched other videos, I have seen people talking about resistors that are common to drift, so again, this is also my reasoning behind spot checking resistors. But I get your point, do more than just a resistor check, understand what its purpose is there...I get it.
 
By the way, the two resistors that I am trying to figure out why they are reading low, they are right off the c1 and c2 input. Again , learning and observing. Could this mean the my input signal, could have damaged them? Is this enough reason to pull these and check them? Any idea how I could troubleshoot this input path?

Thank you
 
As a VERY basic starting point.
1) define what your problem is.
In this case, define "went bad"
  • Does it not turn on, including no lights whatsoever?
  • "Turns on", sort of, lights on but no sound of any kind?
  • Turns on, lights on, but thump/pop/hiss/hum on output?
All these through a bulb limiter just for safety, 40W to 60-75W fine
* Does limiting bulb blink - shine bright/orange/barely visible/”do nothing"?

Notice even the most basic initial test already starts growing many branches 😫
Trying to cover all ramifications soon becomes way too much for a Forum or even 1 on 1 consultation.

And then how do Techs do it?
Basically, they have to take a fresh decision at every branch, at every new doubt, there are not Universal solutions, each amp has its own problems; often similar but hardly exact same as others.

In any case, answer which of initial 4 possibilities applies, might suggest something.
We might get lucky (educated lucky that is, not roll the dice lucky) in the first couple tests.
 
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clip- Not necessarily. There's a winding technique where the resistor is made from two coils wound in opposite direction and tightly coupled magnetically. It's known as Ayrton–Perry winding. That effectively cancels out the inductance.
-end clip

Tom
Certainly better but less effective at high frequencies. I'd use it in an audio path, but not in a feedback loop. The oddest one I ever saw was a big power resistor wound in two opposite direction layers using resistive tape. It was pretty much non-inductive, but had high THD at 10 kHz and above. Magnetic interaction? Not a clue. Somewhere I have an old book that shows at least six non-inductive winding patterns/methods.
 
Hi everyone, I am just checking the resistors in my Crown D150A amp that went bad. I have removed all the outputs and drivers and now I am just spot checking some resistors. I have one that in the schematic/parts lists shows as 2M ohm (confirmed red/blk/grn gold) and I am measuring in place and I am getting about 48k ohm. I am new to electronics, but am I missing something or is this resistor compromised?

Thank you!
There might be something in parallel with the resistance, lowering your reading. Best to remove one end of the resistor from the circuit and measure the resistor by itself.
 
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Actually I was thinking more of high bandwidth stuff. Decent audio opamps have response out to, I dunno, 50 MHz for some. The feedback resistor needs to look like a resistor out there for stability, though a small cap can help roll things off. Metal film, carbon film, oxide and even lumps of compressed charcoal do well, but I'm wary of non-inductive wire-wounds.