Park G10 by Marshall - guitar amp question

[Khron]

Or do the fluctuations indicate something else?

You can't / won't ever get accurate readings of a resistor in-circuit AND powered up. In some cases, you'll get wonky readings in circuit even with the power off.

Rather, with the power on, check the voltage drop across that resistor with the power on.

 

[JohnRoberts]

Ok, so I soldered in the new resistor. With the amp turned off, it reads: 324.2, but with the amp turned on the resistor reading begins to fluctuate between: 324.8 - 325.3
Does that seem normal? Or do the fluctuations indicate something else?
Thanks.

Don't do that.

Typical ohmmeters measure resistance by outputting a small current and measuring the voltage drop. According to Ohm's law the voltage will be equal to the current times the resistance. With the circuit energized the current flowing through the resistor will be the VOM current plus or minus the circuit current.

Getting an inaccurate reading is about the least dramatic thing that can happen. Probing around while the device is powered on can release the magic smoke.

JR

 

[ccaudle]

with the amp turned on the resistor reading begins to fluctuate

You cannot measure resistance with the amp powered, the resistance meter is not made to work that way. You measure the resistance (preferably before installing, but at least with power off), and measure the voltage between the two pins of the resistor with power on. Using ohm's law you can calculate the current flowing through the resistor since you know the resistance and the voltage under load.

 

[CJ]

i think the cheap meter i use has a protection diode or something in it because i have made the mistake of measuring a resistor on ohms with the power on and no smoke. yet.