How can I test for a noisy resistor in-circuit?

[jan.didden]

It's subtle. Perfectly good parts, especially semiconductors, also ract.

Jan

 

[JMFahey]

1) thanks for posting the schematic.

2) it is incomplete, we still need the chopped right half of it.

3) points I suggest grounding which in principle carry audio but have no DC present.

When I mention 2 components, ground their junction which is called a "node": junction of 2 or more components.
On some components (J1) pin identification is straightforward by itself.

So on succession ground, one at a time, nodes:

• J1 hot pin
• J2 hot pin.
• R53/L1
• R58/R81
• R69/R92
• V3 (V4) pin 7
• R107/R108 using a capacitor, 250V, value around .047-.1uF

It will not fully mute noise (if present) but make it much duller, good enough for detection.
* V7(V8) pin 7, again using capacitor same thing.
If capacitor is connected with preamp ON it will pop loud, if preamp is turned OFF, cap is added and then turned ON, it will not pop but noise reduction will be evident
* C57/R79

Guess there is more, but image is truncated there.

Also can't find SW6 anywhere.

But now you get the procedure: audio ground every gain stage or switch/connector along the Audio path, trying what mutes noise, what "does nothing".

Points I suggested cuts path into many sections.
Once a suspect is found, you focus there.

Good luck.

 

[wrenchone]

SOADMT -
I suggest a 100X probe because is rated for the voltages you are likely to see in the circuit, with plenty of safe margin. Look up the max voltage rating on a typical 10X probe - you might be shocked.