Volume pot issue

[theHarp]

Hi all, I'm having a great time listening to my Crack.  If I could figure out an issue with the pot, I think I could not be happier with this thing.

When I adjust the volume at about a zero setting, I hear a slight scratchiness, and sometimes a hum or buzz starts, but it goes away as I progressively adjust volume upward.
Then, about 1:30 through about 7 ish o'clock, the scratching and humming return, and the audio from the left channel cuts out. Past that, again it is fine.

I just checked the connections at the pot, and all seems fine. I spread a couple of prongs a little further apart that looked close to me.
Any suggestions at all would be appreciated.  Could I have a bad pot, do you think?

I really am loving this amp though.

[Paul Birkeland]

Ah, this is a lovely issue to troubleshoot in terms of a learning standpoint!

To start with, I would recommend listening to the amp, then getting the pot adjusted to just the right point where the left channel cuts out.  Once the pot is perfectly in place, tape it down with some masking tape.

Now that you have this spot locked in, flip the Crack over and run through voltage checks.

Additionally, with the Crack off, meausre the resistance between the center lug of each level of the pot and its two adjacent terminals.

These numbers will tell us a ton about what might be wrong.

Also, you should verify that you can reproduce all of these symptoms without anything connected to the Crack.

-PB

[theHarp]

OK, so I unplugged my source, and duplicated the behavior.

Re-tested voltages with the volume knob right at where the "scratchiness" begins:

1     83.5
2     174.5
3     .001
4     174.5
5     83.5
6     .01 (jumps around a little)
7     106.8
8     .001
9     106.4
10   .01 (jumps around a little)
11   .001
12    0
13    174.6
15    196
20    0
21      217.5

a1   83.4
a2   .001
a3   1.5
a4   0
a5   0
a6    83
a7   .001
a8   1.5
a9   .001

b1   82.9
b2   173.6
b3   106.3
b4   83
b5   174.5
b6   106.2
b7   0
b8   0

Also checked resistance in the pot (keeping the black lead on center, and the red on the in and outboard lugs)

upper center to inboard:           16.7 ohm
lower center to lower inboard:  16.25
upper center to outboard:        80.4
lower center to low inboard:     73.7

Please let me know what these numbers mean to you, or if you need any other measurements.
Thanks so much, Dan

[Paul Birkeland]

That shows perfect operation!  It is possible that there is debris inside the potentiometer, I would recommend swooshing in a little contact cleaner to see if that helps.

Even with the pot in the scratchy spot, the pot is still working, and the amp is electrically happy, so that is all quite nice to see.

-PB

[theHarp]

OK, wow, that sure seems to have done the trick.  I got some lubricating contact cleaner from Radio Shack and sprayed it on the contacts and into the pot itself. Now, not only is there no more scratchiness, buzzing, humming, or sound cut-outs, but the volume adjustment is MUCH smoother.

Wow, this amp is great!!

Thanks Paul for your help with this.

[Paul Birkeland]

You're very welcome, enjoy the amp!

-PB