Scratchy volume pot

[ken_j]

After reading the Bassman thread discussing in part some scratchy pots I thought that I would request some help in diagnosing a scratchy volume control in my Mesa Subway Blues. All of the other controls are quiet. The schematic can be found here: http://www.schematicheaven.com/boogieam ... yblues.pdf

Here are the voltages that I found on the pots. I measured all voltages with a Fluke 87 III multimeter. Facing the shaft side of the pot, the lugs are labeled L-left, W-wiper, & R-right. The negative lead of the Fluke was attached to chassis ground.

* The Mid and Bass controls have no connection at the L lugs. The volume control is after the tone controls in the circuit if I am reading the schematic correctly.

With the volume control at the minimum position here are the voltage readings:

Vol. L= 0 W= 0 R= 0
Treb. L= 0 W=0 R= 0
Mid. L= 0* W= 0 R= 0
Bass L= 0* W= 0 R= 0

With the volume control at maximum position here are the voltage readings:

Vol. L= –0.003 W= -0.003 R= 0
Treb. at min L= 0 W= 0 R= 0
Treb. at max L= -0.023 W= -0.023 R= 0
Mid. at min L= 0* W= 0 R= 0
Mid. at max L= –0.003* W= -0.003 R= 0
Bass at min L= 0* W= 0 R= 0
Bass at max L= -0.024* W= -0.024 R= 0

The Highest voltage I obtained on the volume pot was approximately ¾ of the way turned up. At that position the wiper read –0.035 volts. The voltages at the other pots were closer to zero but still all negative.

[soundmasterg]

Is the pot scratchy all the time or only at certain times or when the pot is in a certain location in it's travel? It may just need to be cleaned with some electronic circuit cleaner like DeOxit. You spray the cleaner in the back of the pot and rotate the pot shaft and it cleans up loose dust and dirt inside the pot. Some cheaper pots don't allow you the spray hole to clean them, in which case the pot has to be replaced. You would have to open the amp to see if there is a cleaner hole to spray cleaner in the back of the pot or not.

Usually a DC voltage on the pot issue like in the Bassman thread is a higher voltage and happens at all times, and you also hear more hum often in a case like that. Your voltages are a bit low to be the DC voltage due to a bad cap problem, though it could be.....it depends on the symptoms.

Greg

[ken_j]

The pot is new (purchased from Mesa). I replaced it because I had to turn it a bit and then it would kind of jump on. I also tried cleaning it with Deoxit. Today I put the old pot in to see if there was a defect in the new one and it does the same thing. It only makes noise when turning the control. It dosen't really bother me but I thought if there is a problem I should resolve it before a failure occurs. The new pot is a little better as far as not jumping on but I believe that it is the circuit design that causes it.

Before replacing the pot I tried cleaning the old one along with applying heat from a soldering iron on the lugs. That is a trick I learned years ago at a McIntosh amp clinic.

[beatlefreak]

[quote="ken_jBefore replacing the pot I tried cleaning the old one along with applying heat from a soldering iron on the lugs. That is a trick I learned years ago at a McIntosh amp clinic.[/quote]
Risky trick - It can either clean it or kill it.

[ken_j]

Risky trick - It can either clean it or kill it.

It is done powered down.

[soundmasterg]

The jumping on that you mention is probably due to the taper of the pot. Mesa coul dhave used a linear pot, or they could have used the wrong Log pot. There are Log A and Log B and I forget offhand which is the vintage style that everyone likes, but if they used the other one, then it would have the behavoir you describe.

Given that it still makes the noise then there could be a bad cap. I'll see if I can take a look later at the schematic and see if I can spot a likely culprit.

Greg

[beatlefreak]

Risky trick - It can either clean it or kill it.

It is done powered down.

I realize that. But heat is the killer of electronic components. Using 600-700° from a solering iron for any longer than it takes to make a solder connection risks damaging even a poteniometer - Especially an older one. Besides, while it may remove the scratchiness, it would most likely be temporary, as the heat will help contribute to the buildup of more oxides which cause the scratchiness in the first place.

[ken_j]

It is a method used as a last effort prior to replacement. In the end I replaced it but with not much better of a result. The new pot direct from Mesa was under $2. The only reason I bought it from them is that it had a "D" shaft and it was the only one I could find readily.

[goofyfoot]

Ken, I know this thread is about your scratchy volume pot and some knowledgeable folks have already chimed in. I can't add anything new to this discussion. But I'm thinking you dig your Mesa/Boogie Subway Blues as much as I dig mine. Hope the speed bump is smoothed out for you, brah. Peace, out.....Goofyfoot.

[ken_j]

I am going to replace the 1mfd, .005mfd and the 120pfd caps that surround the Volume pot. I know it is sort of a shot gun approach. I just hope it helps. I like the wheat grill cloth on your Subway Goofyfoot.

[soundmasterg]

It is a bit of a shotgun approach, but sometimes that works fine. Just make sure to get high enough voltage ratings on the caps, and use the same types Mesa used. They probably used orange drops for two of them and a ceramic for the 120pf I bet?

Greg

[ken_j]

Thanks Greg. I have the chassis out of the amp but need to do some circuit tracing to make sure I find the right ones. I don't have a drawing showing component locations. As soon as I get over this nasty cold/cough I have had for over two weeks I will get back to it. I also want to add a pentode/triode switch.