I think i have a grounding issue. The amp does something strange when all pots are max.
For the pot issue it is if Gain1 Gain2 are both max, and I turn G2 down leaving G1 max. The moment I turn G1 down a "hair" it goes away. The hum stops/starts with G1's wiper (see pic below) rotating up to the point where the dimple meets the same position as the pin 3 on the pot. Below that, the amp pops/crackles, and the humming disappears. This picture shows the dimple just below the rotation point where the amp pops into humming mode.
But heres the wierd thing, if i put my finger on the backside of G2 pot, this crackle then humming doesnt happen! Martin manning suggested it might be a bad pot but it didnt ever do this before. The finger makes me think its a grounding issue, but i'm clueless...
If it were a grounding issue, I don't think it would go away with a small turn of the pot. Martin is a sharp guy, I think I would try his suggestion first.
I have read many posts where people have gotten bad PEC pots that are brand new. I personally have had a recent issue with them also.
My problem was that I was using a pair of pliers to bend over the front tab. Problem was the metal for the pin I was bending over was much more heavy duty than the metal on the back of the pot. By the time I got the tab bent over I had dented the back of the pot. The clearance between the back and the working parts inside is minimal, and if you bend the back at all it can make contact and ground the wiper assembly inside. Since the assembly has "bumps" it was working for over half of the rotation, then the bump would contact the back and all hell ensued. Switching out the pot is a pretty simple task. I would try this first.
I will order a pot. However, just to reiterate, it is pot 1 wiper tHat does the weird humming only when both 1 and 2 are max, and holding finger to back of pot 2 fixes pot 1, so if pot 1 was bad, shouldnt it always behave the same?
I also didnt touch pot 1 when i replaced the shieldedrun from the mv. Pot1 worked fine prior to that.
My theory was that the wiper was losing contact when it got to full CW rotation, and that's why I suggested shorting the wiper lug to lug three when the humming was going on. Did you try that? I don't know why touching G2 would stop it though... need to think about that some more. Humming can certainly be indicative of a grounding problem.
Last edited by martin manning on Sun Jan 23, 2011 12:35 am, edited 1 time in total.
I had an Alpha pot that when it was turned all the way down it made a loud buzzing, all I can figure is somehow the wiper lost contact with the trace.
I replaced the pot and the problem went away.
So there is a chance that the pot is bad. I would simply try another one.
Also, on your soldering, try to fill the pot eyelets with solder.
Not so much that it drips out but you want a full eyelet.
The solder joints on the pots probably work fine, I'm just saying as a general soldering technique you should use a bit more solder.
I guess the pot itself is still the prime suspect in my mind. You will probably just want to get a replacement Alpha, but check the bushing size, there are 8mm and 3/8" versions. Other brands will work fine, maybe better, but I like to have all the pots match, having the same shaft and bushing lengths for appearance, as well as the same feel when rotated. A typical audio pot will be about 0.25W, but just look for a 24mm Alpha 500k log taper (A500k) and you'll get what you need.
1224]http://chicagocadcam.com/ChrisHahn/POT.jpg[/img]
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