AB Systems 410C Power Amp Input Level Pots

[stoneattic]

I have an AB Systems 410C power amp that I picked up used a while back and always felt there was a "difference" between the two channels. So I had it apart to clean out the dust, etc, and noticed that the channels have different pots. Channel 1's is physically smaller, looks older and feels a little better quality than channel 2. Channel is labeled as 50KA and channel 2's is labeled as 50A.

I'm trying to determine which is the correct value (original) and am curious what the incorrect value would do. I would guess that at full volume there would be no difference, but that is assuming that it's just a passive level pot, which doesn't appear that way according to the schematics. It appears to control the gain, or that what I believe.

You folk here know WAYYYYY more than I do and I was hoping for some advice.

The company is sort of still around (new name) and I have been able to get some schematics and manuals. I have what appears to be the correct schematic for the 410C but I have not been able to find a manual. i have not been able to find a pot value on the schematics. From the troubleshooting guide with the schematics and via email with the folks at the current AB International I have been able to determine that the 410C is kind of a cross between the 410B and 900 (not 900A) and 9220 (not 9220A). I have attached all the schematics and manuals that I have found.

Any help, thoughts, etc, would be greatly appreciated.

[Steve Conner]

They are both very probably 50k audio taper pots. I've never seen a 50 ohm audio taper pot, and 50k is a reasonable value for a volume control.

However, every pot manufacturer has their own idea of an audio taper. If one of them got replaced, it's quite possible that if you set the two pots to, say, "3", you would get totally different volumes. They will very probably match when turned up full, but at every other setting, there's likely to be an error.

I'd recommend replacing both of them with two new pots of the same make and model.

[stoneattic]

Thanks for the reply. I've never seen a 50 ohm pot either, but my experience is somewhat limited. Replacing them both sounds like a good plan. I'll give that a shot.

[AkkerKid]

Did you fix it?
I'll need to do the same or maybe just jumper the connection and go full on.
How's it sound now?

[stoneattic]

No, I never got around to it. I think there's more wrong with that channel than the pot, which appears to be the correct value, just a replacement.. The clip light comes on that channel pretty often where the other channels doesn't even if they are getting the same signal. We use the "good" channel for our stage monitors and the "bad" channel for our drummer's monitor. So essentially we us it as an emergency spare since we can live without drummer's monitor.