[Jeff Ward]

My PLC-1 has developed an annoying noise (pops, thumps, etc) when switching inputs. Is there an easy way to clean it or would it require service? It is a MK1, I believe.

[CIAudio]

do you get the pop noise changing inputs with no sources connected?

[CIAudio]

I checked my notes and found that this happened on some early PLC-1's. We made a small change to the micro controller programming to eliminate the occasional pop.
If you send it to us we can reprogram it for you (at no cost).

[Jeff Ward]

I did some further troubleshooting today and now I am even more perplexed. I disconnected the #2 input and then switched to it. There was a loud buzzing (similar to a 60hz hum but dirtier) with no input connected at all. I had thought this was just a simple switching noise.

I use the tape loop for a Grant Fidelity tube buffer, and when I leave that off the noise disappears. The unit switches with no pops. What puzzles me is that the buffer is absolutely silent when switched in with both inputs connected (aside from the initial thump). I don't change inputs all that frequently when the amp is powered up, so this was a big surprise.

I only use 2 inputs and the tape loop, just to check I switched it to 3 and four and was greeted with the same loud buzz. Weird. The obvious workaround would be to turn the tape loop off while switching, but I don't understand why this noise happens at all.

[CIAudio]

The hum on open inputs is probably noise being picked up from the tube buffer (try moving the buffer further away). This is due to no load on the input...
If there was a component connected, there would be no hum. You can always put shorting plugs in the unused inputs if desired.

The reprogramming should fix the pop.

[Jeff Ward]

The hum on open inputs is probably noise being picked up from the tube buffer (try moving the buffer further away). This is due to no load on the input...
If there was a component connected, there would be no hum. You can always put shorting plugs in the unused inputs if desired.

The reprogramming should fix the pop.

Thanks. I'll see about sending it to you once we get past the holiday.

[Meicheng]

Thats what I call service, and why I happen to own a couple of CIA products, in addition to the fact that they sound great too.  Dusty here trouble-shooting and trying to help.  Thumbs-up!