Problem with Tube Voicings on Classic

[BradS71]

Hi Guys,

I've been loving my MP1 Classic, but yesterday out of the blue it developed a weird issue. It just started make a low-level crackly, farty sound whenever I play. I fiddled around with it and discovered it only does this on the tube voicings. The solid state presets all sound fine. So I thought bad tube, right? Well, I tried 4 different sets of 12AX7's of various brands and get the exact same results with all. So it doesn't appear to the tubes themselves.

I posted a YouTube video here so you can see/hear the issue:

https://youtu.be/P5i-Hhqy7W4

Any insight/guidance would be very much appreciated.

Thanks,
Brad

[MarshallJMP]

Could be R151, these go bad sometimes and then you can get this.(not always but it can). I'll take some pics tomorrow where to look for it.Do you have a multimeter?

[Dante]

Hi Brad,

No worries bro, you're in good hands. MJMP answered you in less than a half hour!! whow!

I love my Classic too, and will be watching this thread to see what happens.

Dante

[BradS71]

Wow. You guys are awesome. Yes, I have a multimeter. Really appreciate the help.

[MarshallJMP]

Check this resistor with your multimeter, (do not turn the classic on!!) it should be 180k ohm.

[BradS71]

Check this resistor with your multimeter, (do not turn the classic on!!) it should be 180k ohm.

I'm getting 34.5 MΩ! That's 34million Ohms, right? I checked it multiple times to make sure I wasn't crazy. I'm guessing you've found the problem!  :D

[MarshallJMP]

Yeah these go up in value so replace it and it should solve your problem. If not we'll look further !!

[rnolan]

Wow, I hadn't seen a shot of the inside of a classic before, you can see the advance in PCB compared to MP-1/2

[Harley Hexxe]

Yeah Richard,

     It looks more like the PCB of the MP-2 than the MP-1. I remember posting that a long time ago when Jurrie was still with us.

[MarshallJMP]

It doesn't always looks like that, I attached a pic of my classic pcb.

Downside of the classic pcb's is that they are a b**ch to work on.

[BradS71]

Downside of the classic pcb's is that they are a b**ch to work on.

Yes they are! Which is why I still haven't gotten this done. I'm not quite savvy enough to do this one cleanly and I have a good friend who is an ace with the iron and working in tight spots on PCB's that I would rather have do it...but between his and my travel schedules we have yet to meet up and get this done. I will report back once we do. Thanks again!

[MarshallJMP]

Good thinking, if you're not 100% sure let someone else do it.

[BradS71]

Well...I was finally home for a few days and was able to get the unit over to my buddy's place and work on it. We put in a new 180K resistor in R151 and...Boom! It works perfectly!

Thanks so much for the help. Truly awesome.

[rnolan]

  great news  :thumb-up:

[Dante]

Sweet!  :thumb-up: