Strange noise on distortion voicing only

[mgiglio]

Hi, I just bought an MP-1 Classic off of ebay and everything seems to be okay except for the distortion voicing...there is this loud pulsating motorboat kind of sound in the background. The "brown" voicing doesn't seem to be affected and works fine. I can't seem to find the schematics to see how the different voicings are routed or any information on this problem.

[MarshallJMP]

Mail me and i will send you the schematics

[rnolan]

Hey  mgiglio, search for motor boating here, will send you off to a good wiki page (or look up wikipedia on motorboating, good article).  To me, sounds like new tubes will help/can't hurt...
What are your patch settings when you get the motor boating ?

[rnolan]

BTW, clear the unit (unless ther's stuff you want to keep) and load Factory defaults.

[mgiglio]

Yeah, I cleared everything, reset the unit. I also put two brand new tubes in. Neither had any affect. I set the eq to flat and disabled the noise gate, loop and chorus...the motorboat sound turned to a steady loud low frequency hum after the gate was disabled. the motorboat sound was this loud hum bleeding through the gate causing it to chatter. the sound is still there however, just not pulsating. the only control that affects the sound at all is the master gain. neither overdrive 1 or 2's settings make any difference. the master gain just turns the volume of the hum up and down. SS clean, tube clean and brown voicings all sound fine and don't have this background hum.

[Sparker]

Could be a noisy HV power supply, 200V regulator circuit for instance (Q47, Q47, R382 + associated resistors & caps). Could start by looking at the HV supply with a meter, as well as the other PSU voltages while your there.

[mgiglio]

MJMP sent me the schematics. He also suggested checking R382 as my reading on the HV test point was high. I'm going to replace that along with the electrolytic caps and see what that does. These probably don't have anything to do with it but since i already have the correct ones laying around, i'm going to also replace the three LDR's.
I'll let everyone know how it goes when i get all the parts in.

[rnolan]

Cool, hope it works our well  :thumb-up: