MP1 1.38 - Is my Chorus failing???

[skarkowtsky]

Hi everyone,

I have a stock MP1, 1.38. I've never explored the Chorus before, though I've heard all the great things. Indeed it sounds great, but once I dial the Depth past 0.3 is quickly becomes warbley. The preset is custom, S.S, and squeaky clean. I bypassed the rest of the rack to shape the Chorus, so I know the other effects aren't influencing it.

The Rate sounds good throughout the frequency band, but I can't imagine the Depth only sounds good within 3 cents.

Do you guys have any thoughts?

Thanks!
John

[rnolan]

Not heard of this one before  .  The Depth sets the delay time (IIRC using an analogue bucket brigade delay chip) (i.e. up to about 30 to 40 ms for chorus, past that it becomes a distinct delay/echo).  I routinely ran my MP-1 chorus Depth at 100 and Rate = 0 to get a fattening/double tracking effect in stereo set up (sounds great BTW).  How does it sound when you set the Rate to 0 and increase the Depth ??  Maybe the delay chip needs replacing, though may be hard to find these days.

[skarkowtsky]

Hey, thanks for the reply! It's morning here in NY, but I managed to sneak into the guitar room to try factory preset 14 "Sparkling Clean" for a comparison. It's wobbling all over as well, lol.

What would I be replacing internally to restore the Chorus?

Unrelated, but to give you an idea of the current state of my MP1, I recently re-tubed with NOS Chinese 12AX7As from Doug's Tubes. Tone sounds great.

PS Coincidentally, the only Chorus setting I tried prior to last night was Depth at 100 for that widening sound. This was about a year ago and it sounded great. Something has happened since.

Thanks!
John

[skarkowtsky]

My apologies, it's the Rate control that's the issue, not Depth.

[Harley Hexxe]

I think this is a job for MJMP.

           It sounds like a resistor may have gone south.

           Have you opened the top panel and looked inside for anything that may be physical damage? i.e. cracked resistor, or even burnt one, etc.

Just a thought. I think one of those might be the culprit instead of an IC chip/

Harley 8)

[skarkowtsky]

Hi Harley,

I was just inside a couple of weeks ago to swap out the tubes. Everything looked intact, no leaky or ruptured caps, no corrosion or damaged components.

I have a theory about what might have caused this. The unit just beneath the MP1 is a Roland SRV-2000 which throws A LOT of heat. Although it's the normal operating temp for that unit (even states it on the casing), I think it slowly cooked the MP1 over time.

I'm about to re-rack with a blank slot between the two. But yeah, I reached out the JMP, waiting to hear back.

[skarkowtsky]

UPDATE

I appeared to have fixed it. After opening the top and bottom tonight, I did a thorough visual inspection of every component, but no evidence of physical failure. This was also the first time I've seen the bottom of the PCB.

I gave everything a light dust with an old badger bristle lathering brush, while I was in there. For the sake of experimenting, I rolled the new tubes and the old JJs I replaced a couple of times, and to my surprise the chorus sounded great. No warble unless I really push the rate frequency, as expected. Perhaps there was some oxidation or dust in the sockets??

I did notice the large trace on the bottom of the PCB is blistered in spots, though the trace is intact, not tears or splits. Please have a look at the photos and let me know if this is a concern.

I'm not sure if this fix is permanent or temporary, but I'm happy for now.

Thanks!
John

[Harley Hexxe]

   Hi John,

             You rolled tubes and it fixed the chorus issue?!? The tubes don't have anything to do with the chorus as far as I know, but it does narrow down the area in the preamp where the issue is.

             If the traces aren't lifting off the PC board, and they aren't broken, it shouldn't be a concern. It does sound like that might be related to the heat issue you mentioned. The preamp tubes do give off heat too, so I always try to leave the top of the preamp open and try to get some kind of ventilation into the back of the rack for that reason.

             Sorry I can't be more helpful about this.


Harley 8)

[skarkowtsky]

Yeah,I know it doesn't make sense, lol!
Maybe the light cleaning did it, or when I brushed the board I moved something back into place?? I really don't know. If it goes wonky again, out for service it goes.

I'm thinking of replacing the Roland unit with something that doesn't get as hot, too.

[Harley Hexxe]

Hi John,

         You know, theoretically, no solid state device should ever get so hot that you can't keep your hand on it for 10 seconds without pulling it away.

         If that Roland unit gets that hot, it may have some kind of issue going on. Just saying.


Harley 8)

[skarkowtsky]

It's actually considered normal operation by Roland. It gets ripping hot!

Sticker that on the casing below.

[MarshallJMP]

Like Harley already said, the tubes have nothing to do with the chorus, so very strange.

As for the pcb, that's normal with older pcb's. Seen this a lot with the older mp-1's and other older equipment, it came out of the factory looking like that. It has to do with HASL or hot air solder leveling, where a layer of tin is applied on the traces before the solder mask is applied. When it was not applied smoothly you got this wrinkling, it could also happen during the wave soldering if tin was drawn under the solder mask. Nothing to worry about.