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Author Topic: New (to me) MP-1 and no tube sound  (Read 1666 times)

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scole

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New (to me) MP-1 and no tube sound
« on: Time Format »

So I finally rejoined the ranks of MP-1 owners. While I bought the unit locally, the seller says he bought it off Reverb (original listing here) and I think he may have bought it off an ADA Depot member because there's a website sticker on the top panel. Anyways, The unit is v2.01 and looks to be that way from the factory thanks to the label on the chip. Outside of that, a previous owner had replaced the soldered battery with a newer replaceable battery holder (yay!) and the rear input jack seems to have been replaced since it looks different than the other jacks next to it on the rear panel.

The unit has sound with the S.S. voicing but everytime I switch it to one of the tube voicings, it goes silent (well, there's hiss but no signal otherwise). I wondered if, perhaps, the tubes were blown or bad. I don't have previous experience with tubes but here's what I've done. First, I pulled each tube out and inspected them. They both visually look good with no signs of burning. The front tube is a Mullard 12AX7/ECC83:




The back tube is JJ ECC 83S:




With each tube, I used my multimeter to test continuity from pins 9 to 4/5 and they both were fine & none of the other pins showed continuity. I reinstalled the tubes and then turned the unit on and then took this picture of the tubes after 5 minutes:



I know that it isn't guaranteed that the tubes will glow but neither were in this case. I turned the unit off after this & unplugged it. The tubes themselves were slightly warm to the touch but not hot or scalding. Something was rattling around inside the unit so I took the bottom cover off and found a piece of plastic. It looks like it's part of one of the A/B output jack housings:



I've done pickup & guitar electronic swaps but I'm out of my element with this. Where do I start? Here's a short video where I power up my unit to a patch that uses the S.S voicing. I then edit the patch and try to change it to either of the tube voicings.



Thanks!
Steve

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mrguitar

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Re: New (to me) MP-1 and no tube sound
« Reply #1 on: Time Format »

Hmmm, this is a long shot after watching your video, but make sure the gain settings aren't on zero. I suspect you checked that already. I can't remember if turning them to zero will cause this or not but it's the easiest thing to check first. Set it all to 5. :)
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scole

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Re: New (to me) MP-1 and no tube sound
« Reply #2 on: Time Format »

Hey, thanks for chiming in. Like you thought, I did check that and it made no difference. BUT- I do have great news. I ran down to my local shop today and picked up some new JJs and put them in. Once the unit warmed up, the distortion just faded in. Ahhhhh!  :banana-dance:

Pretty relieved that it was as simple as a tube replacement. I was a little mortified after replacing my tubes because I noticed this red wire that was cut and not connected:



I guess its intentional and I have no idea why its cut but my MP-1 roars. I'll take it!
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rnolan

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Re: New (to me) MP-1 and no tube sound
« Reply #3 on: Time Format »

 :whoohoo!: good to hear it's working  :thumb-up:
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MarshallJMP

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Re: New (to me) MP-1 and no tube sound
« Reply #4 on: Time Format »

It seems the firmware is V2.01 but the hardware is V1. The cut wire is normal on these units, so don't worry about that. You will need to replace those output jacks I guess, but at this stage they are easy to replace. Just take some pliers, heat the holes from the top and gently get them out piece by piece. Then clean the solder from the holes and put some new ones in.
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scole

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Re: New (to me) MP-1 and no tube sound
« Reply #5 on: Time Format »

Yeah, I was looking at an MP-1 sale listing yesterday and it had the exact same cut wire and wrap as mine. I was looking at your site recently and while you do carry replacement output jacks, it looked this this specific jack was currently out of stock. Did I read that correctly?

Through my searches on here, I've seen a lot of references to v1 and v2 hardware but not a description of what the differences are (and how to determine which you have). Is one of those differences the presence/absence of the line/instrument switch for the rear jack?

Thanks!
Steve
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MarshallJMP

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Re: New (to me) MP-1 and no tube sound
« Reply #6 on: Time Format »

I'm out of the front input jack, I have the jacks that are busted on your mp-1. Seems the send jack is also starting to crack or am I seeing that wrong? Anyway if you need something just mail me.

The biggest differences are the top switch and the front jack. Also the V1 rear input is a line and not a instrument level input, while on the V2 it's also a instrument level input. Another difference can be the transformer and the white 4.3 ohm 5W resistor, but this was not on all V1 MP-1's. PCB's are also a bit different between V1 and V2.

Then there are things that "evolved" like going from ZETA C4 to vactrol opto's, reset circuit also had 4 revisions, NMI input on the Z80. Firmware went from V1.36 to 1.37 to 1.38 to 2.00 to 2.01. Some V2.00 were a sort of hybrid between the V1 and V2.
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