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Author Topic: MP1 No Overdrive/Distortion  (Read 1996 times)

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MarshallJMP

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Re: MP1 No Overdrive/Distortion
« Reply #15 on: Time Format »

Ok these mp-1's have the "black box" input jack and these can fail sometimes.So try to plug in your guitar chord and wiggle the jack around in the input, see if that changes anything.
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #16 on: Time Format »

I did try that and made no difference. I am not with the unit at the moment. Come to think of it when plugging in the guitar cable it does not have a nice smooth "plug-in" feel. Is there a suitable replacement for it?

Ok these mp-1's have the "black box" input jack and these can fail sometimes.So try to plug in your guitar chord and wiggle the jack around in the input, see if that changes anything.
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MarshallJMP

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Re: MP1 No Overdrive/Distortion
« Reply #17 on: Time Format »

Ok, now this jack has 2 functions,it recieves your guitar chord but there's also a SPDT switch inside which does the switching between the front and rear jack.So if you insert your chord in the front jack it will switch the input buffer to the OD1 circuit. So it also could be that this switch is still closing but the resistance of the contacts is too high.

Now there no replacement I know. I had these but I ran out of them and I can't find them anymore in small quantities. So the best thing to do is the rear jack mod. This involves replacing both front and rear input jacks. The advantage is that these are jacks are available and you can also use the rear input for guitar, now it's a low impedance line input and suited for guitar use.

Maybe you can try to spray some good contact cleaner in it and see if that helps, if it does you know for sure that this is the problem, but even if it helps you will need to replace it because the problem will come back after some time.
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #18 on: Time Format »

Ok, thank you. The more I think about it and read online the more I'm convinced its the jack. The front and rear inputs basically sound the same so it seems to be stuck in the rear input mode.

I guess this is the way to go: http://adadepot.com/mods/ADA_MP-1_v1.xx_Rear_Input_Jack_Conversion_Mod.pdf

Is that correct?


Ok, now this jack has 2 functions,it recieves your guitar chord but there's also a SPDT switch inside which does the switching between the front and rear jack.So if you insert your chord in the front jack it will switch the input buffer to the OD1 circuit. So it also could be that this switch is still closing but the resistance of the contacts is too high.

Now there no replacement I know. I had these but I ran out of them and I can't find them anymore in small quantities. So the best thing to do is the rear jack mod. This involves replacing both front and rear input jacks. The advantage is that these are jacks are available and you can also use the rear input for guitar, now it's a low impedance line input and suited for guitar use.

Maybe you can try to spray some good contact cleaner in it and see if that helps, if it does you know for sure that this is the problem, but even if it helps you will need to replace it because the problem will come back after some time.
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MarshallJMP

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Re: MP1 No Overdrive/Distortion
« Reply #19 on: Time Format »

Yes it is.

Now it's not stuck in rear in mode because then the front jack would not work at all. Also the rear input has less gain. So if the it sounds the same gain wise then this could mean that it's actually the input jack. Although I can't be sure 100%.,it also could also be the input buffer ,but these don't fail often. In 95% of the cases gain problems are either tube related or input related.

Now there's also something else you can try, if you have a boost pedal you can try to plug in your guitar into the boost pedal and connect the out of the boost pedal to the rear input, see if you get more gain this way.
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #20 on: Time Format »

Ok, I will try that tomorrow. Like I said earlier that when you plug into the jack it doesn't feel right at all. It's tight and doesn't have the "click" when plugged in all the way like a good working jack.

I have not found the exact stereo jack in the link I emailed. Would a stereo 9 pin work and just use one set of switching contacts? That should do it....right?

Thanks again!!!.

Yes it is.

Now it's not stuck in rear in mode because then the front jack would not work at all. Also the rear input has less gain. So if the it sounds the same gain wise then this could mean that it's actually the input jack. Although I can't be sure 100%.,it also could also be the input buffer ,but these don't fail often. In 95% of the cases gain problems are either tube related or input related.

Now there's also something else you can try, if you have a boost pedal you can try to plug in your guitar into the boost pedal and connect the out of the boost pedal to the rear input, see if you get more gain this way.
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MarshallJMP

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Re: MP1 No Overdrive/Distortion
« Reply #21 on: Time Format »

I didn't get an email?
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #22 on: Time Format »

Ha....sorry I meant the link I posted not emailed... This:  http://adadepot.com/mods/ADA_MP-1_v1.xx_Rear_Input_Jack_Conversion_Mod.pdf

I didn't get an email?
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rnolan

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Re: MP1 No Overdrive/Distortion
« Reply #23 on: Time Format »

Hey Billy Penn, welcome to the depot  :wave: .  So reading these posts I agree it's the input jack. And, as the input jack takes the most wear over the years, it's a common problem particularly on the early (older) V1 units (with top switch, as you have).  The rear jack mod replaces the front and rear input jacks and also changes the rear jack to be inst (guitar) level, it's currently line level (as MJMP said, if you boost your guitar signal with something that puts out line level and plug it into the rear jack that should give you enough level, but this is a test scenario).

Easiest fix is to do (or get done, how are you with a soldering iron ??) the rear jack mod.  You can buy the mod kit from MJMP here (http://www.marshalljmpmodshop.net/miscellaneous_parts_and_replacement_tubes.htm), it's not a hard mod to do but if your not into a bit of soldering, any decent tech can do it.
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