MP-1 A channel much quieter than B

[Mxzx]

I just bought a MP-1, and know very little about it.  I did download the owners manual and have read that.

I don't have a power amp, so just running into front of a Marshall SuperLead.  I do have the switch on top set to the forward position.  When I plug the amp into A output, it is very weak and thin, when swap to B output it is very loud and full?  I have not tried running both outputs together.

i did swap the tubes around, and that didn't make any difference.

Any ideas?

Also noticed this, is this stock or a mod?



Thanks!

[hal2000]

I only can say something about that orange capacitor (you mean that?). As far as i know, my mp1 was not modded and this section in your photo looks exactly the same as mine.

[MarshallJMP]

yes this cap is standard.

Try to plug in a headphone in the headphone out and listen,are both outs (L and R) the same through the headphone?Let me know.

[Mxzx]

Thanks for the info.

When I first fired unit up, I tried the headphone jack with a set of headphones.  They were the same except one side was cutting in and out, like a dirty pot.  I cleaned but haven't retried it, I'll do it tomorrow and report back.

Also, what exactly does the switch on top do?   I just read the warnings about having it forward for guitar amps, but they don't say what i does.

Thanks

[rnolan]

The switch changes the MP1 A and B outs to instrument level (millivolts) to plug into your guitar amp (clean) input OR line level (~1 volt(s)) to plug into a power amp (which is designed to be driven by a separate pre-amp/mixer etc at line level).  Your guitar amp has a pre-amp and power amp and it's pre-amp is designed to take guitar (instrument) level signal.  If you send it line level, you'll overload the input (badly and may cause damage).  If you plug the ADA set to instrument level into a power amp, it won't have enough herbs to drive it and it will sound week, thin, yuk, noise etc.
The new(ish) ADA site (http://www.adaamps.com/index.htm) has suggested pre-amp eq settings in the ADA APP-1 user manual for a bunch of guitar amps (Marshall included) to provide the cleanest flatest response (no add colour) for people who want to run their ADA APP-1 into a guitar amp.  These are just as valid for MP-1 into guitar amp.  They are on page 15 of the APP-1 user manual (attached), it also includes some other good advice regarding using a guitar amp with these pre-amps.

[MarshallJMP]

Thanks for the info.

When I first fired unit up, I tried the headphone jack with a set of headphones.  They were the same except one side was cutting in and out, like a dirty pot.  I cleaned but haven't retried it, I'll do it tomorrow and report back.

Ok keep me updated.

[Mxzx]

Just tried with headphones and it sounds perfect, even the knob isn't scratchy after the cleaning!

The A side is still very quiet and B side normal with headphones plugged in..

Thanks

[MarshallJMP]

Ok,that narrows it down to a couple of components.Possible culprits are;
-The front vol pot ==>is it scratchy when you turn it?
-Opamp U10 ==>NE5532 opamp,it's located just behind the vol pot.
-The top switch==> you can try to rock it up and down a few and see if that helps.Altough i doubt this switch would case a problem but you never know.
-Caps C56 or C49==>What you can try is set the master volume on 8.0 or 9.0 and set the front vol pot CW (clockwise,fully open).If suddenly the sound comes trough it's usually one of these 2 caps.Now this doesn't work always but you can try it out.
-Output jack.

[Mxzx]

Okay, thanks.  In order of questions:

1. Vol pot was scratchy, but contact cleaner in pot took care of that!
2.  Let me know what I need to check for the opamp
3.  I have moved the switch, doesn't make any difference.
4.   I will try that
5.  Let me know what I can do on output check, guessing check solder connections.

Thanks

[MarshallJMP]

2. Best would be a scoope,or just replace it,it's not an expensive thing,just a few $
5. Don't know if this is going to work but try it.Locate connector B.It's just above the vol pot.It's a small red connector,and in your case the middle wire has been cut and wrapped around the other two.Disconnect it and turn 180° and try to put it in that way.Normally this won't go but try it anyway.This way you will reverse channel A and B.If the jack is fine A should work and B doesn't.

keep me updated.

[Mxzx]

Ok, I'll just replace the opamp. where would one buy such part?

I will give connector B a try and report back the results.

Thanks!

[Mxzx]

Just pulled the plug and switched, and yes the A output was loud and B quiet when switched.

Now what?

Thanks again for all the help!

[MarshallJMP]

Ok so we can rule out the jack.Did you try the volume thing i mentioned?

[Mxzx]

Yes the vol pot was scratchy at first, put some contact cleaner in it, now quire when turned up and down.

Thanks

[MarshallJMP]

Try this first;

Caps C56 or C49==>What you can try is set the master volume on 8.0 or 9.0 and set the front vol pot CW (clockwise,fully open).If suddenly the sound comes trough it's usually one of these 2 caps.Now this doesn't work always but you can try it out.