ADA Depot - A Forum To Support Users of ADA Amplification Gear

Let's Get Technical => Troubleshooting Problems => Topic started by: Billy Penn on July 11, 2016, 07:05:26 AM

Title: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 07:05:26 AM
Hi all. First post here and I was hoping someone could help me out with an original MP1. It seems that the overdrive/distortion is not working. Both 12AX7 tubes are new and known to be good. I notice if I pull both tubes the unit still works but obviously no overdrive/distortion. Has anyone ever had this issue before? Any thoughts on what it could be? Thanks in advance!!
Title: Re: MP1 No Overdrive/Distortion
Post by: Harley Hexxe on July 11, 2016, 08:18:22 AM
Hi Bill,
  Welcome to the Forum. You say it works without the tubes in the sockets? That would have to be a solid state voicing then. Is that what the front panel indicates?
  That seems very strange that you get no overdrive. I assume you raised the levels of OD1 and OD2 and got the same result?

       Harley 8)
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 08:34:52 AM
Thank you for the reply Harley Hexxe! Ok, I just tried it again and when the OD led is lit I do have control over the overdrive (OD1 & OD1). The overall gain and distortion seems low to me. In other words it should have more gain in my opinion. Another note is that the Clip led's do not light up when I strum hard on the guitar. Could it be that the lack of gain is an issue with an op-amp... U4?
Title: Re: MP1 No Overdrive/Distortion
Post by: MarshallJMP on July 11, 2016, 09:11:26 AM
I agree with Harley,if you pull the tubes it shouldn't work on the tube voicing,only the SS voicing.

What settings do you use,and maybe if possible could you post a soundclip?
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 09:46:22 AM
Ok, I just tried it again and when the OD led is lit I do have control over the overdrive (OD1 & OD1). The overall gain and distortion seems low to me. In other words it should have more gain in my opinion. Another note is that the Clip led's do not light up when I strum hard on the guitar. Could it be that the lack of gain is an issue with an op-amp... U4?


I agree with Harley,if you pull the tubes it shouldn't work on the tube voicing,only the SS voicing.

What settings do you use,and maybe if possible could you post a soundclip?
Title: Re: MP1 No Overdrive/Distortion
Post by: MarshallJMP on July 11, 2016, 10:27:20 AM
If the OD1 clip goes on the U4 opamp is fine.
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 11:36:00 AM
The OD1 clip does not go on. None of the clip led's go on. Should I suspect U4 opamp then?

If the OD1 clip goes on the U4 opamp is fine.
Title: Re: MP1 No Overdrive/Distortion
Post by: MarshallJMP on July 11, 2016, 11:45:08 AM
Ah you said that the OD1 led is lit?

You can can try to raise the OD1 trimpot,there's a pic on the forum with a list of the trimpots.Just do a search.
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 11:50:00 AM
The OD LED I was referring to is actually the Dist LED. Sorry for any confusion. I am not familiar with this unit. I will check out your trimpot suggestion. If U4 was bad would I get any sound at all? Thank you very much!

Ah you said that the OD1 led is lit?

You can can try to raise the OD1 trimpot,there's a pic on the forum with a list of the trimpots.Just do a search.
Title: Re: MP1 No Overdrive/Distortion
Post by: MarshallJMP on July 11, 2016, 12:00:19 PM
Don't think U4 is the problem, else you would have no sound at all.

I attached the pic,quality isn't great but just search for the OD1 trimpot,you can turn it up and see if that helps.If you turn it fully CW and you set OD1 at 8.0 or more the OD1 clip led should go on when you play.
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 12:10:34 PM
Thanks for the photo. That does help. Adjusting the OD1 gain at 10 and turning up the OD1 trimpot gets me a little more gain but does not light up the clip LED. The trimpot doesn't affect the overall gain or volume much at all. When I first power up the unit all four clip LED's do blink. Any more thoughts on this? Thanks again..!

Don't think U4 is the problem, else you would have no sound at all.

I attached the pic,quality isn't great but just search for the OD1 trimpot,you can turn it up and see if that helps.If you turn it fully CW and you set OD1 at 8.0 or more the OD1 clip led should go on when you play.
Title: Re: MP1 No Overdrive/Distortion
Post by: MarshallJMP on July 11, 2016, 12:33:53 PM
It's normal at startup that the 4 clip leds blink.

What input jack do you use? Front or rear?
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 02:03:42 PM
I'm using the front input jack. This unit just seems ot have like half the overdrive gain it should have for some reason. Any more help is appreciated. Thank you!


It's normal at startup that the 4 clip leds blink.

What input jack do you use? Front or rear?
Title: Re: MP1 No Overdrive/Distortion
Post by: MarshallJMP on July 11, 2016, 02:32:35 PM
Does it have the topswitch?
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 02:45:02 PM
Yes. One position is low output and the other position is higher volume output. Does not change the gain however.

Does it have the topswitch?
Title: Re: MP1 No Overdrive/Distortion
Post by: MarshallJMP on July 11, 2016, 03:02:54 PM
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.
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 03:05:21 PM
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.
Title: Re: MP1 No Overdrive/Distortion
Post by: MarshallJMP on July 11, 2016, 03:41:55 PM
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.
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 03:47:45 PM
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.
Title: Re: MP1 No Overdrive/Distortion
Post by: MarshallJMP on July 11, 2016, 04:10:09 PM
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.
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 11, 2016, 04:22:50 PM
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.
Title: Re: MP1 No Overdrive/Distortion
Post by: MarshallJMP on July 12, 2016, 03:18:58 AM
I didn't get an email?
Title: Re: MP1 No Overdrive/Distortion
Post by: Billy Penn on July 12, 2016, 04:30:26 AM
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?
Title: Re: MP1 No Overdrive/Distortion
Post by: rnolan on July 12, 2016, 06:38:00 AM
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.