Gain loss

Iron25 · May 05, 2015, 11:11:40 AM

I have been having issues with gain loss. Not the tubes. Had this problem several years ago and don't remember what the fix was...after playing awhile the gain just drops. Did notice my input jack crackling a bit but reseating the plug seemed to remedy that...anybody remember the gain problem fix? Was it a diode?

MarshallJMP · May 05, 2015, 11:15:21 AM

Well this can be the input jack or diode D16.
Here's a pic of a replaced D16 (use a 1N4004 or 1N4007 as replacement).Diodes have a polarity so make sure you insert the correct way (silver ring).

http://www.marshalljmpmodshop.net/IMG_D16.JPG

Iron25 · May 05, 2015, 11:25:00 AM

Cool, I'll try the D16 replacement and report back, thanks man...

MarshallJMP · May 05, 2015, 11:46:49 AM

Ok keep us updated,and ... No problem

rnolan · May 06, 2015, 05:50:51 AM

Sounds like input jack swap is also good idea (while you've got it open)

Iron25 · May 06, 2015, 10:12:16 AM

Yep, replaced D16 with a 1N4004 and all is well. Burned it up for about three hours last night with no problems. Had I had a jack laying around I would have done it, I really don't care for board mounted jacks, it's not if, its when they're going to fail. The part that was in there doesn't look original so I must have replaced it before,yeah, I know I should keep notes...here's a pic of what was there.

MarshallJMP · May 06, 2015, 12:47:50 PM

What's that?Looks like a transistor but it can't because it only has 2 connections.Never saw diodes in a TO92 housing.

Iron25 · May 06, 2015, 11:17:10 PM

Old part, I think my amp tech buddy gave it to me when I was in need of a diode way back when...memory is a little fuzzy these days. :banana-rasta:

MarshallJMP · May 06, 2015, 11:34:33 PM

Ok!

rnolan · May 07, 2015, 04:17:20 AM

I take it the input jack is the early version (mounted onto the PCB ?), MikB is having this issue and getting a replacement may be problematic, MJMP has none now (not sure if you can source them ?). Fix is do the rear input jack mod, makes rear input jack inst (guit) level as early units it was line level and replaces front input jack as well as rear (http://www.marshalljmpmodshop.net/miscellaneous_parts_and_replacement_tubes.htm).

Iron25 · May 07, 2015, 09:27:02 PM

Yeah, early top switch version. Eventually I'm just going to wire in a panel mounted jack on the front when it finally shorts. I don't plug and unplug, it's wired in the rack so it may be awhile yet.
I might the only one who doesn't mind the rear jack being line level.

Maybe it's time to create the just front jack mod?

Thank you for looking out!!!

rnolan · May 08, 2015, 11:41:59 PM

@MJMP, what's involved just changing out the front jack (i.e. front jack mod LoL) ?

I never minded the idea of the line in at the back, but never used it. Although I thought of some uses for it over the years like if you wanted to use MP1 as an insert in a desk channel, or in a line level FX loop, or even on a desk aux send to add some tubeness to a vocal mic (or anything else).

MarshallJMP · May 09, 2015, 02:52:35 AM

You can just replace the front jack,no problem,but you will loose the rear jack if you want too keep it line.Problem is you need a jack with a double throw contact and a long nose.

rnolan · May 09, 2015, 03:16:55 AM

Are the original PCB mounted MP1 input jacks still available at all ?? or are they lost to us now ? In which case it makes more sense to forget the rear line in feature and go with the mod (like MikeB decided).

Iron25 · May 11, 2015, 09:23:57 PM

Really?!?! Hadn't even looked at the schematic...just figured it wouldn't be a problem to keep the line jack as is.
That's a bummer :poop:

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Gain loss