for my next project... Marshall MG100hdfx head

[thetragichero]

so this head has a story behind it...
about seven years ago my old bassist gave me this head to see if I could fix it. my tv was in the living room so it just sat in my garage for three years. he sold it to another friend for five bucks who then gave it to somebody to fix. that was three years ago. i was going to be in town for a concert so I arranged to pickup a working head I had lent him and said I'd give this one a looksee. so now it is back in my hands. gave it a quick testing last night and:

it turns on!
the clean channel passes sound (amplified, even!), albeit with uninspiring tone
with crunch engaged, it is low volume and super sputtery (maybe like a fuzz that wasn't biased)
overdrive channel (od1 and od2) completely silent
didn't test onboard digital fx

so the power supply and power amp both work, the relay switching seems to work. all stuff I don't care to mess with (maybe later on tweak power amp for sound) - I'm in good shape!

so I was thinking... freestompboxes forum has a bunch of schematics of solid state adaptations of classic/popular amp circuits designed to run as 9v pedals
this amp would be for heavy, doom metal kinda stuff so I was thinking Marshall jtm for 'clean' orange otr 120 for 'crunch' and sunn beta bass/lead for 'overdrive' (specific amps subject to change)
quick look at the mg100hdfx schematic shows that most of it is running at 15v. might try to stick a 12v regulator in there since 15v is the maximum voltage rating for the cmos from the sunn schematic. maybe i will edit post later to add links to schems

anyway, this is probably the biggest undertaking so far but thankfully I can work on it in modules
should be fun!

edit:

I'm pretty sure that the amp doesn't work because two electro caps c9 and c20 are missing (leads from c9 are bent together. guessing somebody realized but never decided to replace the missing caps). there's another missing electro on the dsp fx board so that probably wasn't working either

mg100hdfx schematics

plexi pedal

orange otr120 pedal

sunn beta bass/lead preamp thread

[thetragichero]

temporarily soldered c9 and c20 to the leads that were sticking out from the top of the board and all channels (have not tried fx loop and dfx board is in the other room) work. if be an easy fix but that's not any fun! my friend agrees so we gonna get dirty on this one!

[thetragichero]

okay so I have figured out where my preamps will need to connect to the switching circuit (the non - grounded legs of r33 and r4). it looks like only two preamp circuits will fit so the clean/crunch and od1/od2 switches will be repurposed for.... something (I've seen some solid state preamps with trimmers in op amp nfb loop to increase potential gain so a cleverly chosen resistor could be useful, as could a "bright switch")

common mod on these is to swap out some of the undersized power supply filter caps (1000uf) for something a little bit heartier (3300uf or 4700uf) so I've got some 3300uf ordered

probably going to try to sell the dfx board on eBay (after I replace the electro cap that's missing), for the kind of high gain doom metal my buddy plans to play on it, but i think a noise gate would be real handy and fill up a few of the several thousand holes in the front panel so I am investigating some circuits

so we are progressing. I will be building a box with in, out, and power jacks for my breadboard to facilitate testing. will try to tackle this ordeal in modules instead of overwhelming myself by working on everything at once

[sumgai]

Nov 6, 2018 18:34:27 GMT -5 thetragichero said:

I will be building a box with in, out, and power jacks for my breadboard to facilitate testing. will try to tackle this ordeal in modules instead of overwhelming myself by working on everything at once

Always a good thing to bite off no more than you can comfortably chew.

But you also know that documentation, written along the way, is very important to both initial success, and troubleshooting efforts that may crop up in the far-flung future. Which brings me to the point... take pictures to include in that documentation. And since you've been taking them all along (right?), why not post them here for all to see as well?





sumgai

[thetragichero]

I see no reason for cd ins or headphone outs so I'll have a few extra holes to fill so I figured a noise gate would do a high gainer well. will be auditioning several circuits and here's the first:



schematic: here

I have no single op amps so I used half of a tl072. subbed 2n5087 and 2n5089 for the transistors and a 5.1v zener (at least until 3.9vs arrive... I've never seen a zener in series with an led). found two lm3080 in the organ I'm tearing apart so that covered the other obscure part (besides the odd resistor values... had to use two standard resistors in series a couple times)

I've got one or two other noise gate circuits to build so I can see which will go in the amp (found an lm13700 in a box of half completed pedals i purchased on eBay)

[thetragichero]

so here's where we stand...



first two modules (also pictured is my well-meaning, but often quite annoying assistant). solid state emulation of an orange otr 120 amp including the FAC/filter (6p1t rotary switch that progressively cuts bass... probably useless for an amp intended for doom metal but that's okay). this has been tested and will be the 'clean' channel (although sounds pretty righteous with my big muff in front of it).
still have not tested the expander/noise gate (just got a shipment of pots in right before I went away last weekend), and I might end up using one of a half dozen other circuits I've downloaded, but I'll mark it on the + column for now

today it's sorta demo day, in that I will start removing components from the amp that I won't be using to make room for the new stuff, and upgrading the power supply filtering if I have time


opinion question: should I have individual channel volume controls in addition to the master volume or save the two chassis holes (found a nice presence/resonance circuit that emulates a tube amp's interaction with the speaker that I think I shall try to include regardless) and make the individual channels slightly less 'fiddly?'

[thetragichero]

ended up playing guitar for a bit yesterday afternoon with a guy i met online so I didn't do much until last night watching tv...

got rid of the board-mounted pots I won't be using and upgraded all of the power supply electrolytics from 1000uf to 3300uf. here's the old caps on top of the new ones (couldn't believe how much bigger they were)




sourced the jrc4558 ics I'll need for the beta lead/bass preamp (leave it to me to neglect ordering the one thing I for sure don't have in the parts bin...) from the existing Marshall circuit. will snag out the discrete components next. a TON of those nice box film caps I may put to good use. hopefully holiday prep doesn't get in my way this evening!

[thetragichero]

if all goes well tomorrow when I test this, it'll be the sunn beta bass preamp's turn too get soldered onto perf



scratched my head for awhile on how to have a bipolar supply to test this (the Marshall has +/-15v rails so that'll be perfect when it's ready to assemble), even went as far as to purchase a couple charge pump chips
then it dawned on me that a voltage divider with a buffer in the virtual ground (as that has the possibility to vary under load, I have read) was a real simple way. so that's what's on the small breadboard (along with some extra filtering since I'm using a 24v switching mode supply... I have another supply arriving tomorrow that I've successfully turned into a pedalboard supply if this proves to noisy)

long story short: a thousand monkeys working a thousand years on a thousand breadboards will eventually make a fuzz box or something

[thetragichero]

someone asked a question on another forum about a metal zone 'noise reduction' mod and how it worked. basically antiparallel diodes (in that case 1n60p, so less than 300mv Vf) after the input buffer. did some googling and somebody suggested a 1M threshold pot. occurred to me that maybe I was overthinking the noise gate for this amp. did some testing and because I envision lots of dirt pedals in front of this, I dipped into my stash of LEDS. the pot is a MUST for clean playing, as clean high frequency content gets choked off if the LEDs aren't at least partially bypassed. for a quick and dirty one knob gate that will also serve as my input buffer, I'm sold. op amp is tl071




after a bunch of hair pulling trying to troubleshoot, tore the sunn beta preamp off the breadboard and redid it with quad op amps. apparently they do not appreciate it when I confuse the power rails and I ended up frying them. thankfully I use a surprisingly good power supply that has protection if it detects a short. eventually discovered that the Vdd and Vcc pins were internally connected, so I found the capacitor that should've been connected to ground but instead was connected to V+. so now it works! slightly bigger perf would've allowed for a less crowded layout, but this module is done! sounds heavy, I love it!