so recently i picked up a blown up marshall 12 watt reverb combo. when i say blown up? i mean LITERALLY.
this model had a tankless reverb deal mounted right to the amp pcb,
and one of the 3 springs broke somehow.
when it did, it literally took out the amp inside.... the spring melted into about 16 parts!!!
never seen that much destruction in this way before..
most of the amp was destroyed, and much of it had been disassembled by the seller, who gave up on it.
i was like..... hmmm.... 80 bux? i'll give it a shot.
so he sent me the amp, minus the speaker, and when it arrived i was like.... Damn, dude, this is more f'd up than
a soup sandwich.
why do i post this shit on aron's stompbox forum? cuz for all intents, this amp is a damn stompbox with an amp attached... google up the marshall 3005 vero on tagboard. great circuit.
sounds like a marshall in the box, well, cuz it IS a marshall in the box.... but i digress... big surprise, right?
there's a thread on guitarpcb i started about it... here:
https://guitarpcb.com/community/topic/b ... post-23129
ultimately, i had to replace all the electros in the circuit, all the diodes but the 9.1v one, the power transistors, the diode bridge, the power transformer as well... one side of the secondary, that fed the + voltage rail was literally gone, all this stuff got smoked when that spring broke.
so i went on a hunt, found all the parts, and got the amp working again.
the proper transformer for these, btw, is a hammond 166j28 28vac center tapped transformer. its the proper one for the 3005 and the 5005 as well, but in the 5005, due to the reverb, it clocks differently, so it needs to have two holes drilled in the chassis to make it fit, cuz otherwise the amp hanger is in the way.
hook it up the same as the original, bam.
made my own diode bridge with 4 1n4001's. easy breezy. now i had b+ voltage again, a solid 15.1v on positive and negative rails.
i got the proper power transistors, in multiples, and installed them.
removed all three 1458's and replaced with sockets. was hoping they survived, but all three were damaged, so i replaced them with modern 1458's. now the amp would fire. psyched.
the reverb was still dead tho, so i went on a hunt to find another.
this link has a pdf you want to save, that will de-mystify what a pain in the ass it is trying to figure out
what reverb tank you actually need:
http://www.electricalfun.com/workbenchf ... b_tank.pdf
turned out, i had a stack of old reverbs around so i kept trying one til i found one that would work... i got a dcr on each end of 170r, so i looked for something similar.
what was left of the original tank was showing a dcr of 34r/220r. i tried contacting marshall, but they have no records from these suckers nearly 40 years out.
so i went hunting... i could order exactly what i needed from belton.... if i wanted to buy a shit load of them. nope. just need one.
so i used the reverb guide pdf thingy to see what my options were, and came up with the following codes... meaningless? nope... all them numbers/letters stand for stuff... see the pdf
bsn3ba31a
8db2c1d
8bb3d1a
belton
bs3fb31b
8fb3d1b
so i went on antique to try and find a suitable replacement. ended up finding something close enough for rock n roll in a mod tank, 9" small one, three springs, long decay
save ya some hassle... this is the correct pan for this, as well as the 5210 30 watt combo:
https://www.tubesandmore.com/products/r ... y-3-spring
so i got the tank, and some iso'd rca jacks, and added the reverb back to the circuit. boom!!! it lived again!
used one of them new-fangled celestion g10-30's in it, which worked out just right, mainly cuz i am CHEAP. it was like, 35 with free shipping from victor litz musix, aka "the woodwind and the brasswind".
so now the amp was working... as an extra added bonus, since the power supply and amp are identical in this amp and the 30 watters <and almost identical to the 50 watters>
i
so now i had a clean running amp that had killer verb and was loud as hell... perfect for my old guy back. whole amp weighs in about 20 lbs now. love it.
but i was looking at it and going... ya know, if i take the signal after the master volume, and break it, and add a pair of switching jacks, i can add an fx loop for my echoplex...
where the red is circled was where i was intending to break the circuit... thinking that way, i could just be easy about it....
but then i was thinking, shit, what if it becomes a phasing issue or something?
that's when i remembered barry's paramix, which i had bought several boards for years ago in hopes of using with a chipamp dealio like petey twofinger's live amps i was messing with... and realized, with the gain controls and phase switch, would be perfect for my needs:
https://guitarpcb.com/product/paramix-effects-loop/
so i took it an populated it, and made a couple mods... i just jumpered the series fx loop part, cuz i didn't need it in this amp, and was kinda wondering if i didn't already have too many holes in it.
in for a dollar, in for a dime.
also jumpered the dry level of the mixer part so it was full up. i tried using a dual 100k pot to sweep the wet and dry, but in the middle it was dismal, so ended up just jumpering the dry signal half to be on full, and made the wet signal pot b10k instead, which gave a nice and useful sweep to the wet mix pot. perfect.
i kept the phase switch, and used a tl082 cuz it was the first quad i pulled out of the semiconductor drawer.
looking at it, i pondered how the hell i was gonna power it. wilkie on barry's forum said the jellybean was good up to 18v and i was like, cool, so how am i gonna do this?
that was the eureka moment... i had the diodes/caps for the bipolar supply right there... so i simply ran a ground from one of the 22uf cap's ground side to the ground on the paramix, and the 15v+ to the 9v in on the board.
then i pulled out the 082, lifted pin 11 so it wasn't in the socket, and connected the 15v- directly to the pin.
BOOM! now i had the same voltage swing as the rest of the preamp! worked out mint, tho i DID have to remove the 47u electro, which caused problems with the phasing switch... if ya flicked the switch, you'd get a pop.....pop....pop....pop.... kinda thing.. i realized that was the only electro on the board, so i just snipped it off. popping ended.
c9 in the marshall, a 220nF cap was the easiest insert point, so i grafted in the daughterboard there... i cut one side of the cap, the side to the master volume, and then soldered leads to the two ends of that cap's cut lead, and hooked in the fx loop. boom.
tested it... now i had a functioning fx loop in the amp. drilled a couple holes, and mounted the phase switch, send and return jacks <iso'd, important!!! use wah wah style 1/4" jacks> and was like, cool! so now i got reverb and a useable fx loop in this little beast, and have doubled the output power, to boot. and the "overall level" of the daughterboard, i made a 100k trimmer. i keep it all the way down, tho it can be used to juice the amp more at the expense of a bit of reverb or delay or whatever.
i made r5 22k instead of 12k, thinking it would be handy to have a bit of a "bump" for volume. this meant i had to raise r8 on the daughterboard up to 220k from the stock 100k to keep the "wet" level up a bit more.
now that trimmer? you could hook it up to bypass it, or switch to the trimmer setting for a lead "boost" if ya wanted to. i've toyed with it, but haven't bit yet. you could use a high z cable and a simple dpdt or 3pdt footswitch, but its gonna be a long run and may cause signal degradation or popping or noise from rfi etc. so if i DID do it <which i still may> i'd likely use a 15v relay hidden in the amp to keep the noise down, but that's another story and this is already too long.
so now it was just about perfect. the reverb i stuffed in a bag, under the speaker like in a fender, and used regular rca cables to hook it up. so the amp works now, the reverb is restored, and it has a functioning effects loop while looking bone stock.
btw, the phase switch is important and useful! when i first tried my 2399 echo, if the switch is down, and the echo is out of phase, it doesn't work well... but up, in phase, it's perfect.... so its worth keeping.
so finally i was like.... ya know....
what would make this sucker PERFECTER would be if i added a 9v output jack to the back to power my cheezy echo pedal with.... so i drilled one more hole, and mounted a 2.1mm power jack <iso'd, of course.... anything like this, you want to isolate it from ground so you don't get hum.... "ground" in this case is the neutral potential between the +/- rails>
so first thing i thought was.... hey! i can add a l78l09 right to the pads for the electro i'd snipped off, run a couple wires, and boom! 9v, right?
well... it worked... sorta. its amusing to hear what happens to a 2399 when starved for current. the echoes go all over the place at random pitches.. kinda cool. but not really...xin loi! <sorry, but not really>
the l78l05 was supposed to be good for 100ma current, but... no. when the guitar was barely on, it would work... but once ya hit the amp hard, and it sagged, the echo would freak out.
so i go to myself, self, yer a dumbass... of COURSE it ain't working right, you used a dang regulator with no capacitor support... so i made another daughterboard, this time with a 100r resistor in line as a " choke", a 10u cap on the input and a 2.2u cap on the reg output. much better... until the amp sagged.... echoes all over. i thought it may be the resistor, so i bypassed it. better, but ,,,
not really, same shit it was doing. echoes all over the place.
so i said... nope.
and i went instead with a simpler idea... a 9.1v zener diode between ground and the + rail to drop the 6 volts or so from the 15, a 10uf cap, and a 2.2nf "snubber"....also between ground and +.
boom! bingo! now i got a solid 9.45 vdc output on the back of the amp to run a couple pedals with. done! now it all works great.
here's a couple pics:
the dc9v jack goes just to the right of the mix pot. these pics were while it was in process.
i WAS having an issue with oscillation before i went with the zener for the 9v supply. i couldn't turn the amp above 8 without oscillation. that cured it, now i can dime that suckah, nice and quiet, no noise, no nothing.
but i had left c9 in, and had flying leads connected to it. it was dumb. no need... the paramix had the same value output cap, so i just snipped out the original cap completely, and soldered the daughterboard to the leads of the defunct cap. done completely.
mo pix, including the defunct regulator...
and finally, some videolic evidence of my heinous debauched molestation of this poor small amp:
[youtube][/youtube]
[youtube][/youtube]
so... amazing the stupid shit ya can do with stuff if ya wanna get creative. yes, i was bored. but the end result was def worthy!
