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Messages - pinkjimiphoton

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Amplifier Discussion / Re: Marshall 3005/Lead 12 Power Trans Question
« on: September 01, 2021, 11:53:15 AM »
i build guitar effects for a living. the 1458 is my favorite chip in many distortions, it really sounds different from a lot of the others, i mean for all intents its a dual 741, but to my ear, it sounds great. so i tend to keep a couple hundred kicking around in the semi conductor junk drawer.
i DID socket it, so i could mess around with it if i wanted to, but i always go back to the 1458 in all of my own personal circuits. i figure if it was good enough for the guv'nor, its good enough for me ;)
i've tried just about every dual opamp available in distortion circuits. most folks prefer the jfet based ones, but to me, the '58 is the only way to fly.
proper transformer arrives tomorrow, so i'll install that and button it up.
thanks for the support!

Amplifier Discussion / Re: Marshall 3005/Lead 12 Power Trans Question
« on: August 31, 2021, 06:04:06 PM »
gotcha. my bad. and my apologies to the forum.  :dbtu: :dbtu:

well to finish up, yeah the 1458 was the issue, but not the one i thought it was.
previous owner had tried to replace the chip apparently, and killed the pad for pin one of the opamp.
so i desoldered it, replaced with a socket and ran a piece of wire from pin 1 of the socket to r7/8.
took some voltages, now i was reading a lower voltage than i expected, +/- 5vdc.
close enough for rocknroll to not fry the chip. replaced it. dead. put in a fresh 1458n out of my junk drawer and fired right up.
thanks for the support and patience.
rock on!

Amplifier Discussion / Re: Marshall 3005/Lead 12 Power Trans Question
« on: August 31, 2021, 04:11:39 PM »
well, its about the only thing i would expect to drain enough current there.
it looks like the "replacement" xformer was installed, but the 2 1k resistors that feed the +/- rails to pins 8 and 4 were never adjusted to account for the voltage change.

gonna go pull it before i leave for tonite's gig. thanks for the advice, sir! :dbtu:

Amplifier Discussion / Re: Marshall 3005/Lead 12 Power Trans Question
« on: August 31, 2021, 03:31:41 PM »
hi gonz,
mainly cuz this thread was already a wealth of good information. it looks like someone tried to take jm's post about beefing the power supply and apply it, but they didn't do too well. :duh
data sheet says a max voltage of 18 v for a 1458, and right now its reading about 20v/-20v at the power rails. i wouldn't think that small a voltage increase would pop it, but ... the power amp IS working. the preamp is completely dead.
i've got signal from the output of the MV on. nothing past pin 3 on the 1458, so, to my rookie eye it seems like the jelly bean took a hit from this... the transformer was only connected on one side, and had ripped off some of the wires from the power supply jack as it flopped around, so it may well have damaged something else, or shorted somewhere at some point and popped it.
if i don't get it, i'll start a new thread. just seemed relative to this one.
thanks man!

Amplifier Discussion / Re: Marshall 3005/Lead 12 Power Trans Question
« on: August 31, 2021, 01:02:18 PM »
zombie bump!!.... errr... zombie woof?

hi guys
got one of these on my bench. someone tried to "upgrade" it with an oversized xformer, and appears to have messed this up into the proverbial soup sandwich.

i have the amp semi working... the power amp, anyways. i can get a bit of a low-volume honk out of the speaker if i touch one of the leads at c8.
i replaced all the electros, and checked the diode bridge. seem to be getting about 19v/-19v at the power rails coming off the rectifier...
but that's it. there's not much in there, really... so i tried audio probing it last nite.
i have sound up to the input of the 1458... but that's it.
since i can't seem to get a reading for any voltages at pins 4 and 8, and no output after the 1458, should i assume the opamp is toast? a shorted opamp would make it so i get no voltage readings anywhere, wouldn't it?
just thought i'd ask someone with better knowledge before desoldering it.

one of the two power transistors in the actual amplifier seems to get warm after a few minutes too, the one on the negative rail.... i'm assuming a shorted opamp may make that happen?
i ordered fresh bc148's for it, fresh zener etc etc and a PROPER power xformer for it, a hammond 166j28 28vct transformer that will actually fit the footprint inside the chassis with proper foot spacing for the mounting holes.... and look forward to getting it running.

i appreciate any thoughts, guidance, and advice! peace!

Amplifier Discussion / Re: marshall 5212 channel bleed fix?
« on: February 19, 2020, 01:05:23 PM »
thanks, phatt!
awww, shoot. just hoping it would work out there.. seems i wanna tie the clean channel in after the "master volume" of the dirt channel to get it independent, but ..... oh well, guess that won't work.
i DO have room to add another jack... the panel is punched for another input, tho covered with the gold overlay sheet. but that seems like a lot of work.
frankly, the whole reason i use these amps is cuz i love the dirt side, i never use the clean cuzza the dirt bleed thru... my venerable randall rg120 had that bleed issue as well, so i may have to rebuild the switching section.
but for all intents, i am fine with just the dirt side and reverb. i DID find a thing somewhere about modding these circuits to fix the bleed problem, but it seems to have vanished from the net recently.
i may try your suggestions tho... would love to separate the "channels" to more independance.
i'll try contacting marshall and see if there was a tech support ticket for this dealt with... if the guvnor were still with us, be easier.... i remember him actually picking up the phone one time when i called there decades ago... but now, they're tighter than a billionaire with the last yuppie bisquit to steal in their hands. lol.

again, thanks... will letcha know how i make out. waiting on parts to arrive to recap it, and one of my roadies busted off a pot the other nite loading it up. ouch.


Amplifier Discussion / marshall 5212 channel bleed fix?
« on: February 18, 2020, 05:01:03 PM »
hi folks,
restoring two of these that i picked up. replaced all the electros and tants, and they are now nice and solid and dependable... so far ;)
but i get a bit of channel bleed happening from the dirt "channel" into the "clean" channel when i have the gain pegged on it, even with a proper footswitch hooked up. the clean channel sounds great only if i turn the dirt gain to zero.
been messing with pedals and amps for years, but never really dealt with this kind of situation for the switching before, and not really sure how/where to look, or if there may be a modification i need to make possibly... maybe add a snubber to ground somewhere or something?
here's the schematic

also, another question. it seems to me marshall dropped the ball on the design in one crucial way...
its not really two "channels" like in, say, a fender amp or something, looks more like its just switching gain stages? or am i as confused as i think? ;)

the problem is that the clean volume is affected by the master volume. the gain channel has gain and volume, both of which sound best cranked. it appears they figured  you'd crank the dirt gain, lower the dirt volume and then set the master for both channels.. but it sounds better with the gain and volume cranked, and the master lowered.
BUT then the "clean channel" gets overdriven and sounds bad... like a silverface 70's master volume that doesn't belong on any fender.

is there a q&d mod that could be made to take the output of the clean channel volume right from the wiper and dump it just post the master volume instead? that way, the master could be used to really dial in the distortion, and the clean, well, ya just turn it up, it doesn't really NEED a master volume imho.

thinking breaking the node at r10 and r9 and ic1a pin 2, and instead connecting the freed side of r10 to the node at r27 and the fx send jack?

would love advice opinions and guidance if anyone can spare a few minutes to take a look

Amplifier Discussion / Re: Marshall 5203 220-117v mod?
« on: February 13, 2020, 06:39:48 PM »
turned out already set up for 110v. very groovy! pretty good shape. had to graft a new volume pot in and caig it, and replaced all the electros... amp was marked 88, so i figured i should. fired right up, sounds boss.

thanks for the support guys. ya'll rock!

Amplifier Discussion / Re: Marshall 5203 220-117v mod?
« on: February 12, 2020, 06:00:21 PM »
hi guys,
just checking in... amp arrived today, seems to be intact mostly <g>
gotta busted pot, so gonna throw it on the bench and fire it up. it looks like it has a normal
110v plug with it, so i may get off lucky with this, but thanks to your help, should be good.
i will post back how i make out.
again, thanks!

Amplifier Discussion / Re: Marshall 5203 220-117v mod?
« on: February 05, 2020, 09:19:37 PM »
thanks guys!
 i won the auction for cheap, look forward to resurrecting the beast!
to me, this is the best series of amps marshall ever made, tonally, and i've owned plexis and 800's. all the tone without the earbleed glass. love 'em!

Amplifier Discussion / Re: Marshall 5203 220-117v mod?
« on: February 04, 2020, 12:49:31 AM »
beautiful. thank you, enzo!! exactly what i needed to know.
have an awesome day, mate!

Amplifier Discussion / Marshall 5203 220-117v mod?
« on: February 03, 2020, 09:00:05 PM »
hi peeps,
long time lurker, first post i think...
i have one of these amps

i'm hoping to pick up, but just realized its a canadian model set up for 220v, and i live in the usa where its 117v
(give or take)... looking at the schematic, it looks like i should be able to just move a couple wires on the power transformer to choose, and maybe change the fuse value to a slightly higher one..
but not sure looking at it, which way would this be implemented?
sorry if a dumb question, but it looks kinda like they jumper two parts of the power transformer together... would i just use one half of the pt primary? or do i need to remove something?
i'm a fair hand with a fuzzbox, but this is pretty much my first real rodeo with this kind of thing so i appreciate any advice i can get. thanks so much!

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