Author Topic: old Marshall 30w transistor schematic please  (Read 9920 times)

pinkjimiphoton

Re: old Marshall 30w transistor schematic please
« Reply #20 on: March 06, 2022, 03:28:02 PM »
I'm in the process of shifting PC's ATM so I can't do any tracing.

As far as I can tell, your board *is* a 30W unit.   It has two TO-3 output transistors and
heatsinks with fins on all four sides.

What I  can tell you is the Marshall schematics you find on-line are 'sort of' correct.   I don't know what the hell goes on with their schematics but here's the general way forward using schematics and boards from that era.
- Find the schematic which matches the board as close a possible.  (not straight forward)

You will then find:
- the schematic mostly matches
- the parts designators on the PCB don' t match-up with the schematic!
- when the schematic doesn't match you will find minor mods to the circuit
- in some cases the shape of the circuit matches the schematic but the part values are different.

All-in-all a big mess.

At least two people have put-up revised schematics for the 12W versions.


So start with Solid-state, 30W, no reverb, which is this schematic,



You can see it sort of matches this part of the circuit,

https://postimg.cc/9R2GGxj1

The top half of the board is the power amp you can see the 10 ohm, 39 ohm and 1k5 ohm resistor.   Then notice
the designators of the 1k5's not match up with the schematic.  You can also see C21 on PCB (100uF 25V) doesn't
match with C22 on the schematic.

The lower half of the board is the preamp:

R1 is 220k on the schematic and 1M on the PCB.
Next you have the 2x68ks on the PCB so some small change there.
The other parts around IC1 match-up with the schematic.

I think you get the general idea.  If you take that schematic you can kind of fumble through the PCB.

One difference is your PCB is marked JM78-2 whereas the schematic implies the PCB is JMP27A.

FWIW, the 50W models have a different transformer (larger, higher VA) and probably a higher voltage.
The TX would be likely be 70VA to 100VA.   The back panel on the amp is marked 72VA but you
can't read too much into that.

hey bro, yeah i can grok the pc thing, i just had to get a new one recently and am hating life lol

the 50 watters do use a bigger transformer, but its the same one in the 12-20-30 watt models, believe it or not. for all intents, the actual amplifiers are all but virtually the same.
the diff is actually the speaker load!
16 r, 12 watts
8r, 20 watts
4r, 30 watts..... seriously, if ya look at the schematics, they're almost identical!

i think the main dif between the posted common schematic is everything is off a little bit from the way the input is set up. it puts all the parts numbering off a couple numbers. most of it is VERY similar, in term of layout, but some of the part values are different.

i'm gonna shoot marshall an email and beg for the schematic, i'd rather at least try that than trace it on my own. if they can't help, i'll bite the bullet.
i've begun drawing it up, and taking notes... gonna probably take a while, tho, cuz i really suck at doing this stuff.

on a side note, i'm not getting notifications on forum posts all the sudden, really weird!!

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Rob Strand

Re: old Marshall 30w transistor schematic please
« Reply #21 on: March 06, 2022, 05:21:21 PM »
Quote
the 50 watters do use a bigger transformer, but its the same one in the 12-20-30 watt models, believe it or not. for all intents, the actual amplifiers are all but virtually the same.
the diff is actually the speaker load!
16 r, 12 watts
8r, 20 watts
4r, 30 watts..... seriously, if ya look at the schematics, they're almost identical!

I don't doubt it.   The fact they aren't exactly the same is the source of screwing up the part designators.

This was one of the "corrected" schematics for the 12W.    IIRC, there was another schematic by a Japanese guy.



Not sure how useful it is as it suffers the designator mismatch issue.    Notice also, even on this corrected schematic  the part values have the 330k from the original schematic, whereas your PCB clearly has a 1M input resistor.

It would be interesting to see if Marshall can give you a matching schematic.    Perhaps quote your PCB number of  JM78-2 if it helps.   Otherwise I suspect they will send you the schem which is already on the web.
Send:     . .- .-. - .... / - --- / --. --- .-. -

anotherjim

Re: old Marshall 30w transistor schematic please
« Reply #22 on: March 08, 2022, 07:26:05 AM »
I wonder about the <500k input resistors in these SS amps and indeed all others with opamps with 741 DNA. Even up to the RC4558. Possibly they were designed before or during the appearance of overwound hi-z humbuckers from DiMarzio et al.
But what was the rationale for not going with 1M resistors? To satisfy the input bias current of those opamps? So if you wanted 1M, you'd probably best change to a TL072, but that probably defeats the 741 sound that Jimi likes.


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Comma a question.

teemuk

Re: old Marshall 30w transistor schematic please
« Reply #23 on: March 08, 2022, 08:27:19 AM »
Lower noise and a bit more treble rolloff in the input? Can't see why that would hurt in a high-ish gain amp.

Many Peavey amps work great with about 500k too and the classic Fender input stage has mere 66k if you plug to the other input jack.

pinkjimiphoton

Re: old Marshall 30w transistor schematic please
« Reply #24 on: March 08, 2022, 12:58:20 PM »
Quote
the 50 watters do use a bigger transformer, but its the same one in the 12-20-30 watt models, believe it or not. for all intents, the actual amplifiers are all but virtually the same.
the diff is actually the speaker load!
16 r, 12 watts
8r, 20 watts
4r, 30 watts..... seriously, if ya look at the schematics, they're almost identical!

I don't doubt it.   The fact they aren't exactly the same is the source of screwing up the part designators.

This was one of the "corrected" schematics for the 12W.    IIRC, there was another schematic by a Japanese guy.



Not sure how useful it is as it suffers the designator mismatch issue.    Notice also, even on this corrected schematic  the part values have the 330k from the original schematic, whereas your PCB clearly has a 1M input resistor.

It would be interesting to see if Marshall can give you a matching schematic.    Perhaps quote your PCB number of  JM78-2 if it helps.   Otherwise I suspect they will send you the schem which is already on the web.

i did contact marshall, no reply yet.  i'm in process of tracing the mess. got the input stage, part of the preamp, and tone stack so far.
we'll see what happens. i can only go so long before my eyes just give out. anything i need to see detail on is problematic.
the tone stack matches the schematic, at least the layout does... the values are a bit different.
i gotta pull my other 30 watter apart and see if its the same.
i haven't looked at it in a long time, hoping it IS a match so i can use its front panel to help me draw up where to put the holes for the blank chassis i bought last nite.

still not being notified. very weird. i'll post news when i find some.

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pinkjimiphoton

Re: old Marshall 30w transistor schematic please
« Reply #25 on: March 08, 2022, 01:07:29 PM »
I wonder about the <500k input resistors in these SS amps and indeed all others with opamps with 741 DNA. Even up to the RC4558. Possibly they were designed before or during the appearance of overwound hi-z humbuckers from DiMarzio et al.
But what was the rationale for not going with 1M resistors? To satisfy the input bias current of those opamps? So if you wanted 1M, you'd probably best change to a TL072, but that probably defeats the 741 sound that Jimi likes.

tl072's sound like ass in this circuit. they don't clip gracefully, they tend to sound like broken glass... and they have just plain too much high end response. maybe good for some things, but in this circuit, all that means is an assload of hiss. not good.

one of my 5212's that had come in had been modified. it sounded good clean, but if cranked was so damn hissy it was horrible, brittle, and unuseable. i don't wanna have to use a noise gate... this thing was so damn noisy tho i had to. the guys in the band bitched. if i had to mic it outdoors, the sound guy and audience would bitch.

i opened it up, saw a bunch of sockets. every single one was loaded with 072's. i replaced them with the proper chips, and the amp came to life. better warmer distortion, no more broken glass or hiss.

the mojo-istic belief that them jfets sound better is just more internet hubris. its simply not true. in SOME circuits, they can sound great. but not in these marshalls. they're horrible in this circuit. i tend to find that tho i DO like mosfets in distortion, i totally despise jfets unless they're clean. they just plain don't sound good.

yeah, they may mimic a tube's amplification curves somewhat, when CLEAN, when they clip? they clip super hard and ugly. no bueno!
the 1458's don't have that broken glass thing, and don't waste power producing loud hiss a couple octaves higher than the guitar signal resides.

funny, many of my friends tend to agree on this. there's really a world of tonal difference. almost any dirt pedal will sound better with a 741 or 1458 than any jfet dirt circuit. ymmv, and should, of course...

but i'm working in the real world with these things, not playing in my room or at my bench. stuff often sounds vastly different at stage volume.

i think the biasing is to make 'em sag a bit more. these chips also sound better when browned out some. i'll stick with the guv'nor on this one! ;)
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anotherjim

Re: old Marshall 30w transistor schematic please
« Reply #26 on: March 08, 2022, 02:34:38 PM »
I'm with you Jimi, I wouldn't use any TL chip if I think it can run in to clip. The thing I don't get is why not fit 1M. MXR D+ is 1M and a 741 although a dirt box is perhaps a poor example.

Too much treble? We have a tone knob on the guitar.

Too noisy? Ermm, as I understand it, the quietest resistor is no resistor.
In series, that's 0 ohms or wire. In parallel, it's infinity ohms or nothing. So 1M or higher even isn't noisy in itself for the parallel fit, but I think opamps have a spec about input noise that's dependent on the source impedance which is lower = quieter. I don't understand the mechanics of how that works but a lower value parallel input resistor will lower the source impedance seen by the opamp  -  guitar pickup impedance is relatively low but can go higher: 250k if I turn a 500k vol knob on a Gibson with one pup on to half resistance and 125k with a Fender 250k.  A 330k input resistor will roughly halve the source impedance then. This is when the TL072 beats others at 1M , it doesn't get so noisy if the guitar is at a high impedance.
I've modded a Peavey to 1M because it was dull with high output 'buckers but I never considered changing the stock input opamp since the other half is in the overdrive circuit.

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Comma a question.

PRR

Re: old Marshall 30w transistor schematic please
« Reply #27 on: March 08, 2022, 10:31:41 PM »
> a lower value parallel input resistor will lower the source impedance seen by the opamp

AND lower the desired signal. (Or rather: you will turn-up to get the desired level.)

Trying to "lower node impedance" with a shunt resistor is self-defeating.

The shunt resistor should be as high as possible.
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pinkjimiphoton

Re: old Marshall 30w transistor schematic please
« Reply #28 on: March 09, 2022, 02:26:24 PM »
I'm with you Jimi, I wouldn't use any TL chip if I think it can run in to clip. The thing I don't get is why not fit 1M. MXR D+ is 1M and a 741 although a dirt box is perhaps a poor example.

Too much treble? We have a tone knob on the guitar.

Too noisy? Ermm, as I understand it, the quietest resistor is no resistor.
In series, that's 0 ohms or wire. In parallel, it's infinity ohms or nothing. So 1M or higher even isn't noisy in itself for the parallel fit, but I think opamps have a spec about input noise that's dependent on the source impedance which is lower = quieter. I don't understand the mechanics of how that works but a lower value parallel input resistor will lower the source impedance seen by the opamp  -  guitar pickup impedance is relatively low but can go higher: 250k if I turn a 500k vol knob on a Gibson with one pup on to half resistance and 125k with a Fender 250k.  A 330k input resistor will roughly halve the source impedance then. This is when the TL072 beats others at 1M , it doesn't get so noisy if the guitar is at a high impedance.
I've modded a Peavey to 1M because it was dull with high output 'buckers but I never considered changing the stock input opamp since the other half is in the overdrive circuit.

well, remember, this is supposed to be in an amp that is supposed to be a marshall, so they're likely using the values they chose to get as "close" as they could tonally, as well as in the effective range of the controls. its not a stompbox, so perfection ain't likely. it was built to meet a price point! ;)


i think you're misunderstanding me. when you push a tl into clipping, it clips hard and ugly.

the reason it has more noise/hiss is because its effective frequency range goes much higher than the 741 can. so you're putting out an extra octave or more of treble, above the range of the guitar, which manifests itself as a really loud, annoying HISSSSSSSSSSSSSSS that won't go away.

the only way these amps really sound good is with the gain pegged. below that, they're nothing really special. but when you crank the gain to get the max distortion possible <which cleans up really well when ya roll your guitar back> that extra treble gets distorted and manifests in a most unmusical way.

its not the resistors. that's not the issue. the issue is the amount of gain, and the jfet based chips clipping hard and ungracefully, but most especially the frequency range.

a 741 when clipped acts very differently from a tl071. in the case of this particular circuit, the "worse-er" specs sound way better.

peeps make "laws" that they often don't understand... often based on internet hype and mojo.

who in @#$% wants a "hifi" distortion? that's kind of defeating the purpose.

even a klon(e) sounds better with the bjt based opamps to my ear. warmer distortion, better cleanup from the guitar etc.

i see peeps add tl's cuz the internet tells them they sound better. not based on them LISTENING CRITICALLY to the results. to me, if ya put a tl072 in a toob skreemer, it just sounds even worse than the stock 4558 does. too much glass, too much high end, too hissy. no bueno.

so tho the resistors may make a difference in the biasing of the circuit truly, ultimately it still comes down to the magick jelly bean in there.
the XX58's just plain sound more musical in distortion than the more "hifi" jfet ones.

jfets sound great clean. but they really sound like crap in most cases with dirt.  they sound ok til they flatline. and when they do, they make a horrid racket, at least to my ears.

on a side note, i got a reply from marshall.... the ONLY schematic they have is for the one that's already out there.

so far, i've got the input stage and the tone stack drawn up for this specific circuit. its gonna take a while. glaucoma and long traces that twist around in box shapes tend to really mess me up.

rock on!
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anotherjim

Re: old Marshall 30w transistor schematic please
« Reply #29 on: March 10, 2022, 05:30:56 AM »
I was only trying to discuss why the input impedance should be low since 1M was already a standard and nobody really took me up on it! I'm not convinced by any arguments for lowering it. I wasn't ever suggesting you should change the opamps, only mentioning the TL07x as a way to have 1M input without too much noise then your character opamps can take over. There are tube amps with TL072 input buffers and they can sound pretty good.
Oh, and I got my pot resistance maths wrong earlier too. Really surprised I got away with that. A 500k volume pot becomes a 125k source at half resistance, doesn't it?


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Comma a question.

pinkjimiphoton

Re: old Marshall 30w transistor schematic please
« Reply #30 on: March 10, 2022, 12:01:02 PM »
I was only trying to discuss why the input impedance should be low since 1M was already a standard and nobody really took me up on it! I'm not convinced by any arguments for lowering it. I wasn't ever suggesting you should change the opamps, only mentioning the TL07x as a way to have 1M input without too much noise then your character opamps can take over. There are tube amps with TL072 input buffers and they can sound pretty good.
Oh, and I got my pot resistance maths wrong earlier too. Really surprised I got away with that. A 500k volume pot becomes a 125k source at half resistance, doesn't it?

you're speaking to a monkey with a breadboard, bro..... what? lol
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PRR

Re: old Marshall 30w transistor schematic please
« Reply #31 on: March 10, 2022, 01:02:38 PM »
...A 500k volume pot becomes a 125k source at half resistance, doesn't it?

Yes, and who cares? Pot at half, you have already thrown-away half your signal level.

If you play LOUD, nobody hears your hiss.
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pinkjimiphoton

Re: old Marshall 30w transistor schematic please
« Reply #32 on: March 10, 2022, 02:12:17 PM »
...A 500k volume pot becomes a 125k source at half resistance, doesn't it?

Yes, and who cares? Pot at half, you have already thrown-away half your signal level.

If you play LOUD, nobody hears your hiss.

hahahah, oh yes, they do. that's what led to me opening the amp and discovering the culprits... tl072's where 1458's should be.
with the gain cranked, it was unuseable, and believe me, i wasn't being quiet.
outside in particular where i needed to mic it it was especially bad.
once i changed the jellybeans back to what they should have been, the hiss literally disappeared. like turning the gain from dimed to rolled back to about half way. significant change in noise and hiss.

when i use these suckers, believe me, they're cranked. gain and volume on 10, tone stacks roughly 6 across the board, and master volume between 4 and pegged. i go in and blueprint 'em with 1% metal film flameproof resistors to get the noise down. it makes a big dif.

but the change was so significant even the deafest members of the band noticed it immediately, as it had become a bit of an inside joke. and the sound guys always bitched, lol...

no more problem.

the tl072's so many people think "sound better" simply don't in this application. they're ok with the master volume way down, but once you're half way up, the noise makes it unuseable without using a noise gate, which completely destroys the tone that makes these worthwhile in the first place.

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pinkjimiphoton

Re: old Marshall 30w transistor schematic please
« Reply #33 on: March 19, 2022, 01:48:02 PM »



thanks to tassieviking on ssguitar, appears they worked up a schematic from my pics








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Rob Strand

Re: old Marshall 30w transistor schematic please
« Reply #34 on: March 19, 2022, 06:55:05 PM »
Quote
thanks to tassieviking on ssguitar, appears they worked up a schematic from my pics

Cool.  Thanks jimi and tassieviking.

I have *not* checked the trace/schem, however, the input stage didn't look 100% correct.

The no input cap is correct but the arrangement at the output of the first stage of IC1 looked wrong.
I posted the correction below.   The fix actually follows the other 5010 schem fairly closely, the only
difference is the other 5010 schem has an extra 100R on the output of IC1.


Bug on schematic Rev 1, 2022-03-19

(IC1 pin 7 goes to C4 (100p) and R6 (22k) as shown)

But then from IC7 pin 7 it should go,

IC7 pin 7 to C2+
C2- to paralleled C1 (47n) & R4 (10k)
other side of paralleled C1 (47n) & R4 (10k)
   to VR1.1 (ccw)
   and to IC1 pin 3
Send:     . .- .-. - .... / - --- / --. --- .-. -

Tassieviking

Re: old Marshall 30w transistor schematic please
« Reply #35 on: March 20, 2022, 08:38:10 AM »
I can't make out the detail around C1, C2, R4 very clearly, but I believe I am right.
Here are some of the pics I worked with, I have mirrored the track side so it easier to compare to the component side.

I did find the inputs wired wrong, the middle pot reversed and R18 wired wrong, fixed for draft 2






« Last Edit: March 20, 2022, 08:45:58 AM by Tassieviking »

Rob Strand

Re: old Marshall 30w transistor schematic please
« Reply #36 on: March 20, 2022, 10:41:09 AM »
Quote
I can't make out the detail around C1, C2, R4 very clearly, but I believe I am right.

On the PCB, the lower sides of C1 and R4 have separate tracks, which are easily visible.   
What's hard to see is the short track which joins the lower side of C1 and lower side of R4.

As drawn the circuit cannot work:
 - IC1 pin 3 has no bias path to ground.
 - the volume control (VR1) cannot make the volume go to zero.
    That's the purpose of R4, it appears in series with the output of IC1 pin 7
    so VR1 can short the signal to ground when VR1 is set to the minimum position.

It's a common circuit (also used of Peavey amps) and is used on the "other" 5010 schematic
given earlier in thread (C6 and R6, R5 not present):

https://www.drtube.com/schematics/marshall/5010.gif


EDIT:
It occurred to me later Jimi posted a few pics of that area of the pcb earlier on in the thread

For example, on the right hand side of this pic you can see a thick track joining C1 and R4.  It's the same thickness track that joins the top of C1 and R4.
(this pic is not flipped to emulate an x-ray view)
https://postimg.cc/t1W3ZK8Y
« Last Edit: March 20, 2022, 07:36:45 PM by Rob Strand »
Send:     . .- .-. - .... / - --- / --. --- .-. -

pinkjimiphoton

Re: old Marshall 30w transistor schematic please
« Reply #37 on: March 21, 2022, 12:36:48 AM »
gentlemens....

here's the deal. tassieviking's schematic, the last one, has only one mistake.

r4 10k and c1 47n are in parallel. one node connects to negative side c2/gain pin 1, the other to pin 3 of the 1458.

other than that, matches the circuitboard i have on my bench as far as i can tell completely, and am absolutely sure the preamp is right other than that one spot.

its hard to see. there's a couple traces that may not look connected cuz of the way the board was laid out, and cuzza that sticker that i hadn't removed from an inspector.

here's a pic, i drew lines to show the connections, and filled in the blanks with the part #'s and values



















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Rob Strand

Re: old Marshall 30w transistor schematic please
« Reply #38 on: March 21, 2022, 01:39:28 AM »
Quote
here's the deal. tassieviking's schematic, the last one, has only one mistake.

r4 10k and c1 47n are in parallel. one node connects to negative side c2/gain pin 1, the other to pin 3 of the 1458.

other than that, matches the circuitboard i have on my bench as far as i can tell completely, and am absolutely sure the preamp is right other than that one spot.
Good on you Jimi!  Thanks for checking.

Send:     . .- .-. - .... / - --- / --. --- .-. -

Tassieviking

Re: old Marshall 30w transistor schematic please
« Reply #39 on: March 21, 2022, 04:31:38 AM »
I stand corrected, I could see a darkness in that area, but I could not see if it was a trace or not on the photos I was checking.
Thanks for the new photos pinkjimiphoton, it cleared it right up.
I have updated the schematic to draft 3, but I had already made the inputs as stereo jacks and I can't be bothered to change them back right now.
If I make up a PCB I always use stereo jacks just for the added strength so the solder is less likely to crack with use.
It might be silly, but I feel why not add the extra 50% solder connections to hold the jacks steady.
Once we have the schematic correct, I will post it up as a final.

Then the fun might start, what mods could be made if I make up the PCB and go to get some made ?
Effect loop for sure, but what circuit do I add, reverb ?
But that's for later
Here is Marshall 5010 PCB 78-2 Schematic draft 3 :