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Marshall 5010 Needs Help

Started by El Scorcho, March 18, 2018, 09:32:21 AM

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J M Fahey

The YT Video you linked to does not show crossover distortion at all but something which scares unexperienced guys a lot:

* tube amps have a *smooth*  and continuous non stop variation from clean to distorted, imagine you are rolling a wheeled cart along a long ramp, you will go from a point to a somewhat higher or lower one with no hitch, going from, say, Volume 5 to 6 or 4 may mean going from 5% distortion to 6% or 4%, no big change (since you are never *really*  clean to begin with) , you can´t find a "frontier" since there´s not a defined one, and your ear accepts it.

* SS amps (*all*  of them by the way) are very very very clean before clipping (say 0.5% or less  :o ) to harsh distorted (10/20/30/50%) as soon as they clip, easy to see on a scope where you see a sharp edge at the clipping point.
Going from clean to dirty now is like hitting a staircase step: very definite, very sharp edge, "your cart bumps" on that edge, it is very annoying.
What the video guy does is play basically clean , rise volume until just the peaks distort, the resulting  mix is as harsh as putting mustard on your candy, you are jumping up down all the time between the top and the bottom of the vertical wall stair step.

* experienced SS amp Guitar players simply do NOT play over the step, they just roll their cart either in the clean area or in the fully distorted one.

Here´s one example of an experienced player testing the Marshall Lead 12, your exact preamp circuit, same gain, EQ, distortion mechanism (which does NOT use diodes of any kind but slams Op Amp against the rails, like a mini SS Powwr Amp driven to 11) .
In fact yours is the improved version, with higher power (same circuit but higher rail voltage, larger transformer and a 12" speaker) plus 2 Factory "Mods" :  they added a single transistor buffer to better drive the power amp and an adjustable presence control.
Of course yours may have some improperly changed parts values , but we have lots of doubts about the circuit.
I am certain circuit is the same (they used it on a ton of same era amps, why would they make a different one for this particular model?) BUT parts designation in the drawing and on the actual PCB might not match, so I ask you to print schematic on a large sheet (even better on 2 sheets and tape them together) and follow it part by part, following tracks where they go, then write actual (PCB) labels and actual part value on the paper one.
Then take a nice sharp picture of corrected schematic and post it here.
If in doubt with some part value write what´s printed on its body, don´t guess (as when you called pF what was actually uF).
That is a killer amp.
Power amp *might* be different, I´m puzzled at the "MosFet" label on front panel because those used TO3 metallic Darlingtons (MJ2501/3001).
They did also make a MosFet one, but those are VERY expensive transistors, justified only on the 100W one, definitely not in a way cheaper 30W one.

This guy ONLY plays amps full blast, with a high output humbucker equipped guitar and if not a combo, plugs them into a Marshall 4 x 12" cabinet.

To make it clear that even the cheapest cheesiest amps can sound GOOD when properly used (without any silly Mods) here´s:

J M Fahey

Started comparing your PCB and the published schematic and just fron the very beginning I see parts designations will NOT match, so you will have to apply what you see on PCB to the paper version.
Actual *circuit* (who connects to who else and how) must match 99%  :o
Just as an example and the first I checked, you go on: paper shows a single jack connected to one of Op Amp inputs through a 0.1uF cap "&C2" ; your amp has 2 input jacks which are mixed by mixing resistors #R1 and #R2 , 68k each .
I am calling paper circuit parts "&"  and actual PCB ones "#" because plain labels R1, R2, C1, C2, etc. differ on both and can be very misleading.

Also checked that yours has Darlington power transistors (MJ2500/3000), the early versions of standard MJ2501/3001 and not MosFets, which further confirms it´s the basic same circuit.

El Scorcho

Thanks for having a look at my post, I've seen some of your other posts on these Marshalls, and was hoping you'd chime in.  Those Johan Segborn videos on YT inspired me to buy another one. I would LOVE for mine to sound like that. I had Lead 20 years ago that sounded great, but I couldn't hear it very well over a drummer. This particular 5010 has the volume, but an unpleasent sound. What's frustrating is the sound is 'in there', but with a crappy coating that can't be dialed out.
    I've been busy with exams, but I'm ready to open it up again.

El Scorcho

Here's what I've got so far....

Jazz P Bass

I believe that TR2 is the incorrect transistor.

The schematic calls for a BC212, which is a PNP transistor.

Your notation states that a BC182 is installed in that position.
BC182 is an NPN transistor.

Also, I do not know what is going on there at the LTP (TR4 & TR5) but those two transistors should be the same type & hopefully from the same batch so that there characteristics match.


You will find that the PCB printing sequence does not line up with the schematic used.
Although they are *Almost the same* there are a some differences.

*Elscorcho* has likely just assumed the PCB numbering is the same as Schematic but if you swap the numbering it makes sense. :tu:

D1&D2 are not at the input they are right before the power amp on the input to VR5 (master volume), maybe Diodes were removed at factory and a cap across the master volume to block excessive treble. Makes sense as the diodes would surely restrict the max volume level. So the cap is likely a factory rethink after pcb's were made. (not uncommon). xP

@ Elschorcho;
As JMFahey has noted it's close but not quite the same. Your best bet is to redraw the circuit. Yes it's tedious but you will learn a lot in the process. 8|

You can use the existing schematic as a guide but don't cheat triple check every connection. (wise to do it over several days)
These are single sided semi translucent PCB's and if you hold them up in front of a strong light you can easy see the tracks while looking at the component side of the board. This allows you to work out where the tracks go. I've done this hundreds of times myself to back track odd circuits that have no schematic.
At the end of it you will have an accurate schematic and a better understanding of how the amplifier works.
You will also help others who may have you model amplifier.

BTW, when you quote components please write down *EXACTLY* what is written on the component. i.e. if it says 273 write that,, don't try to convert it to what you think it means.
A capacitor might read 2u2 or 2.2uF which means the same thing.
just write down what you read as most teck guys will know what it means.
hope it helps,, Phil.

El Scorcho

Interesting development.

I found the exact same Red McKenzie version of this amp for sale locally, and contacted the seller to see if he would be willing to let me take a picture of the circuit board to compare it to mine. While I didn't expect him to allow it, he was kind enough to send me number of google links, which I DIDN'T expect, one of which provided some very useful insight.

Apparently the Red Meckenzie version uses a JM69 circuit board, and the only available Marshall schematic for the 5010 is a JMP27A board.

One of the links provided by the gentleman was from an auction site where someone gutted his 5010 to convert it to a tube amp, and was selling the old board - which in this case happened to match mine, a JM69.

Zooming in on the picture confirms Phatt is correct - D1 is a cap, and D2 is vacant, just like mine. Also, I appear to have a number of factory caps (blue in the pic) replaced with Orange Drops; C 1,4,6,8,9,12, and 16.

Jazz P Bass

El Scorcho

I'll go back and find that image, jazz bass. In the meantime, I was on the Marshall forum trying to find out anything I could about the JM69 pcb and not having much luck-until I stumbled across a post about a guy with a similar problem to mine, only with his bass amp. The schematic was for his 5503 30 watt bass amp listed a certain pcb, but his had a JM69 board instead, and the components didn't match. So I ran down that schematic, and found it is much more similar to mine. In fact, the transistor values that seemed out of place on the 5010 schematic match. I think the Red McKenzie version of the 5010  is essentially a bass amp.


That 5503 schematic is not much different, they just call out the transistors differently.  You do not have that many transistors, and you have D3 and D4 in the power amp instead of a transistor for the biasing circuit. 
Also, look at the number printed on TR1 in your amp, is it a FET (2N3819) or a bipolar type (BC184) ?

Here are links to those pics.  They do all show a cap in D2 position and no D1 used.
The orange caps look like the rectangular type rather than orange drops.


El Scorcho

In my amp Tr1 is an N3819, though according to the (inaccurate) 5010 schematic it should be a BC184.


Ok then, if it's 2N3819 then the 5503 schematic is a better match for the preamp.
I am still working on translating a schematic but have no idea when I'll be able to finish it.

El Scorcho

El Scorcho