Marshall 20/20 Schematics?

[Robert M. Martinelli]

Hi all,
A friend just brought in a Marshall 20/20, the unit doesn't work, he says I have all the time in the world to fix it ( he owns several amps ), At a first glance the thing doesn't look like a standard Marshall circuit ( there's also an ECC82/12AU7 inside ). I think I can handle it, but things would be ( quite obviously ) easier with the proper schematics. Does anyone here have them?

TIA for your time and help!

Best regards

Bob

[Enzo]

Try these.


(For U.S. Marshals only)

[Robert M. Martinelli]

Enzo,
I'm speechless!

Should you ever decide to run for president, I want you to know I'd vote for you if I could!

I only have a little doubt left ; I pretty much know what to do with the schematics ( more or less ) but....what I'm I supposed to do with the pic ?
( I'm no US Marshal ) BTW, isn't that Tommy Lee Jones ?

Tusen takk ( A thousand thanks in Norwegian )

Take care

Bob

[Enzo]

Your Marshall is not a US MArshal, since it is in Italy. SO you don;t need Tommy Lee.


Tommy Lee Jones indeed, star of "US Marshals"

[Robert M. Martinelli]

You got me!

Your honor, to my partial defense, I have to say I haven't seen that movie,
( though I've seen "the fugitive" and "M.I.B." ) - I was too busy destroyin......er.....I mean....repairing/modding amps' n guitars

Cheers

Bob

[KevinOConnor]

Hi Guys

It's not surprising that this amp keeps failing.

Look at the 100R screen resistors on the tube board R7,9,11,13. These should be a minimum of 1k-1W flame-proof each.

Have fun
Kevin O'Connor

[J M Fahey]

Fully agree and add: 18 (eighteen) connectors !!!
With corresponding flat wire ribbons.
They look to be the lighter .1" ones, not the stronger .156" type, because as a "security measure" the ones carrying high voltage leave alternate pins unused, presumably to increase the separation.
A recipe for disaster, in a short or somewhat longer time.
It amazes me that Marshall copies Bugera instead of the other way round Mysteries of Nature !!
As a side pet peeve, I hate when they spell "torroid" instead of "toroid" which means "doughnut shape"
In fact, "torroid" means "something soon to be toasted".
Well, maybe they imply something there

[KevinOConnor]

Hi Guys

The use of 0.1" connectors and cables does not reflect "lesser" quality compared to 0.156" connectors. The currents in all parts of this amp are within the capability of the smaller pins.

Voltage capability of typical 0.1" connectors is 250V between adjacent pins. Again, this is all one needs for this amp. Skipping a pin for extra voltage isolation, like for the connections from the plate windings of the OTs or PT, is standard practice.

What you don't want to see in any connector in an amp are IDC connectors - which stands for "insulation displacement connection"> This is where the wire is just pushed down between two knife edges that cut into the conductor. The connection is considered reliable enough for telecom use - where single-strand is the norm - but it is asking for service issues in mobile gear like guitar amps.

Most of the Western manufacturers have freely given their production to the East to remain competitive. Behringer is based in Singapore and mostly lives off the seat of other designers. The competitive edge of the market tends towards unhealthy, monopolistic competition, rather than healthy co-operative competition, so business ethics take a hit but prices to consumers go down but further the consumer suffers with poorly built products. Some of Marshall's current engineer group used to work with Behringer.

On the schematic, "Torroid Int'l? is the name of the transformer company. Whether they spell it like that or Keir made a mistake, it does not bode well as you point out. Toroids are otherwise good to have in most gear.

have fun
Kevin O'Connor

[Robert M. Martinelli]

Hi All,
the unit I was repairing showed a completely different kind of failure: ALL (and I mean it) the PCB tracks bringing the +HT have arced, and the arcing caused the PCB material around to carburize. I had to cut them out, mill and clean with my trusty Dremel all the carburized material and then rebuild the tracks using an insulated wire in their place.
Seems like the PCB designers placed these tracks too close, and the "dielectric stiffness" wasn't enough to keep the tracks from arcing. Now the unit is fully operational, and it regularly pushes 22W RMS per channel into a dummy load.
Has someone else encountered a similar problem on one of these units? Just curious....

Cheers

Bob

[imaradiostar]

Is it just me or is that about the worst schematic you've ever seen?

jamie

[Robert M. Martinelli]

Er....well....no, Jamie, that way to draw the schematic HAS to have its own logic, I'm sure it has one.....I.....Oh, well, OK, I surrender....you're right!


Maybe it's only us old f@rts that don't understand the exposed talent of these new geniuses....or maybe, and more simply, the guy(s) who drawn it never repaired/maintained an amp....

I'm getting bitter by the minute, must be something I ate, or I'm getting old, or....

Cheers

Bob

[imaradiostar]

I guess the logic is that it represents the goofy build method of the amp with its many connectors. As far as looking at the schematic and understanding what is happening- well, that's a challenge. It's as if they need a different page with the schematic draw in a more conventional way.

I'm 31 and I already feel like I'm getting old. Maybe I'm just a Luddite.

Jamie

[J M Fahey]

Hi Jamie.
Now *many* big names do not post the *real* working schematic but the individual sections which actually are traced *inside* the PCB, straight from the PCB drawing program.
They do *not* provide the connectivity among them, nor external components which are chassis mounted but are not part of any PCB.
They sometimes not even label connector pins, other than the pin number.
You don't know what they carry.
An absolute lack of respect for the Technician, besides the errors and certain waste of time it carries.

[KevinOConnor]

Hi Guys

Modern schematics reflect the demands and limitations of computer-aided design, including drawings that result in PCB manufacture. There are two approaches to the over-all schematic presentation, and which a company uses depends on the scale of their business and the interests of that business.

basic PCB design software can be used to generate the trace and component layout directly, but this is only suitable for small boards or hobbyist one-offs. The "usual" approach is to enter the schematic and generate the board from that, but you have to abide by the software and computer limitations. Every component on the board, every connection between components, every terminal on those components - including connectors - has a unique designation in the schematic capture. The computer cannot allow two things to have the same "name" - the name being R1, R2, etc; different than the "value", where both R1 and R2 can be 100k, say.

The software is only "interested" in what goes on the boards. If you drew the entire schematic including things that are off the board, then you may encounter an error list that says all these things are not connected. Actually, as long as, say the PT is connected to the switch and the power connector, it does not really matter at the end that these components are not mounted on a board; this just incurs an extra step before you can do the CAM processing to generate the files the board house needs. That extra step is to cut and paste the actual boards to a different file, then run the CAM processor on that file. Accepting this extra bit of work allows the designer to generate a complete schematic.

Countering this, is the fact that the designer will do a bit of figuring the old way, just with a pen and paper. It is easy to figure out that the assembly will require three boards interwired or that plug together. From there, the individual boards, or the panel of three boards, can be laid out as he knows them to be mechanically.

Yes, most of this results in schematics that are not as easy to read as older hand-generated ones. On the plus side, there cannot be obfuscation as the schematic has to truly represent the circuit to be produced. back when Randall Smith hand drew his schematics, there were errors on them - some of which I believe were deliberately there to mislead copyists. New CAD drawings of the same feature are drawn correctly.

With PCB design in hand, the designer can add features that he wouldn't have bothered with in the hand-wire days. Adding parts to a PCB is easy, where adding more simple circuits to the hand-wired amp eventually makes that amp a nightmare to wire. Been there; done that.

Another trap PCB designers fall into is the temptation to eliminate all wiring. This greatly improves assembly time and lowers the cost to the consumer, but most often makes repair more troublesome. PCBs are designed "in-house", by the guitar amp company, then manufactured elsewhere as it is a specialized industry that uses a lot of smelly chemicals. Stuffing the board and wiring it into the chassis is usually done by the amp builder, but the stuffing portion can be contracted, too.

Have fun
Kevin O'Connor

[J M Fahey]

Fully agree, but would like to add something.
This schematic, and many like it, is fine *to design the board*, and of course, as you say, must not show parts not into it.
This is the one used to trace and fabricate the board.
Good.
BUT
They should draw another schematic, a functional one, for the service technician.
Yes, the service technician may print those three separate, incomplete schematics, into 3 separate sheets of paper (I doubt many have the required A3 or "double letter" printer needed to put all 3 on the same sheet of paper), glue them, and pencil the lacking interconnections, which he'll have to imagine because he would need to have a 4th drawing showing them.
Besides that, he'll often , for example, have to guess the pot values out of thin air, if they are not on the board and thus in none of the supplied schematics.
It may take, what? , 1/2 hour ? to do that? .... with possibilities of mistakes ....
Why does not the Factory do that once and for all, and send a proper *service* schematic on request?
[end of ranting mode]