Working out an amp from gut shots

[mjj]

Reasonably new to all this. Wondering how feasible it is to work out a schematic from gut shots alone. Been looking at some pics of a board W/ reasonably clear component labels - just wondering how likely you are to go awry with wiring under the board and so forth.

[ChopSauce]

Which amp?

[pompeiisneaks]

You can usually do it, under board jumpers can be found with a multimeter in continuity mode, but you would do well to draw it up and share so that people can look and see if something seems off. Also there are quite a few here that like reverse engineering amps and would be happy to assist with photos of the guts.

~Phil

[Phil_S]

Pictures only? You'll be doing some guessing because you won't be able to actually trace the circuit. Sometimes, it isn't obvious what's connected to what. If it is a known amp, you should be able to find a schematic. That would make a big difference.

[pompeiisneaks]

Pictures only? You'll be doing some guessing because you won't be able to actually trace the circuit. Sometimes, it isn't obvious what's connected to what. If it is a known amp, you should be able to find a schematic. That would make a big difference.

No I meant assist. I.e. schematic drawn with photos, to correct anything that seems 'amiss'

~Phil

[ChopSauce]

How is it that I guess the OP was mostly expecting to read "YES" and won't post anymore ...

Pictures only? You'll be doing some guessing because you won't be able to actually trace the circuit. Sometimes, it isn't obvious what's connected to what. If it is a known amp, you should be able to find a schematic. That would make a big difference.

This is something of what I had in mind. If it is close enough to something well known - hence my above question - there should be more possibilities to make an educated guess about the hidden. Even in that case way, it proved to be very difficult to reproduce some circuits closely derived from famous amps, here, in the past. That also required several experiments involving buildind a prototype and modifying it a few times.

[Matthews Guitars]

It varies from "challenging" to "absolutely ridiculously impossible".

Try to derive a complete and accurate circuit from just a few clear photos of the internals of a Fender Champ.

And then get a print of the schematic and grade your work.

Unless you're already very familiar with the Champ circuit, very few people will score 100 points.

Now I'm staring at a Mesa Mark IV's internals.

And I'd just tap out. Nope. It's tough enough to follow the official schematic with the amp in front of me on the bench.

Pre-PC board vintage Marshalls are sort of easy to figure out, but ONLY because the jumper wires are on the component side of the board. But even then,
you have to know a few things, like those wires that run to the filter capacitor that's hidden under the main board.

I've totally rebuilt a couple old Marshall Superleads, I mean literally rebuilt them as if they were being built from scratch, and I know them better than
anything else. But I don't think I could rebuild another one using only photographs and have it work properly on first power up.

[pompeiisneaks]

I don't disagree fundamentally with what you're saying here, but many of the dumble amps were reverse engineered from general amp knowledge and just high quality photos. I get that you sometimes can't say where a jumper may go, or how it interacts, but you can see pretty clearly where pin X is on tube Y and that it connects to an eyelet at location Z etc.

The PCB types of course are nigh on impossible without direct probing with a DMM for sure.

~Phil

[Matthews Guitars]

Sure, you can get close using photos, particularly if you have enough of them. After that, if your skillset is sufficient, you can draw out the schematic of what you've built, apply your electronics knowledge, and try to fill in the blanks using that knowledge. And of course there's always the possibility of asking for help.

It can be done. I've had to do it myself, fairly recently. I got the challenging task of completing the rebuild of a partially completed Fender 400PS, and it was made more complicated by the fact that the guy who started the overhaul job had decided to rearrange the amplifier and left no notes on what he was planning to do. And he's dead so he's not of much help. So I had to figure out how to complete rebuilding it when the layout didn't match any diagram or photos of an original. The basic audio path wasn't hard to complete, but the reverb and tremolo circuits are literally not even the same schematic as original, to say nothing of their layout. That's actually a job I have to get back to and get those circuits working properly. It's going to be a wire chase. One at a time.

[thetragichero]

so many replies and still no pictures from the thread starter....

[Phil_S]

so many replies and still no pictures from the thread starter....

It was his first post. Did we scare him off? I feel certain we just meant to set expectations and frame the task, not scare him off. There seem to be plenty of offers to participate. It could be a fun group-think project. I hope he comes back.

[thetragichero]

i for one am interested in what amp it is

[pompeiisneaks]

It's entirely possible they are used to forum software emailing you if your threads are updated... and we don't. If they're used to 99.9% of other forums they'd expect notices about replies... so yeah... why I am bothered by the state of things here... in a nutshell.

~Phil

[sluckey]

He was last on the site this morning, May 24, 2022, at 1:18 am.

[mjj]

My bad!

Sorry to leave everyone deep in the lands of mysteries.

The whole line of thought came about from looking at the current Jim Kelly Line amps, for which Jim posts pretty detailed gut shots but which there's not a tonne of other detail out there.

That said, it was very much idle questions at this point - I'm just working on my first build at the moment (a 1986 Plexi) and I'm about a million miles away from starting an ad-hoc build of an unknown amp.

A better framing of my initial question would be "Hey, you guys have figured out some amazing stuff given a limited amount of information, how does that work? What's the least you can work with?"

The answers in the thread so far have really helped to elucidate that, so I appreciate it!