Should I trust factory schematics?

[theguitartech]

Im currently trying to get the reverb working on a Marshall TSL100 (http://music-electronics-forum.com/t26543/) and I'm running into a problem I didn't expect- The schematics I have dont match the board in the amp.

My schematics were sent to me from Marshall and match the issue number/part number of the board. The general layout is correct but there are many components who's values dont match up (all resistors) or were ommitted completely (a couple of zener diodes and a capacitor)

Im new to working on electronics.....Is this something that is common? I have the correct schematics for my board and the next revision- both have the same discrepencies. Do I restore the board to the values listed on the schematic or were these changes made after the schematic was made? All of the components appear to be factory installed. Is it possible the factory used the wrong parts?

[52 Bill]

Factory revisions are common and sometimes never documented, so the change may have been from the factory. If the reverb ever worked on the amp, then the missing parts had nothing to do with your problem.

The real question will be, was someone else in there working on this before you? In my experience, one of the worst situations is when someone else has tried to fix something by removing, replacing, moving, substituting parts, and then leaving the amp in a mess. Do you know the history of the amp? Does it look like someone else has been "repairing" this before?

Now to your problem, what are the symptoms, and what have you checked so far?

[exclamationmark]

Marshall schematics are usually pretty good (ie, accurate), but the layout has of their schems has always confused me. I suppose when working on a pcb it's easier to read. Not sure about the TSL, but they may be going the ENGL/Mesa route and purposefully leaving proprietary 'mojo' values out of their schems, and anything not related to imminent safety or function of the amplifier. Although, if you built an ENGL to factory schematics it would probably kill you...

[Enzo]

I don't think Marshall would bother to waste the time to sabotage the schematics. It wouldn;t even slow down any competitor who wanted to "steal" any of it. Hand the board to an intern and say, "Trace this out and verify the schematic." Problem solved.

I agree with Bill completely. Revisions happen all the time, and many are not documented. And knowing that the unit was working and failed in the form you now see, is a big advantage over someone has been changing parts so you don;t know if it EVER worked in its present form.

No, please don;t try to modify the thing back to some other revision. Chances are the changes have reasons behind them, like circuit stability, component reliability, etc. And sometimes the board design is used in more than one product, and certain parts may not be needed in some situations. For example, the only zener diodes I se are across the inoput to the return stage, and as such they are just limiters to keep the signal from going over about 3 volts.

If the thing used to work, as Bill said, those missing parts were not involved, nor were component value changes.

In electronics, it is about the circuit, not the parts. If all you see different is resistor values, then the circuit is the same. Resistors affect the gain and freq response and stiff, but it still should work.

What are you trying to fix?

[Gtr_tech]

TSLs had the issue with the wrong grid stoppers in the output section. They should've been 5.6k and there was 220k or something stupid stuffed into those positions. Thats the common factory boo-boo.

[jamesmafyew]

There are a bunch of undocumented changes to the later TSLs -- all the ones I've noticed are good common-sense things. An example is that all the schematics I've seen have a 10k at R77, yet most mainboards have a 3.3k there. That's better, gives you a wider range of bias voltage. There are a number of things like that throughout.