Mesa Boogie Express 5:25 or 5:50

[dobrota]

I'm looking for it for long time. Thanks !

[QJerry]

here you go

[Jazz P Bass]

Mesa Boogie Express 5-50 Schematic

Repost of the file.

[igaar]

Thank you so very much, this does have some differences in how things are arranged and switched on and off compared to other units. I fixed the Reverb problem when I took the chassis out of the cab, the cab holes were drilled in the wrong place to line up with the chassis holes and there was a support screw that did not go into the top of the cab in about the middle of the chassis where it should have, (did not put that one back in) also found a soldered lead bent over hard touching another part of the circuit on the switching board in the back of the chassis, put it all back together and reverb works without adding tremendous gain causing oscillations making the amp not usable. Now i noticed the Contour controls seem not to work, this one has no EQ added to the front of the chassis. It's a good sounding amp even if the contour controls don't work. will connect the foot switch to see if that has something to do with it.

[thegroup98]

Hi I have the same problem with the contour. ... you then solved the problem
If yes I would like how did
Thanks Angel

[igaar]

hi Angel, I'll look up the schemo, it was 4 years ago when i fixed that bugger. But a Contour Control is usually connected to the Slope resistor in the tone tank, it's the resistor just before the Treble cap connected to the Bass cap at the beginning of the tone tank circuit if that makes any sense to you if you have the schemo to look at. the contour control could be connected in parallel or series to that slope resistor, could be a bad control, broken wire, bad solder on the board in the tack section, could also be one of the tone caps is leaking DC voltage into tank, that would mean the other tone controls may sound very noisey when turned and not working correctly either. Let me know if you have the schemo, I'll pull up mine and we can discuss this more intelligently
.

[igaar]

leave it to Boogie to reinvent the wheel! this is no Contour control that i'm familiar with, has nothing to do with the standard tone tank, page 5 shows the whole thing, there are transistors control it for either channel connected to 2 100 pots, you can look there for connection issues, it looks like the actual contour control in that matrix circuit to the bottom left and it is passive but it does connect to the differential amp directly above that. This used to be the old 4 band EQ that the Mark series amp had in them with 4 separate bands however this info does not tell you How it works, the Mark series had this section on a separate board, the Express may have it all lumped together, I don't know, I would look very closely at where the 2 100k pots are to see if there loose or badly soldered wires there. The contour could be working on one channel, if so then its a connection or wire issue, if not it could be the differential transistor amp or some other active transistor in the circuit or some low voltage circuit that drive all the transistors in that part of the amp, does it switch channels? Do channel lights come on??

[igaar]

Look for +12 volts on transistors Q1,2,3 and 4 on that page 5. If you don't know what transistor are or how they work, you are going to need much more help. Iff the 12 volts is gone, look for a fuse or the 12 volt circuit or verify 12 volts at the source output back on page 2 center of the page at the 7812 volt regulator output plainly marked +12 volts. If you have no voltage there but you do have DC more like 16 to 18 DC there then the reg is out. If there is no DC before the reg, then look for AC coming out of the Power Transformer, if you don't have AC coming from the power transformer then the bad news is that low voltage winding burned out, I don't see a fuse there either so if this is so that would be bad news, I kinda doubt though the problem would go that far. I could a low voltage cap in the 12 volts also like C60 or C48. could also be R34 or C25 next to Q1 at the Contour circuit. you should start checking at the collector of Q1 and work your way back to where ever you find +12 volts DC and then you will have the answer.

[Luthin You]

Hi , guys , any chance you could re-post that schematic ? So hard to find !

[Enzo]

The one in post #3 just opened fine for me.