not a mesa dealer no schematic.they have made changes in the circuit in question. he told me of a couple of parts they changed but wouldn't give any other information except sorry we can't let you change the parts. send it back to the factory....now a 15 minute job for me is going to turn into a 30 day wait for the amp!don't forget it's on the opposite coast. wish i could of helped with a schematic or service manual. i would of been helpful instead of a pita...thanks for everyones help!my good friend and tv repairman of 40 years big jim gave me the sugestion to raised the resistor off the board to cut down on heat radiating to the circuit board and the ceramic cap and resistor in front and back of the 10k wirewound.guess they must of herd you jim!!!...later....ed

 heres one for the pros!!!i replaced the 3.75w 10 k resistor on the output board.it was getting hot due to a transistor having broken legs. someone previous to me must have gave it the push test and broke a couple of legs. i replaced the transitor only as i didn't have a resistor at the same time i replaced the transitor.the resistor still checked good at 10k.the amp worked for a bit.then was hooked up to a single extention chord with the rest of an entire bands equipment.i guess a couple of other elcectronic items the band was using had problems also. the amp sounded good at the sound check then had noise and lower power so it was replaced with a different amp for the rest of the night. the amp was taken apart again and the 10 k resistor was replaced with an exact replacement 10k 3.75w wirewound.everything checks out component wise ,but the resistor get hot 300 degrees plus when checked with a infared temperature gage.it also has 90v+ on one leg and  -89.7v on the opposite leg. i notice there is a trim pot on the board would it have anything to do with the heat or is it just for biasing the transistors.is it possible that the 10k resistor is an isolation resistor?wondering if mesa did any upgrades possibly a 5w 15k resistor or something larger.possibly a design flaw.where to go from here.any ideas...thanks....ed

GOT THE AMP WORKING IT HAD  BROKEN TRANSISTOR LEADS.THE 10K RESISTOR NEXT TO THE TRANSISTOR WAS GETTING HOT.I JUST PUT THE TRANSISTOR IN THE AMP AS I HAD TO WAIT FOR BACKORDERED RESISTORS.THE AMP HAS A 10K 3.75 WATT WIREWOUND.MOUSER SENT ME 10K 3 WATT RESISTORS AS THAT IS ALL THEY HAD.IT DOES SAY ON THE SPECS IT IS GOOD TO 3.75WATTS ,SO I'M PROBABLY OK. I'LL HAVE TO TAKE THE AMP APART AND SEE WHATS GOING ON.HOPE THAT THE RESISTOR WAS WEAK AND IT ISN'T A DESIGN FLAW.POSSIBLY THEY HAVE UPDATED THERE CIRCUITS ON THEIR NEWER MODELS. THIS IS THE SECOND TIME THIS AMP HAS BEEN DOWN SINCE IT WAS BOUGHT.SOUNDS GREAT WHEN IT WORKS.WILL KEEP YOU POSTED.....

guessing it was the tl074 quad chip as i had no signal on one of the outside output legs.there was signal on the other 2 chips but it was very low.since i had the board out and time is of value i replaced all 3 ic's.....ed

got it going.found another bad ic.i was injecting a signal into the input with my mjf signal tracer injector (love this thing for non digital solid state circuits)i had volume on the hot leg of the speaker jack when it was in my load box that was controlable from the front panel. you would swear it was working fine.when you plugged a guitar and speaker cabinet while playing a guitar i had no sound. after replacing a couple of suspect ic's with sockets (tl072,tl074 quad chip, & ne5532)bam it got a lot louder signal in the speaker connector of my load box. i pulgged it into a speaker cabinet and hooked up a chord and bass and working good as new....thanks for the help....ed

does anyone know where i can get a schematic for a marshall mb-450h amp head.i put a signal in the input and get a signal at the speaker that is controllable by the volume controls on the amp.when i hook up a guitar and speaker i get nothing.

jm where do you insert a signal on a fet transistor (gain,source,drain).i have a mfj signal injector tracer.if i put  a signal into the source terminal ground the second lead on the tester would i get the output at the drain terminal on a good fet?....thanks....ed

where can i get a schematic for this amp. it powers up but no sound. i found 2 bulged  47uf capacitors and replaced them along with 2 bad ic.i have signal to the output board  jumper wire now but nothing after that...thanks...

just took the amp apart again to try to fix the volume drop issue.i plays good for a minute or 2 then the volume drops off.then after 20 or 30 seconds it comes back to full volume. any ideas....ed

thanks jm. i'm always looking to upgrade my testing skills and you and enzo are in a league far above me. i'm still learning.i've been fixing most things i work on ,but now and then you get a few bugaboo's.if it was playing guitar i might be better able to help.iv'e been playing 39 years...as always thanks for your help jm.

i place an ab switch in parralel first so i can place a tuner in circuit without it stealing signal, then a  wah wah followed by a compressor,distortion,eq,phase,then delay or reverb...hope this helps...try a menatone blue collar pedal.one of the orignal ones without the external battery compartment.it's the only overdrive pedal i have used that maintains the original sound when the knobs are adjusted.i have hundreds of pedals and this pedal blows the tube screamers out of the water. this is the pedal that put menatone in the pedal business.it put's everthing i have tried by the wayside....also try a orange squeezer kit for a compressor from general gadjets,they also make a good kit for tube screamer people. it is a really good pedal also close ,but not a blue collar.for the money though it's a great overdrive pedal.the blue collar goes from clean to marshall like tones.hope this helps!!!....edwardeffect1

i noticed in a post someone recommended using the send outlet of the effects loop to run a signal into the input or return  jack of the effects loop of another amp.if the amp was playing right the preamp would be good on the amp in test. if it didn't get proper sound it was recommended to do the enzo signal tracing test....what is this test....thanks...i use a mfj injector/tracer.it works excellant in testing ic's,output chips,and signal tracing the amp itself.i just fixed a alto powered monitor and i had signal on the output chips output ,but it was really low in volume. when i touched the input jack where i input the signal for the signal tracer it was also extremely low in volume. i thought i had a low battery in the tracer. when i unplugged the input jack with the injector leads still hooked up and touched the positive side of the jack the signal was loud as it usually is when testing. something was loading the circuit. i got my ohmeter out and found 4 shorted zenier diodes.i replaced them and bingo got a loud signal all the way to the output chip.the alto is back and running.could you explain enzo's system of testing...thanks...ed

thanks again jm for the reply....ed!

does anyone know the value of u1 voltage regulator.i believe it is a lm1086.all the regulator voltages are marked on the circuit board except u1...thanks...ed!!

i believe i read a previous posting somewhere about checking ac voltages on the chip.i have an oscilloscope.how do you do it?i know all output chips are different,but there must be some simularities in  the testing proceedure!