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Hey everybody. I've got a mesa Maverick on the bench that's giving me hell. It's mine, so I'm not charging anyone for it. I'm getting sound, but not much. The rhythm channel barely does anything, but the lead does a lot more, but still significantly low output. All sound cuts in and out until I tap the chassy. I thought it was the main PCB, but then it worked when I tapped the power tube PCB, and then I noticed that it doesn't matter where I tap it. It comes back just by tapping the chassy. When it's "working", the EL84's have a blue glow in part of the tube, but when the sound cuts out the blue goes away?! I've replaced all tubes, filter caps, several diodes, 3 optoisolators, some pots, all screen resistors, resoldered any crappy looking joints, checked the OT. Where the schematic and PCB said there should be a 220 uF cathode bypass cap, there was a stock 220 ohm resistor on top of the 75 ohm cathode resistor, which I have replaced with the cap. I've tried plugging a cable into the fx loop and not very thoroughly checked some relays. I'm at my wit's end. I need advice. Someone please help!!!
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Throwing parts at an amp is never a successful strategy.
HAve you checked the pot covers? The metal covers on the rear of the pots are held in place by bent over tabs. If something smacks a knob out front, sometimes it pushes the backs out, and the back is what holds the wiper assembly against the resistive element. Just a thought. Any pot shafts seem looser than others? Get a finger on the rear of each pot, is that cover loose? Do with power OFF.Education is what you're left with after you have forgotten what you have learned. -
I will check that out Enzo, and get back to you. I was hoping you would respond. You're a jedi at this shit.Comment
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Nah, just a tired old guy who has done this a long time.Education is what you're left with after you have forgotten what you have learned.Comment
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It looks like the tubes are losing emission. Check by tapping and monitoring the voltage across the 75R cathode resistor. The voltage should be constant. If it's not check for bad solder joints in the cathode, screen and anode circuits. Does the cathode voltage remain constant when the tubes are 'wiggled'? If not check the solder joints to the tube pins.Originally posted by evil79 View PostComment
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Try connecting the effects loop send and return jacks with a patch cable or a guitar cable.These switching jacks are a common problem in Mesa amps.I've seen this same problem in a few Mk type amps.Comment
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You've got an intermittent connection, since tapping the chassis affects the situation, right? A chopstick might help localize the issue. You can press on the circuit board(s), ground connections, pots, etc., until you chase the problem to a smaller area of the amp.Comment
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Originally posted by Dave H View Post
I will try that this weekend and let you know. Thanks for the advice!Comment
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I've definitely tried that several times. I know that's a common problem with mesas.Originally posted by stokes View PostComment
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I've poked around very thoroughly with a chopstick and still can't isolate it to a specific section. Same thing happens no matter where I tap.Originally posted by xtian View PostComment
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Return jacks are not a common problem with Mesas, they are a common problem with any amp having return jacks.Education is what you're left with after you have forgotten what you have learned.Comment
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Do you have a voltmeter and know how to use it safely on an open powered up amp?
It’s important to troubleshoot by following the observed clues not just try all the things that “could cause” the problem.
That is a significant troubleshooting clue. HUGE. Proceed on the path suggested by Dave H.Originally posted by evil79 View Post
Also monitor the screen voltage (EL-34 pin 4) and the plate voltage (EL-34 pin 3) while it cuts in and out. Depending upon what you discover we can suggest the next steps if you do no find the intermittent yourself.Originally posted by Dave H View PostLast edited by Tom Phillips; 01-09-2016, 05:46 PM.Comment
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Loose ground.... If you can make it happen by tapping, your losing a grd to one of the circuits. I'll be curious to what you find.Comment
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What Tom meant was:Originally posted by Tom Phillips View Post
"Also monitor the screen voltage (EL-84 pin 9) and the plate voltage (EL-84 pin 7)"
And I agree with Dave and Tom. The power tubes are losing emission. It could be a shared ground (as 1ampman suggested) given that you have problems with the individual channels as well.
Mesa's probably aren't any more prone to effects loop jack problems as any other brand, but they ARE known for cracked or burned board traces. This could cause the problem you're having. No guarantees, but look at ground traces associated with the power amp and/or power supply.Last edited by Chuck H; 01-09-2016, 10:02 PM."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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What you need to do is trace the signal from the input to the output and see where its goes awry. For that, you need a small signal generator and a scope.
If you can't do the above, try this to isolate the issue to the preamp or power amp.
- Plug your guitar in and feed the effect send to another amp/pa. See if you get a good, proper signal.
- Plug your guitar in the effect return and see if decent sound comes out.
Without these, its just a shot in the dark... not really a nightmare. You need to be methodical when troubleshooting issues like this.
You could start by cleaning all connectors and tightening all ground lugs.
Dave H has good suggestions too.Last edited by AmpFix; 01-10-2016, 05:39 PM.Comment
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