Mesa Boogie Mark 3 Gain Loss

[sixstring63]

Hi, It seems a lot of you have some Mesa experience so I'm hoping you might be able to help me out. I have a 1985 Mesa Mark 3 60 Watt combo that I purchased new and it sounded great for a number of years but then started to gradually lose gain on the rhythm 2 channel which also affected the lead channel.I've changed all tubes countless times over the years and no real improvement.In 1994 I retired the Mark for a Tri-Axis because it had lost its crunch.I recently joined a small Blues band and did not want to drag a big rack and cabinets to small clubs so I though I would dust off the Mark and try to get it working good again,the amp fired up fine but still the same old lack of gain,bought all new tubes and replaced the filter caps on the main power board and still no cigar plus the lead tone is muddy and boomy with no bite.I'm new to doing major repairs but have been doing a lot of research trying to learn how to repair and trouble shoot myself.I've been playing for over 30 years and have a good electrical background and some basic electronics.I have built my own stompboxes and metronomes and that kind of thing and feel comfortable changing out components but I am just at a standstill on what to look for on this gain loss .The crunch channel on 10 is as clean as a Fender Twin where it used growl like a Marshall,it is clean and bright but cold and sterile almost like a clean transistor amp.I really am getting into learning how to do this stuff and going to try a kit amp soon but would like to try and save the Mesa,Any ideas you might have or help on this problem would greatly appreciated.

Thanks, Sixstring63

[Enzo]

You might have an open plate load or coupling cap, and the output is now just crosstalk. I'd verify the power supplies are right, then I'd go down the row of 12AX7s and measure plate and cathode voltgaes on each, looking for missing or way high plate volts and missing cathode volts. That will tell you if a tube section has gone sour. Not thinking the tube itself, I am thinking the support parts.

Certainly in a Mesa, a bad opto can have a bad effect on your sound.

Once the basics are covered I apply a signal to the input and follow it through the amp.

[pdf64]

I fixed a MkIV with this same fault awhile ago that had me puzzled. It ended up being a coupling cap gone oc. I seem to remember it was a tantalum type in a low voltage part of the circuit. I say tantalum, just guessing, one of those small cylindrical things which you might think was a resistor which Mesa use.
Best to get a schematic, analyse the signal paths for the various channels, then maybe map it out on a print out of the pcb overlay or similar. Work out what the ball park signal levels should be along the path. Then inject a signal and see where it takes an unexpected dive.
Good luck - Peter.

[Enzo]

Those are ceramic.

Tantalums always have a polarity.

Tantalums usually look like plastic rain drops - dipped in epoxy. Like these:
http://www.avx.com/docs/Catalogs/tap.pdf
http://www.vishay.com/docs/40020/199d.pdf

SOme look like miniature electrolytucs
http://www.vishay.com/docs/40015/150d.pdf

And of course there is always surface mount crap where they all look alike - too small

[mac1amps]

I'd also check to make sure that the front-end preamp tubes are "firing-up". I've run into a number of Mk series Mesa's that had problems with the DC heater supply, which only affects the first preamp tube...(or is it the first two?.....I don't recall). Otherwise, it may also be "strayed" value plate load resistors.

[sixstring63]

First Tube

Just from a visual and touch it seems the first tube is not even getting warm ,I came to this conclusion myself. It seems to be in the first gain stage somewhere but now to track it down with my very novice trouble shooting skills.I have been learning a lot thanks to all the people here with great knowledge in this area. I'm going to put a meter on it and check some voltages and see what I find.

Mike

[sixstring63]

Voltage

I checked plate and cathode voltages on all the preamp tubes and everything seemed ok compared to the only barely readable schematic that I could find online and also used a Mark4 schematic for some values I could not read as I have been told they are similar except for the tone stack config. So what next? I'm a rookie at this and stuck again.
Mike

[Enzo]

I'd be back down to the last line in post #2.

[sixstring63]

Use a scope ?

Should I follow the signal through the amp with an Oscilloscope ? Forgive me for sounding ignorant to these suggestions as I am trying to learn as I go . I do have a scope that I got with a electronics home study course I started 15 years ago while laid off from work but only made it 1/3 of the way through as I was called back to work ,got married ,bought a house ...........etc. I always wanted to do this but did not have time until now. I do not have a tone generator to run a signal with ,would a keyboard holding note work ? You guys have been a big help so far.
Thanks ! Mike

[pdf64]

Yes a keyboard would be fine, turn any modulation off and get the purest tone you can, somewhere around middle C, about 100mV level.
Or use your pc soundcard, download signal generator shareware.
A scope or a DMM will be fine, you're just looking for ac and dc levels. Those Mesa boards tend to be tightly packed, think about your probes, best to get miniature insulated clip on tips for them, otherwise the probe tip could easily slip and short adjacent component legs together.
Assume you've already found the schematic
http://www.schematicheaven.com/boogi...ogie_mkiii.pdf
Not good image quality, Aspen Pittman's Tube Amp book has the same with slightly better quality.
Has the clean channel lost gain as well? There's not much used by 2 and 3 that isn't used by 1 also. Peter.

[chipaudette]

Hi Sixstring,

Glad you posted this to the wider community.

Your comment that the "first tube is not even getting warm" really leaps out to me. Is it glowing at all? That first tube should get just as warm as any of the other pre-amp tubes. If it's not getting warm, it makes me think that the heaters are having a problem. If you've already swapped tubes, then the problem must be with the wiring in the amp that goes to the heater pins on the tube.

I'd bring out your multimeter and check the voltage across the heater pins of your first tube. I'm pretty sure that the heaters are AC, so you'll have to set your multimeter to measure VAC (not VDC like most of the other measurements that we make inside our amps). You should read something lie 6V (AC).

Let us know how it turns out!

Chip

[sixstring63]

Cold Tubes

Thanks Chip , I'll check that out as I have not found anything else yet. There are 2 tubes in the preamp that have no glow at all not matter which tube you put in those spots.
Mike

[NorCalTuna]

I have one of those amps, a 100 simul long chassis with all the toys- and it's totally rippin'! it's 95% of a IIc+ and definitely worth fixing up. I got mine from an engineer selling his collection of big amps- I also had to pass up a minty SUNN that day! Mine had weak gain because of a damaged pot and a bad Effects Send jack, which is worth checking on yours too.

Reading the thread though, the clue is right there: the first two tubes don't "light up" or heat, and then I looked at that schematic from heaven(?)... it's so old, it's of no help: at least one of those filaments (v1 according to this snip) is dc powered, a' la marshall lead 15. Here's the part you need, from a schematic I got with my amp (No manual, but a schematic- I love engineers) which I suspect will help solve your issues. Do your footswitches work? also, isn't the r2 relay powered the same way? does your amp sound like a grizzly fender but not quite a burly mesa (the relay bypasses the traditional 3.3 resistor and adds a bypassed 390k to ground instead)? that snip belongs at the end, just line up the 100k resistors.

as to why the other tube isn't heating, I'm thinking about taking mine apart to see if the DC supply is to the left (with the power supply at large) or to the right, after V2... could there be a open wire or trace? or does your amp power V1 AND v2 off that circuit, which might have overloaded or broken it? There were so many variations in those amps you may want to post pics of your particular one. That and I love mesa pr0n.

Good Luck! sounds like you're gonna be going by the weekend.

[sixstring63]

Heater Voltage

I checked the heater voltage on V1 and could only get a 1.3 V AC reading and a -1.2 DC and about 3.2 volts AC on the rest of the Valves.I found some things that look suspect around the V3 heater pins and around the 4 resistors towards the back of the chassis around the 3 large caps,I checked those resistors with a meter and they did not seem to match with the value of the bands even after factoring in the tolerance. I'll attach a photo with the areas that look funky circled in yellow. Does this board have traces on both sides ?

Thanks ! Mike

[sixstring63]

Should I be getting the full 6 volts to the heater pins ? I get 3.2 VDC to pins 4 and 5 to pin 9 on V1 and -1.6 from pins 4+5 to ground and all other tubes are 3.2 VAC to ground. The heaters do not seem to be getting enough voltage to me.The tap from the PT is 6.4 V across both leads that also run to the power tubes which seem fine. I'm kinda running out of ideas. The voltage to my Ch. 2 foot switch is only 3.2 V and to the Lead foot switch with it is around 13 V and a big difference in LED brightness. Mike