Mesa M-pulse 600

[Valvehead]

Working on a bass head = Mesa M-Pulse 600. It seems to have very low volume. Sounds like 2 watts thru my 4x12 cab. This thing should be VERY loud. It uses 2, 12ax7 in the preamp , and 12 output chips.

What Ive done so far :

cleaned all jacks--checked to make sure they are switching / contacting.

checked all op amp type 8 pin chips. they all have +15v and -15v

replaced the 2 tubes.
cant find any chips that are getting hot. None of the 12 output chips get warm.

moved all the wires around , hoping to find a bad connection

Loop tests-- using the mesa slave out to a 50w tube amp return, its not very loud.

mesa send to 50w amp return = the 50w amp does get fairly loud--definitely pushing its 50 tube watts


50w amp send to mesa return = not loud at all.

Any ideas on where to look next ? Maybe start pulling the boards to check solder joints ?

[Alex R]

I think before pulling the board I'd want to check the functioning of the output section and the power supply. Check the main power rails are ok. I put a new PT in one of these two weeks ago. As far as I recall the rails are something like +-70vDC, but i can't check now and I never had a schematic. But checking the rails are equal voltages would be a start. And that there is no DC on the output. Power amp problems usually result in silence and blown fuses but you never know.

If the power amp checks out you have a smaller section of the circuit to look into - the part between the return and the power amp.

[Valvehead]

thanks alex---

all output chips have 90vdc on them. No dc on the speaker jacks .

[Alex R]

Seems a good chance the power amp is ok then.

v2 forms most of the driver section between the effects return and the power amp. You've tried a new valve - a good next step might be to check the pin voltages on v2. Pins 1 and 6 should have high voltages, well over 100vDC, one higher than the other. Pins 3 and 8 should have lower voltages, one in the 1-2vDC area, the other higher, maybe 50-100vDC. One half of v2 is a standard gain stage, the other a cathode follower.

There's a schematic on this thread http://music-electronics-forum.com/t33690/#post312824

[Valvehead]

just found something very odd...

measuring the tube sockets---no tubes in.. plate pins are ok -- 500v --seems high?

but on one socket ... i get 500v on pin 2 ! also on pin 3 i get 230v with the tube in.

i havent looked at the scem...but that seems very wrong--that kind of voltage on a grid and cathode pin ???

[J M Fahey]

just found something very odd...

measuring the tube sockets---no tubes in.. plate pins are ok -- 500v --seems high?

but on one socket ... i get 500v on pin 2 ! also on pin 3 i get 230v with the tube in.

i havent looked at the scem...but that seems very wrong--that kind of voltage on a grid and cathode pin ???

Well, *please* look at the schematic !!

Otherwise, what are you doing inside that amp?

500V "no tube" are to be expected on V2b plate and V2a grid, it's a cathode follower after all.

230V "with tube" is normal for V2a cathode, same consideration as before.

all output chips have 90vdc on them. No dc on the speaker jacks .

Maybe you mean *some* output "chips" have +90V on them, and others have -90V .
Not the same thing, a 180 V difference.

Loop tests-- using the mesa slave out to a 50w tube amp return, its not very loud.
mesa send to 50w amp return = the 50w amp does get fairly loud--definitely pushing its 50 tube watts
50w amp send to mesa return = not loud at all.

This means: Preamp good, Power amp low gain or driving signal lost in the path.

1) enable effects (pull switch/FSW ? )
2) set wet/dry pot to full wet.
3) feed 400Hz or 1KHz sinewave at effects return.
Beware you *might* need up to 1 or 2 V RMS, so you need a good signal generator.
My usual suggestion of an MP3 tone from a cheap MP3 player does not hold here.
4) scope output of amp properly loaded , rise driving signal until it starts clipping.
Measure output AC voltage.
I expect something in the range of 35/45 VAC.

Post results.

Do not retouch solder or replace parts at random, trying to catch the problem by sheer luck, troubleshoot first.

[woodyc]

This will probably be of no help, but...

I've seen several of those that have had the same problem, namely a shorted or pulled down +15 supply. If thats the case, the bad part is not easy to find. And its even harder if you have a schematic because its NOT on the schematic. Its labelled C44 on the board, a 15V cap, tantalum I think. If you have the board out, its probably a good idea to replace it with an electrolytic even if it isn't bad.

Also many thanks to vintagekiki for posting that schematic. And besides the missing C44...

There is also a limiter that isn't on the schematic. Its in the signal path between between C61 and the FX-MIX wiper. Its real similar to the other limiter (the one that is on the schematic), except this one uses both halves of the opamp.

The wiring and part numbering around IC12 is also incorrect on the schematic.

[Valvehead]

i dont own a scope and my tone gen is mysteriously dead...powers on but no output.

woody-- i will look at what you suggested--thanks !

can you tell me the location of the parts you mentioned ? Most of the caps are not labeled on the board ! Is C44 a big mustard yellow tantalum cap ?
its labeled

173D +
156M

whats the value ?

[woodyc]

...can you tell me the location of the parts you mentioned ? Most of the caps are not labeled on the board ! Is C44 a big mustard yellow tantalum cap ?
its labeled

173D +
156M

whats the value ?

First, check a pin 8 on any opamp. If its near +15V you may not have this issue.

I think C44 is 15u/15V. And its kind of a superfluous filter cap you can just snip out and not even replace. The only pic I have is the bottom of the board, and I put an X where I think it is...

[Alex R]

well c14 won't be your problem as your +15v supply is ok. Nice to know about though, thanks woody!

[Valvehead]

ok--i did check all the op amp chips they all had +15 and -15 .

[J M Fahey]

Dear Valvehead; testing part by part is an endless task, unless it's something as simple as a speaker cabinet or pedal PSU.

You have a working amp, which seems to have lost sensitivity.

The shortest path is to test it *functionally*, in this case feeding it known audio, and measuring what comes out. (as suggested in post #6).

Until you do that, you will be running in circles.

If you don't have the tools, well, you don't, but then refer servicing to somebody who does .

[Alex R]

True.

We could however look at the v2 DC voltages with the tube plugged in. Pins 1,2,3, 6,7, and 8.

This amp uses a tube stage to drive its power amp.

[cgoepel]

WoodyC - Thanks!!!!!!!!!!!!!!!!!!

I have a customer's M-Pulse 600 on the bench with a short on the +15 volt rail. I first replaced the LM7815 because it was shorted to ground as well. This amp has 15 devices (luckily they are socketed) tied to the +15 rail. It is extremely hard to tell which one of 15 or 20 devices in parallel is actually shorted. Based on WoodyC's post pointing out C44, I certainly would still be scratching my head looking for that shorted tantalum cap. The original is a 15 uF rated at 15 volts. Guess what? There's no excess reserve or stress factor built into the design. According my days of calculating reliability and failure rates for military electronics, C44 had a high probability of failure based on it's rating of only 15 volts for a 15 volt circuit. I am posting the component side of the M-Pulse preamp board to show where C44 is hiding. Thanks to you guys who know your stuff and are willing to take time to post. Charlie G.

[Neurot]

Hi gang, count mine in the statistics for this failure. My head powered up fine but I was getting no signal from any of the outputs. C44 at fault here. Well, not much of a statistic, if the component is wrongly designed in, for sure it will fail from fatigue. Still, with this poor consideration from Mesa, this is a kick-ass amp. Thanks for troubleshooting this one, I had not used it since 3 yrs and my kids were eager to rock with it. This made our day!!!