Mesa Boogie 5:50

[leydenjar]

Why might I have 2.5 vdc on the C node of this power supply?

I've checked the e caps. I've checked cotinuity between solder pads. I've checked C51, C43, the choke, and yes the 6k8 in the dropping resistor string is good.

Please note: my variac is at 53 vac not 120vac.

Express525-550 schem.pdf

[Jazz P Bass]

What is the resistance to ground (amp Off & Drained) of node C?

[leydenjar]

What is the resistance to ground (amp Off & Drained) of node C?

232 ohms.

[Jazz P Bass]

232 ohms to ground, huh?
That really does beg the question 'where is that coming from?'
There should not be a path to ground.
If you look at the schematic the C Node goes to the preamp plate resistors.
You could start by pulling the preamp tubes.
Turn the gain controls & see if that 232 ohm reading changes.
I am thinking a bad coupling cap.

[g1]

Try disconnecting or removing the 2 caps at node C (C53B & C54). What happens to the voltage?

[leydenjar]

Okie doke. Pulling tubes made no change.

Turning master vol 2 all the way down lowers my 232 ohm resistance reading to 163 ohms.

I tried pulling C53B and C54, no change in DC voltage at node C.

I tried measuring resistance from all preamp plates to ground and I got readings of approx. the plate load resistors on the respective plates.

Measuring resistance from the solder pads where the caps connect isn't giving me a clue either.

[Jazz P Bass]

Turning master vol 2 all the way down lowers my 232 ohm resistance reading to 163 ohms.

I think C4 (.001 coupling cap) is shorted somehow.

[leydenjar]

Plate side of C4 resistance to ground reads 104k.

The other side of C4 to ground reads 860k (the gain pot).

C4 also checks ok with a cap meter.

Not shorted.

[Enzo]

Let us clear one thing up, you cannot test caps with a hand meter. Because it doesn;t measure shorted on an ohm meter tells us little about the cap health. Caps that look fine at the volt or two your meter uses may leak like a screen door at anything over 20v or 40v or even 240v. Bottom line is they have to withstand 400v, not 4v.

You have 187v at node B, and from there we have 4k7, 5k6, and 6k8 all in series with the 232 ohms. Look at them as a voltage divider across 187v, and calculate what you expect at node C. I get 187v x 232/(4k7+5k6+6k8+232) And that comes out to 2.5v. Just exactly what you have.

Find out where that 232 ohm is coming from and your voltages will fall back into place.

Is the amp burning up parts? Or blowing fuses? If not, why are you running it on half voltage with a variac? If it IS burning things up, then what part is burning at full mains? That would provide a clue as to where this load is coming from.

Unsolder and lift one end of that 6k8, removing node C from power. DO the other nodes come back up to 300-400v land?

Your preamp tubes are all separated from this node by 100k resistors, so the tubes/sockets/coupling caps don;t seem to have any way to get 232 ohms onto node C. No way in the world the master volume control could affect that given the circuit in the diagram, so my money is on a short in the circuits. A solder bridge, or a component with longer legs leaning against another or grounded, even a loose piece of hardware, or a hunk of wire trimming.

[Enzo]

Let us clear one thing up, you cannot test caps with a hand meter. Because it doesn;t measure shorted on an ohm meter tells us little about the cap health. Caps that look fine at the volt or two your meter uses may leak like a screen door at anything over 20v or 40v or even 240v. Bottom line is they have to withstand 400v, not 4v.

You have 187v at node B, and from there we have 4k7, 5k6, and 6k8 all in series with the 232 ohms. Look at them as a voltage divider across 187v, and calculate what you expect at node C. I get 187v x 232/(4k7+5k6+6k8+232) And that comes out to 2.5v. Just exactly what you have.

Find out where that 232 ohm is coming from and your voltages will fall back into place.

Is the amp burning up parts? Or blowing fuses? If not, why are you running it on half voltage with a variac? If it IS burning things up, then what part is burning at full mains? That would provide a clue as to where this load is coming from.

Unsolder and lift one end of that 6k8, removing node C from power. DO the other nodes come back up to 300-400v land?

Your preamp tubes are all separated from this node by 100k resistors, so the tubes/sockets/coupling caps don;t seem to have any way to get 232 ohms onto node C. No way in the world the master volume control could affect that given the circuit in the diagram, so my money is on a short in the circuits. A solder bridge, or a component with longer legs leaning against another or grounded, even a loose piece of hardware, or a hunk of wire trimming.

[leydenjar]

So I've just pulled up the circuit board on this amp and I may or may not have corrected my problem.

Do you guys ever carefully power up amps with the pots and jacks NOT screwed back onto the chassis to avoid having to assemble / re-assemble over again in a situation like this?

[Enzo]

It REALLY depends on what I am doing. I have powered up a spread out Peavey Classic 30 board set and fired it up. Took most of the clip wires I own to get all the things grounded that needed to be, but I did it. Yes, if I have some purpose that will be served by doing so, I will. Comes up a lot less often on tube amps for me, but give me some Peavey solid state thing, and I will flop the board over onto a folded towel or stack of newspaper in a heartbeat.

If you lifted the board and the 232 ohms is now gone, set it back down and see if it returns. If you had a flake of solder or something causing the short, and it fell away, then the amp may well be fixed.

[leydenjar]

I had a resistance reading that i thought was reasonable from node C to ground with the pots and jacks loose. So I reattached the jacks and pots and powered up the amp. No dice. Same voltages, same 230 ohm reading from node C to ground.

So I pulled the input jack and resistance at node C jumps to 1k but that still doesn't bring node c voltage up to anything near what it should be.

By the way, if I don't keep the mains voltage low the amp burns up the power supply dropping resistors.

I pulled all the other pots one by one and resistance stays the same

[Enzo]

So power off, measure resistance from node C to each thing that might be involved grounding it. The node is obviousloy shorted to something, and that something has paths to ground. Since you go from 232 to 1000 ohms I have to think wherever the short might be, it then has several parallel paths on down to ground.

COnsider that input jack. How on earth might it be connected in any way to B+? I can;t think of any. So there must be some odd path between the B+ node and that jack. Not a dead shoprt, but low enough.

Stop using voltage, it just stresses things further, you already know that 232 ohms is a serious problem, so no power, chase it with ohms. You reported that the master volume affected the readings, and ungrounding the input jack, Schematically those have nothing at all to do with any high voltage circuit, so the failure will not likely be on the schematic. COnsider that if two copper traces run side by side on the board, they may have nothing to do with each other in the schematic, but if they short together, all hell breaks loose. The schematic may help give the readings some sense after we find the problem spot, but probably won;t help us solve this. What the schematic does do is let us find readings that are way out of place that MIGHT point us where we need to go.

How is this thing constructed? Are ther any ribbon cables between sections? Ribbons can get plugged in off a pin. Pin headers can get bent so one is bent down touching its neighbor.


A couple questions: Is this the real commercial Mesa amp? Or is it a home made clone? Did this amp used to work and just one day found itself like this? Or has it been worked on for some other problem and during the work now has this problem too? Either at your bench or at some place else?

[Enzo]

As to powering up a loose board: no more power up the board. Resistance only, looking for the mysterious 232 ohms.

Maybe this: Pull the board, and with all the jacks and controls no longer grounded, measure node C to circuit common. (Not chassis) Now with clip leads, ground each thing in turn - the input jack, the master volumes, whatever. Which things affect the resistance from node C to common? And which don't?