Mesa Boogie .50 Cal+ hum problem

[soundguruman]

I hammer cause I know what goes wrong with them.
It becomes predictable.
But Chucky is right, the cathode bypass caps go bad too, and so do the LDRs...
The wires break off the board...the ground buss on the controls is probably not soldered properly.
(even though it appears to be correct, quite often the soldering is defect)
BUT the whole thing about this amp is that you can't fix it without removing the circuit board.
AND when you do remove the board, you are sure to spot the spot that is arcing...
That's the usual suspect on most Mesa amplifiers...arcing, burning.
You get used to it after a few trials...

[Chevy]

Gents,
Thanks so much for all the suggestions.... see if I can answer the questions here:

Yes, the schematic posted appears to be the same one I have, and V2B cathode R is indeed color banded as 1k5.
There is pretty much zero hum at the speaker jack with the master at zero.
Voltages on V2B are 206V plate, 1.7V cathode.
I tried to ground the grid of V2B to verify that the GAIN pot is working and grounded, and there was no effect on the hum.
The hum on V3B plate measured around .5V p-p.
I don't have any real good 12AX7's.... but I put a NOS JAN 12AT7 in V2's position, and didn't see any significant change in the hum level... there was a bit I could see due to the gain change, but overall not significant.
Soundguruman: Aaargh. I HATE taking the pcb board off this #$%@&&*!!!! amp. What a POS!!!! OK... calm down for a sec... breathe.... I did find and lift R104 and R105, and don't see any carbon tracks there. Just on the off chance that there ARE carbon tracks lurking on the underside of the pcb, that I didn't see on my previous journeys there, I thought that they should ALSO be conducting if the DC power was ON or OFF (carbon resistive path, right?), so I should be able to see some 60HZ on the plate of the V2B stage even in Standby... logical, no? Well, there is absolutely zero (as far as my scope can tell) AC of any sort induced onto the plate while in Standby.

So... there is a ghost in the $$%&**!!! machine, far as I can tell. Short of pulling the pcb and once again looking for carbon arcs, or goosing the filament supply up to about 20-40VDC or so, I'm out of ideas.

[soundguruman]

Gents,
Thanks so much for all the suggestions.... see if I can answer the questions here:

Yes, the schematic posted appears to be the same one I have, and V2B cathode R is indeed color banded as 1k5.
There is pretty much zero hum at the speaker jack with the master at zero.
Voltages on V2B are 206V plate, 1.7V cathode.
I tried to ground the grid of V2B to verify that the GAIN pot is working and grounded, and there was no effect on the hum.
The hum on V3B plate measured around .5V p-p.
I don't have any real good 12AX7's.... but I put a NOS JAN 12AT7 in V2's position, and didn't see any significant change in the hum level... there was a bit I could see due to the gain change, but overall not significant.
Soundguruman: Aaargh. I HATE taking the pcb board off this #$%@&&*!!!! amp. What a POS!!!! OK... calm down for a sec... breathe.... I did find and lift R104 and R105, and don't see any carbon tracks there. Just on the off chance that there ARE carbon tracks lurking on the underside of the pcb, that I didn't see on my previous journeys there, I thought that they should ALSO be conducting if the DC power was ON or OFF (carbon resistive path, right?), so I should be able to see some 60HZ on the plate of the V2B stage even in Standby... logical, no? Well, there is absolutely zero (as far as my scope can tell) AC of any sort induced onto the plate while in Standby.

So... there is a ghost in the $$%&**!!! machine, far as I can tell. Short of pulling the pcb and once again looking for carbon arcs, or goosing the filament supply up to about 20-40VDC or so, I'm out of ideas.

Like I said, there's no way to fix it unless you take it OUT.
AND it is a supreme pain in the posterior. Times TEN.
There is no amp I hate working on more, and no amp that sounds better when it works right.
It's a love hate relationship.

In case you give up, PM me and you can ship it here.
But I hate working on them, and love testing them.
Usual for this problem is under $200. Yes it takes about 4-5 hours to do it all...just the basic repairs.
Complete service is about $600, (including all new caps and tubes, new LDRs, etc...etc...new Teflon wire, etc...)

Send it to Enzo, He fixes anything.

Wanna step into this? Where eagles dare ye ma-ties....

[J M Fahey]

Voltages on V2B are 206V plate, 1.7V cathode.

Well, this is something strange.
1.7V across a 1K5 cathode resistor means 1.13 mA
Which across a 100K plate resistor means 113V
Which added to the 206V plate volts you measure, mean 319V
Only problem is that at supply point C, where this voltage comes from, they claim to have 258V.
Please confirm. Some of these measurements are wrong, or some parts are way out of spec.

[soundguruman]

Well, this is something strange.
1.7V across a 1K5 cathode resistor means 1.13 mA
Which across a 100K plate resistor means 113V
Which added to the 206V plate volts you measure, mean 319V
Only problem is that at supply point C, where this voltage comes from, they claim to have 258V.
Please confirm. Some of these measurements are wrong, or some parts are way out of spec.

No, it's the tubes. There is a huge variation.
Use another tube, and the voltage will change.
The voltages are not amazingly accurate on the schematic.

[J M Fahey]

I didn't even mention the tubes.
I'm talking about voltage across *resistors*.
Ohm's Law, you know?

[soundguruman]

I didn't even mention the tubes.
I'm talking about voltage across *resistors*.
Ohm's Law, you know?

The voltage across the resistors varies according to the tube installed.
Each different tube produces a different reading.
*and the bias adjustment radically effects this too.
The circuit is only measured with tubes installed. Otherwise the readings are meaningless.

The voltages on the schematic are for whatever tubes were used at the time.
Whatever the bias "was" set at...we might not know.
If you use different tubes, you will get different readings.

I know that there is a substantial variation from the schematic voltages, but that's the tubes...
and the bias adjustment loading down the B+

[Chevy]

SG Man

You didn't acknowledge my point about the resistive path being there with the power on or power off... if it conducts, it should always conduct if it's just a resistor... so why don't I get any AC voltage whatsoever on the plate when in Standby?

[soundguruman]

I think I did mention that AC will be on the plate if the carbon spot has formed, with the standby off.
However it will also arc grid to plate, and form the carbon spot there too.
Is the grid voltage going way up with the standby on? That would be the indication...

[J M Fahey]

If you get 1.7V across a 1k5 resistor, you must get 113V across a 100K resistor which is in series with it and, obviously, passing the same current.
What do tubes have to do with it?
Or to get more to the point: does anybody think that Ohm's Law aplies differently in tube circuits than anywhere else?
Just sayin'

[soundguruman]

So anyhow, if you observe an abnormal voltage on plate cathode grid of V2a or V2B
it's an indication that the carbon has formed on the board.
but it probably will be a high voltage...
The resistors r104 r105 are mounted right on top of the plate voltage track,
and that's where the insulation breaks down.
you can see that area in my second photo, right between V1 and V2.
R104 (1.5M) is in parallel with C8 (180pf) and couples V2A grid to the input jack.
When the input is unplugged from the guitar cable, that resistor, sitting on top of the plate track, is grounded by the jack internal switch.
That's when it arcs, I suspect, and the carbon track starts to form.
R105, 330K or 680K (depends on version), couples V2A grid pin 2 into LDR two. I do not suspect that is what burns, although the resistor is still right on top of the plate track.
I have also observed that same plate track arcing through the fiberglass, into the surrounding tracks.

[Chuck H]

"My" meter sucks for reading low voltages. It has no zero calibration. You do need to do subtraction for a figure that represents the meters accuracy%. So is that 1.7V actually 1.7V. A small variance here can really skew the math on the plate load figures.

All else being equal, I agree that there's something hinky about the voltages. And it could indeed be a carbon track on or in the board. SGM continues to approach this as if the carbon track is there and needs to be discovered by the OP. I think it would be fun (not for the OP though) if the carbon track were there. Serious cap feathers for SGM. But the only way to know if there is a conductive path on or in the board is to look for it. Even testing won't always reveal this sort of thing because it might only become problematic with high voltages. I'd still try another tube. You can even use a tube that's in the amp now. Pull V1. If the hum is still there, as it should be, stick the V1 tube in the V2 socket. Any change??? Hum? Voltages?

[soundguruman]

"My" meter sucks for reading low voltages. It has no zero calibration. You do need to do subtraction for a figure that represents the meters accuracy%. So is that 1.7V actually 1.7V. A small variance here can really skew the math on the plate load figures.

All else being equal, I agree that there's something hinky about the voltages. And it could indeed be a carbon track on or in the board. SGM continues to approach this as if the carbon track is there and needs to be discovered by the OP. I think it would be fun (not for the OP though) if the carbon track were there. Serious cap feathers for SGM. But the only way to know if there is a conductive path on or in the board is to look for it. Even testing won't always reveal this sort of thing because it might only become problematic with high voltages. I'd still try another tube. You can even use a tube that's in the amp now. Pull V1. If the hum is still there, as it should be, stick the V1 tube in the V2 socket. Any change??? Hum? Voltages?

Yes you always try good tubes first, on this amp there is not a huge amount you can do because all the parts are mounted inaccessible.
I always wanted to cut out an access plate from the chassis, like on an old Ampeg amp, so that the board would be accessible.

come to think of it, plugging in an unterminated guitar cable
to un-ground the input jack...(cable only, but no guitar plugged in)
With the amp on, standby on, there could be high voltage on the input jack...
if that resistor 104 is burned to the plate track.
Funny I just thought of that.
I must still be drunk from last night.

[Chevy]

Sorry for my slow response... have very little spare time to work on this amp...

Ok, here's what I measure on V2B (preamp stage right after the tone stack), with my Fluke 111:
B+ is 330V, plate is 207V, cathode 1.7V, grid 0.0V.
Here is for V2A (first preamp stage for clean channel): B+ 335V, plate 202V, cathode 1.6V, grid .002VDC (weird...)

Soundguruman: I'm not debating the residual carbon trace/burn issue here, but I am very much interested in proving that that is the problem before I take this amp totally apart again. So, bear with me here... the issue is possible conduction between the heater traces and other traces, creating the 60 cycle hum, right? I figured if I lifted the 100K plate R's on V2B and V3B, as well as the two 100R heater resistors off the pcb, I would isolate the tube and heater tracks electrically from the board. The heater track(s) should have an infinite resistance then, to all other traces on the board. Then I could measure resistance between the heater track and the plate, cathode and grid, and actually see if there is any conduction caused by carbon.
Well, I did this, and there is zero conduction, (infinite ohms), between the heater traces and the plate, cathode and grid of V2B. Same with V3B.
Please keep in mind that the Gain pot is set down to zero, and I have grounded the grid of V2B, so V2A isn't even in the picture. The noise / hum comes from V2B it seems.
Unfortunately, after all this, I still don't have any idea where the hum is coming from, except maybe the filaments themselves. But then, if that were the case, the .50 Cal+ should hum like a bugger right out of the factory, in all cases, right?

[soundguruman]

Sorry for my slow response... have very little spare time to work on this amp...

Ok, here's what I measure on V2B (preamp stage right after the tone stack), with my Fluke 111:
B+ is 330V, plate is 207V, cathode 1.7V, grid 0.0V.
Here is for V2A (first preamp stage for clean channel): B+ 335V, plate 202V, cathode 1.6V, grid .002VDC (weird...)

Soundguruman: I'm not debating the residual carbon trace/burn issue here, but I am very much interested in proving that that is the problem before I take this amp totally apart again. So, bear with me here... the issue is possible conduction between the heater traces and other traces, creating the 60 cycle hum, right? I figured if I lifted the 100K plate R's on V2B and V3B, as well as the two 100R heater resistors off the pcb, I would isolate the tube and heater tracks electrically from the board. The heater track(s) should have an infinite resistance then, to all other traces on the board. Then I could measure resistance between the heater track and the plate, cathode and grid, and actually see if there is any conduction caused by carbon.
Well, I did this, and there is zero conduction, (infinite ohms), between the heater traces and the plate, cathode and grid of V2B. Same with V3B.
Please keep in mind that the Gain pot is set down to zero, and I have grounded the grid of V2B, so V2A isn't even in the picture. The noise / hum comes from V2B it seems.
Unfortunately, after all this, I still don't have any idea where the hum is coming from, except maybe the filaments themselves. But then, if that were the case, the .50 Cal+ should hum like a bugger right out of the factory, in all cases, right?

Did you measure all these points with the MEGAOHM scale?
The typtical carbon trace will measure like, 2.2 megaohms.
If your meter does not have megaohm scales, you may have missed it.
If the meter is set to the lower scales, it won't show at all.

Connect a guitar cord to the input jack, but no guitar.
Use a DC voltmeter to measure the open end of the guitar cable.
IS there DC (probably a higher voltage) between the tip and sleeve of the guitar cable?
IF there is, that resistor laying on top of the plate track is arcing.
Check that first. That's where it burns.

I have one open on the bench right now...
and soooo...I will take some readings and make some notes, and pass that on to you.

The heater tracks are each tied to ground with a 100 ohm resistor.
This balances the heaters
there is no center tap for the heater winding.
as long as those 100 ohm resistors are tied to ground...anything tied to ground will show resistance to the heaters.