Mesa Boogie .50 Cal + hum problem

[Jazz P Bass]

[QUOTE=soundguruman;236058]DISCLAIMER
Also, spilling beer inside of the amp should be discouraged, and is not recommended.
QUOTE]
Is this sage advise limited only to beer?

[Chuck H]

Is this sage advise limited only to beer?

+1
I once saw Dean Markley's wife set a glass of wine on an amp with a rear loaded chassis (face plate oriented 'up' like a tweed Fender). Of course someone tipped it over. And of course the amp fizzled, smoked and popped it's fuse... I had just done a cap job on that amp

[soundguruman]

And thanks to Dean Markley's wife, and others like her, we have job security....
May God speed the beer spillers.

[soundguruman]

Water has a chance of drying, you need something that leaves sticky residue, and attracts dirt.

[Chevy]

So... can I just order up 4 5C1's and pop them in there... any worries about that?

Guys, Can I just go with all 5C1's for LDR's 1 thru 4?

The 5C1 has a 50M "off" resistance, compared to only 400K for the 5C4. Also, the cell voltage rating for the 5C1 is 100V, but only 50V for the 5C4. (This seems low to me... for either of them.)

If someone can please tell me if these will work... I think all will be good.

Thanks kindly,

[soundguruman]

chances are that the problem was a carbon track arching on the board. That could cause hum arching between the grid and the high voltage or the high voltage and the filament track.
The LDRs might work fine, but I change them whenever I take the board out, or at least put them on the other side of the board, so that if one goes bad, it can be changed.
Don't forget when installing that the LED side is polarized. It is marked + and -must be installed facing the correct way.

[Chevy]

Thanks, question is... will the 5C1 ldr's work in this amp in LDR positions 1 thru 4?

I don't know if this is the right part or not; I am going to replace LDR's 1 thru 4. Can someone tell me if the 5C1 will do the trick ?

Thanks,
Chevy

[soundguruman]

It's true that 5C1 will work in any position, just not quite as good. I usually use all 5C4s.
Mammoth has 5C4 if you want to order them. I don't know anybody else except order them from Boogie.

[soundguruman]

Hi Guys,

Having a hum issue with this amp... 60 cycle stuff that's pretty annoying. I've just changed the power supply filter caps, but the problem persists. I've replaced V2 and V3 with new tubes today, and no change in the hum issue.

I've decided to trace the 60cycle signal on a scope, back from the Master volume, as the hum goes away completely when the Master is turned down to zero. The hum follows the Master volume knob setting, and is there even with the Gain knob (the first volume knob on this amp) at zero. So, this seems to eliminate V1 and V2a completely from being part of the problem. Scoping the power supplies at the top of each plate R...., this shows that V2B, V3A & V3B plate supplies are totally clean at the most sensitive scope setting.
The most hum signal appears at the plate of V3B, the driver for the reverb section. There is a little 60Hz at the grid of V3B, maybe .004V p-p, if my scope is accurate at that level. At the plate of V3B, I see about 200mV p-p.

So, it seems that the 60HZ first appears somewhere between V2B and V3B. It would seem the tubes are at fault, but changing them didn't fix it. (I also see some 60Hz on the bias supply, but with both grids getting the exact same signal, they should zero out, right? Also, this is way after the Master volume pot.)
Any ideas?

Thanks much!

The important thing to remember when replacing the LDRs is that this requires a very high level of soldering skill, and some really nice soldering equipment.
For that matter, pretty much any part on this circuit board requires above normal effort and tools to remove and replace.
Remember:
If you do not have the experience to work on delicate, double sided PC boards, Have an experienced person help you or do the job for you.
It would be worth while, because the board is easily damaged by a person with limited experience / and or rudimentary soldering equipment.
This is not a circuit board to practice on...

[Jazz P Bass]

Wow.
Another one.
The sage advise just keeps on flowing.
Gonna have to start calling you Ben
Anyway, yeah, there are certainly circuit boards that have really small pads, the amplifier heat can dry out the epoxy that holds the copper traces to the underlayment ... and the beat goes on.
Technique, good soldering equipment & most of all a good solder sucker is what is required.
I am not saying it cannot be done with dodgy equipment.
It is the risk of damaging the board that must be taken into account.
In other words, a competent tech can be in there & out again & you would be hard pressed to tell that anything was done to the amp.
(other than the fact that now it works correctly

[soundguruman]

Yes this amp is very easily damaged, take your time, above all.
I have a $400 soldering station, 6 different tips, and it's still a challenge to get it just right and perfectly clean.
Now you can see why a lot of techs mount the LDRs to the opposite side of the board, there's lots of room.

I have repaired for 36 years, but like to pass on info about Boogies. This is an amp that most techs are afraid to work on.

[cranehazard]

I have a 22+ with a hum problem replaced. Power resistors and caps. Still humming the circuit board. is black all around the power section. Is this the carbon traces in the board you talk about? Should I keep trying to fix it (drill, cut, ect. ) it would be hard to isolate those tube sockets. What do you think?

[soundguruman]

I have a 22+ with a hum problem replaced. Power resistors and caps. Still humming the circuit board. is black all around the power section. Is this the carbon traces in the board you talk about? Should I keep trying to fix it (drill, cut, ect. ) it would be hard to isolate those tube sockets. What do you think?

Um, yes, you can have carbon traces on the board from overheating or arcing, and these traces can sometimes act like extra resistors in the circuit.

Post clear pictures and I can maybe tell you. several clear close ups?
Have you carefully checked under resistors in preamp section?
Have you verified that the LDRs are working?
Have you swapped out the tubes?
Have you CLEANED the preamp out / poweramp in jacks?
Have you verified that the switch contacts on those 2 jacks are closing?

It's very easy for the wires to break off the board. Are you sure they are all connected?
Another thing that comes to mind is the solid ground buss wire, it's soldered to the controls, volumes, reverb, presence.
sometimes that buss wire has cold or cracked connections, and the controls are not grounded...

anyhow, if you give up you can ship it here and I can fix it for you, soundguruman@yahoo.com in beautiful sunny Grover Beach, CA.
OR take it to the service center...it can get up around $200+ shipping to fix one of those (even more), due to the fact that the board must be removed...etc...

Anyhow, not an easy amp to work on, and if you do not have the test equipment, tools, and experience, this is "not" the amp to learn on.
It's for "advanced" level technicians. Seriously, most techs won't work on these, and you will learn why when you take it apart.

Yes, you will see resistors laying on top of circuit tracks, and sometimes these resistors burn down to the board.
Check carefully, with a light and a magnifier, to spot a burned area under a resistor. Lift the resistors up, off the circuit tracks. There should be a space in between the resistors and the board.

Sometimes there will be a burned spot between two circuit tracks, usually in the preamp.
This occurs between plate voltage track and grid track, or between plate voltage and filament track...
It will be a very small burn spot. You have to bypass the burned area...remove the carbon by cutting...