Need help with my Mesa Mark V. No power on V1, V2 & V5

[Redline]

Please help me with this complicated beast. I have power to all other tubes. I checked the tubes. They are good. I also don't have any front panel lights. I found a schematic on here but this thing is going to be really difficult to trace. Is this a common problem with these amps? Where should I start? Please help me avoid sending this thing to Cali. Thanks in advance!

[The Dude]

A good start would be to post a link to the schematic you found so we can have a look.

[Redline]

I'll try. I'm pretty much computer illiterate.

[Redline]

file:///C:/Users/User/Downloads/mvschemo%20(1).pdf

[TimmyP1955]

That link is to the file on your hard drive - no good here. Click Go Advanced below, then Manage Attachments, then navigate to the file, click Add Files, and navigate to the file.

I suspect that unlike the rest, those three tubes are fed by a 12VDC filament supply, giving you 6VDC across each heater. If so, there will be a TO220 regulator IC somewhere on the chassis.

[g1]

On pg.1 of the schematic, there is a LM2940 regulator powering the heaters of V1, V2, and V5.

[Redline]

On pg.1 of the schematic, there is a LM2940 regulator powering the heaters of V1, V2, and V5.

Weird... I can't find that one. I found an LM780. That's all.

[The Dude]

Weird... I can't find that one. I found an LM780. That's all.

I'm supposing you mean 7805 (5V regulator). I'm guessing, but I would suppose that the LM2940 might be screwed to the chassis, maybe under a board? It would get a bit warm supplying filament current to 3 tubes. You should be able to trace it from that bridge rectifier circuit that precedes it.

[Tom Phillips]

Weird... I can't find that one. I found an LM780. That's all.

To be clear. It was just mentioned as being in the circuit. This does not mean that the LM2940 in your amp is bad. Troubleshooting needs to be done to determine what exactly is wrong with the amp you have.

[Gregg]

I found an LM780. That's all.

The one you found (circled in blue) is on the back panel PCB and is there to supply 5V to the LM3914 chip and the foot controller.
The 12V one (circled in red) is bolted to the chassis.

[Redline]

Sorry for the delay. Work has been crazy. I had to pull that VR IC to check it but it tested good. Back at it!

[g1]

What is the voltage going in to the regulator?

[The Dude]

Yep,.....easier to check in circuit. It's simple voltage in and voltage out.

[Orbeck]

Too hard for me, even if I am trying to follow all the steps.

[g1]

Too hard for me, even if I am trying to follow all the steps.

Did you locate the regulator like circled in red in post #10 above?
Here is the pinout, what DC voltages do you measure at pin 1 and pin 3?