Mesa Nomad 45 heater issue?

[boogiebomb]

So I picked up a Mesa Nomad for free, no output, and no glow from the power tubes. Upon closer inspection, no glow from the preamp tubes (They were behind the shields) and only V1 is glowing! I believe V1 is on a separate circuit, and not wired in parallel with the other preamp valves.

Also, the molex connecter from the power transformer was pretty burnt up on the two heater wires. I took out the two heater leads from the molex connector, and soldered them to the pcb. The main pcb has a pair of pads that go to the EL84 board for the heaters, so I soldered the heater leads there. V1 still has heater glow.

I checked the heater pins of the other preamp and power tubes:

I have the correct voltage to ground (3.1VAC) from EL84 pins 4/5 and from 4/5/9 to ground on the 12AX7's, but no voltage between the heater pins themselves, unlike V1 which has voltage between the heater pins.

[boogiebomb]

UPDATE:

I just double checked my work, and here's what I found:

EL84 heater pins 4 and 5: 2.9 VAC to ground, but none between each other.
12AX7 V2 to V5: same as above, but 3.1 VAC.

I also don't get any voltage between the two heater wires coming from the power transformer. Each one gets the 2.9 VAC to ground however.

[pontiacpete]

Here's the schematic:

I think you should be measuring DC on the heaters,
There's a bridge rectifier on the heater winding.

[boogiebomb]

Thanks, I already have the schematic. Bridge rectifier is supposed to come before V1, and after V2 and up, according to the schematic. Not sure if that's actually how it's laid out. That's partly why I'm confused as to why they aren't lighting up. No DC between the heater pins, except for V1.

[pontiacpete]

maybe a connection issue.
Did you check connections with an ohm meter?

my schematic isn't showing up anyway, what happened there?

[boogiebomb]

I did check connections with the continuity setting, there is continuity between heater pins, and continuity between the heater wires from power transformer. If there are any tests I should perform, please let me know. I am a little familiar with tube amps and have dealt with some heater problems before, but this one is a bit more complicated.

[pontiacpete]

Did you check the VAC across the heater winding?

[boogiebomb]

No VAC between the two green wires. 2.9VAC from the green wires to chassis or the green/yellow on the jumper, which is the ground.

[Jazz P Bass]

my schematic isn't showing up anyway, what happened there?

The site has 'lost' a bunch of files.
If you have saved a file, on your computer, that is part of the 'lost bunch', the site remembers that file by it's size.
So to reload a 'lost' file you must change the file size before you upload it.
I have the same file that you tried to upload.
So I reprinted the file using Cute PDF Writer.
The file size is now different.
The upload was successful.
What a PITA, huh?

[Jazz P Bass]

No VAC between the two green wires. 2.9VAC from the green wires to chassis or the green/yellow on the jumper, which is the ground.

If there is no continuity between Green & Green, then the transformer is bad.
But then your + - 3volt footswitch supply would not work.
Nor would V1 heater.
You got me.

[pontiacpete]

I must have missed this bit, thanks Jazz!

[Jazz P Bass]

I did check connections with the continuity setting, there is continuity between heater pins, and continuity between the heater wires from power transformer. If there are any tests I should perform, please let me know. I am a little familiar with tube amps and have dealt with some heater problems before, but this one is a bit more complicated.

Rather than 'continuity', you may be better off with an actual resistance reading.

[boogiebomb]

Continuity between green and green, but only 3 ohms of resistance? V1 still lights up.

[boogiebomb]

I guess I may have to pull the pcb to see what's going on on the underside of the board. For kicks I measure 34 ohms between v1's heater pins, with no tube inserted. No ohms between the other preamp tubes.

[km6xz]

From green to green you should be reading 6.3 volts AC between pin 4 and 9 of V2, V3, V4 and V5
Since the same winding is powering the heater bridge of V1, you know the transformer is fine. So figure out where there is a break between V2-V5 pin 4/5 going back the one of the green wires and pin 9 to the other green wire. Some of your other measurements suggest you might have been in the wrong meter mode.