Mesa Nomad 45 Solo function not working

[xtian]

Solo function not working. Schematic attached, see page 6. J175 FET is meant to short out (bypass) the 250K solo pot normally, and open when the Solo boost function is engaged. This is not happening. I read 0.6vDC on the gate of what I'm guessing is an N-channel JFET? This voltage does not change when pressing the Solo button on the footswitch.

Can anyone find a datasheet on this FET?



Schematic: mesaboogie Nomad 45.pdf

All low voltage supplies measure nominal. Not sure where to look next. I can't find the circuitry shown at lower right of page 6:

The Nomad is poorly labeled and difficult to suss. Any suggestions welcome!

[The Dude]

Here's the datasheet.

https://www.mouser.com/datasheet/2/308/J175-1120772.pdf

That said, if the gate voltage isn't changing, I wouldn't expect the FET to switch either. Is the voltage coming from the FS to that 1N4007 changing?

[xtian]

I would love to find out, Dude. But there's no map. While the PCB has some part numbers, they're mostly obscured by the components. And the components in question, like the 1N4007 you as about, are not numbered anyway.

I tested DC coming back from the FS over the DIN cable, and found changing DC voltages for the Channel function and the Reverb function, but no changes on any pin for the Solo function. Then I realized all that circuitry on page 9, Footswitch, is actually stuffed into the footswitch!

I resoldered the connections at both ends of the DIN cable and retensioned the female DIN pins. No help.

[The Dude]

For clarity:
You said the SOLO (switching) function is not working. I'm not sure what you meant. Does the SOLO function always work or never work? In other words, is the FET shorting across the pot all the time or never? If the FET shorts across the pot even with a gate voltage, the FET is likely bad.
Does the SOLO LED light?

[g1]

Just had a Nomad 55 here. Can you tell me how strong the reverb is on this 45? On the 55, with reverb at '10' on the clean channel, not much there. Like less than a blackface Fender with reverb set to 3. Still could be normal, I guess. Some marshall with ss reverb circuits don't impress either.

The J175 (p-channel) is 'on' (low resistance) til you get a positive voltage to the gate to turn it off (which enables the 'solo' pot). Looks like if the transistor shown below the Fet is on, solo is disabled. So maybe that transistor is shorted? Or maybe the voltage isn't getting through the 100K resistor?

[xtian]

Owner picked up the amp today, unconcerned that the Solo function is not working. J175 was always low resistance, thus the Solo pot had no effect on the volume. The way the footswitch (FS) works, the amp sends 5vDC and 0 (ground ref) out over two conductors of the DIN cable, and a switching matrix in the FS sends various voltages out over three conductors, one for Channel, one for Reverb, and one for Solo. The Channel and Reverb conductors showed changing DC values in response to button presses as expected, but the Solo conductor would not change, even though the Solo LED on the FS was lighting when button pressed. Malfunction in the FS?

Reverb was set pretty deep on all three channels--sounds like it was definitely louder than your 55, @g1.

[Jon Snell]

I have worked on many Messas with switching issues and it always boiled down to the J175s failing.
If ... the mains is powered on when the valves are cold and it coincides with a spurious mains spike, the HT goes high very quickly and the Solo pot is high resistance, there is a possibility that the Drain Source breakdown voltage can be exceeded, (capacitors taking time to charge etc). Fitting a 27v zener Drain to Source will cure this phenomenum.
Not the easiest for a lazy techie to change, dead easy for the committed engineer who takes the time to remove the board first.

[Mick Bailey]

Just had a Nomad 55 here. Can you tell me how strong the reverb is on this 45? On the 55, with reverb at '10' on the clean channel, not much there. Like less than a blackface Fender with reverb set to 3. Still could be normal, I guess.

I've had two 55s for tube replacement recently and the reverb was lacking compared to a Fender. Interestingly, both owners were using a separate reverb pedal.

[g1]

Not the easiest for a lazy techie to change, dead easy for the committed engineer who takes the time to remove the board first.

You can try to make it sound as nice as you like, but nothing 'dead easy' about removing the board first. It's quite time consuming and the price of that time gets passed on to the customer.
If you want to imply that the huge number of techs who cringe at mesa boogie (or just refuse to work on them) are doing so because they are 'just lazy', that's just insulting.

[Mick Bailey]

Why remove the board? I rarely find this to be necessary other than for relays. For a J175 I snip the legs and use a desolder station on the component side. If I'm lucky it sucks out the solder and the remaining leg with it. If not I poke it out through the board and shake it out to recover it. The new component is soldered in from the component side as the board is double-sided. Takes about as long to do as describing it. I guess I'm one of those Lazy Techs.

[g1]

Why remove the board? I rarely find this to be necessary other than for relays. For a J175 I snip the legs and use a desolder station on the component side.

I was thinking the same. The caps also require board removal.

[The Dude]

I agree- snip the legs and use a desolder station on the component side. Why would you bother pulling the whole board to change a part that's so easily accessible from the top side? It might be different if it weren't a double sided board. Pulling this particular board out for FET replacement is like lifting a car body off the frame to change a tire. It makes no sense and stands a chance of creating even more problems.
Also, arrogance is extremely unattractive.

[Familyortiz]

I have worked on many Messas with switching issues and it always boiled down to the J175s failing.
If ... the mains is powered on when the valves are cold and it coincides with a spurious mains spike, the HT goes high very quickly and the Solo pot is high resistance, there is a possibility that the Drain Source breakdown voltage can be exceeded, (capacitors taking time to charge etc). Fitting a 27v zener Drain to Source will cure this phenomenum.
Not the easiest for a lazy techie to change, dead easy for the committed engineer who takes the time to remove the board first.

I am working on a Nomad 55, solo function not working. Found that the previous hack took out the J175 but did not install one. I can only assume the previous one died. Thanks for explaining the failure mechanism. A new J175 and 27v zener should arrive today.
I would like to PM you about your opinions on opening up the drive ch on a Bugera V22, as it is farty and lacks tone. regards, mike

[g1]

I am working on a Nomad 55, solo function not working. Found that the previous hack took out the J175 but did not install one.

According to schematic (see attached), the fet is only needed to turn the solo off. So if it is removed, that would leave the solo permanently engaged. Has a jumper been installed where the fet is supposed to be? That would disable the solo function.

[Familyortiz]

According to schematic (see attached), the fet is only needed to turn the solo off. So if it is removed, that would leave the solo permanently engaged. Has a jumper been installed where the fet is supposed to be? That would disable the solo function.

Thanks for the correction. It was stuck in Solo mode. Just installed the J175 and all is well! To be honest, I have the zener, but my measurements show a different orientation of D and S from the silkscreen and I need to get the zener orientation right so I figure, it's working to spec and if blows again, I'll install it. regards, mike