Mojotone MOSFET switching board causing Buzz

[Just Mike]

I've connected the power leads to 2 different points (htr 6VAC) and I have it grounded to the chassis, but it buzzes pretty bad when I take the amp out of standby. Any ideas?

Mojotone Complete Mosfet Switching Soldered Assembly
Thanks!

[dwmorrin]

Its not clear what you're up to.
What's the amp? The GA5 mentioned in the documents you linked to? Did you build the amp? Did it ever work (not buzz)?

[Just Mike]

I've installed it in a Peavey VTM 60 which doesn't buzz when the switcher isn't installed. The switcher functions as it should, just causes buzz when I hook it up to the heater supply. I'm using it to turn on and off selected dip shitches on the amp.
From the MOJO document;

For inputs onto the board there are:

6vac x 2
These are 2 pads that need to be wired to your heater supply in a tube amp – or any 5-9 volt ac supply in a non tube amplifier. The Mojo mosfet switcher uses a floating ground for the CD4049 but has a center tap on the power supply that gets referenced to chassis ground.

Ground
Ground on the board needs to be wired to the earth ground of your amp, or the device being switched if you are floating the ground.

[dwmorrin]

Can we narrow down what kind of "buzz" you're getting? Is it a 60Hz or 120Hz hum type sound, or something else?

The schematic is attached. I'm not familiar with the amp, but the schematic uses 2 different ground symbols. Maybe the amp's "ground" scheme is tricky. Maybe you've got a ground loop going. Exactly where you tied the MOSFET sources to may be the issue. We might need more details of what you've done to go further.

[Just Mike]

B+ is off of V2 pins 4&9. And I've confirmed that both grounds on the schemo are tied together. Buzz is classic 60hz and is not effected by volume controls.

[dwmorrin]

And is the MOSFET switcher "ground" connection going to the same "ground" as the source, or is it going to the chassis ground?

Is it obvious when inside the amp why there are 2 ground symbols on the schematic?
Can you try a different "ground" for the switcher? The documentation suggests some applications require a floating ground.

[dwmorrin]

I've confirmed that both grounds on the schemo are tied together. Buzz is classic 60hz and is not effected by volume controls.

I'm guessing you've created some kind of ground loop. If you're just testing the ohms between grounds, this doesn't mean you can just start throwing in a new circuit between filaments and switches and expect no issues.

If you've got a chassis ground to the switcher board, I would try lifting it and putting it right at the "device" being switched, as the documentation words it.

EDIT: Can you indicate on the layout (page 2 of the schematic) where you made the cut in the trace? Maybe you cut off some other component accidentally?

[Just Mike]

I'm sure I cut just the land where shown here.

I'll try moving the ground in a day or two. It's bedtime and I won't have time tomorrow to look at it. Thanks for the pointers. I think we're onto it. I'll post back when I get back into it. Thanks for your help dwmorrin!

[drewl]

I'll try and find some notes I took at the time, but I had the same problem using their switching board in a Marshall clone I built for a customer.
I ended up building my own relay switching boards I believe.

[Just Mike]

Could the buzz be coming from the UN center tapped heater lines? I wonder if I could use a transformer to power the Mojo board seperately? And if I bypassed the rectification on the Mojo board would it be safe to use a 9VDC transformer?

[MarkusBass]

Have you looked at the GA5 and VTM60 schematics and compared them? The heaters supply in both amps is different: in GA5 there are two resistors (100 Ohms) to the ground as a hum balancing circuit. In VTM60 there are no such resistors, there is R9 hum balancing trimmer (on the power amp board) and the heather windings are elevated (connected to positive voltage - check R10 on the power supply board). By connecting the switching board you are most probably shorting the elevated voltage. So the circuit will work in a typical tube amp but it will not work in an amp with elevated heaters. In order to make it work you have to remove the heater elevation. This requires few changes but definitely not cutting the ground. But ... the heater elevation is done on purpose so maybe you should analyse the schematic of the amp first.

Mark

[Just Mike]

I'm going full bore with a 6vdc power supply and bypassing the on board rectification on the MOJO board. Stay tuned!

[MarkusBass]

I'm going full bore with a 6vdc power supply and bypassing the on board rectification on the MOJO board. Stay tuned!

Have you got the schematic of the Mojotone switch? The reason that I'm asking about it is that it seems to me that the switch may be supplied with a symmetric power supply, something like: -3.5V, 0V, +3.5V. If this is true, single 6VDC power supply won't work . Of course, you can easily check on the PC board if it requires symmetrical supply.

Mark

[Just Mike]

Success! After way too many hours inside this thing I got it to work properly. I used a Radio shack wall wart and gutted it. It has a nice little PCB that I mounted to the chassis by the power xfmr. I used some nylon standoffs and drilled a couple of holes in the side of the chassis. The wall wart I had was one of those multi voltage ones so I had to do a little cypherin' to figure what resistors to use to drop the DC voltage to 6v.
Needless to say MarkusBass, there is no negative rail in the Mojo board but thanks for the heads up. The documentation for the mojo board was a little vague until I got it. There are some jumper options to change it from T/S to T/R/S footswitch operation and not all switching outputs are the same. Some are on to off and others are off to on...
Then there was the fact that I can't just put a MOSFET on the ground buss. the dip switches need to be individual, otherwise the circuits are tied together. So I pulled the dip switch bank out and replaced the ones I needed with MOSFETs.
I'm not a pro tech, I'm a guitar player with enough knowledge to be dangerous, but every time I build or mod an amp I screw it up a couple of times and keep plugging until I figure it out. I play out with my band and the best complement I can get is when other guitar players ask me WTF am I playing thru because their Marshall-Peavey whatever doesn't sound that good...Awesome!