Fender Frontman 15G 60hz Hum

[Chevy]

Hi guys,

Been away a long time... feel remiss in not being around and contributing, but been ill and out of the game for a while... but wondering if I might ask a question here:

Have this Frontman 15G with an excessive hum, without anything plugged in. Volume control doesn't affect it much. Sounds either 60 or 120HZ, not sure which. When I touch both the ground sleeve of the input cord and the metal handle bolt (connected to chassis I guess) the hum drops dramatically.
Checking for AC voltage between the input ground and the handle bolt, I get like 3VAC on the DVM (while not doing the grounding thing above).

Other than the hum, the amp seems to function ok.

The schematic says there is no connection from earth ground to electrical ground. I don't quite understand this sort of design, and how is it supposed to work properly anyways? Any ideas? Can I just connect a cap from earth ground to electrical ground and call it a day?

Thanks,
Chevy

[pdf64]

I can't see any circuit between the circuit ground and electrical ground on the schematic either. I can't believe that's correct though, so assume it's an oversight on the drawing.
I would copy the arrangement on other Fender amps, with a 10ohm resistor between the 2 grounds, see
http://www.fender.com/support/amp_sc...ic_-_11x17.pdf
at the input jack sockets.
When / if you examine the board closely, you'll probably find a bad connection, but failing that fit the resistor as a mod.

[Enzo]

Oh there are sneaky ways to bond the circuit common to the chassis. Most usual one is through the bushsing of one of the jacks. Some mounting posts for the circuit boards complete a path from common to chassis in some amps. Or the heat sink might be connected to common and also the chassis. I forget, are the pots in that like other Fender amps, with the support brackets on either side? Those are used to connect common to the chassis in some amps. See of there is continuity from common to any of those brackets. Lots of sneaky ways. And all of those ways can be defeated by loose hardware.

Pull the power cord from the wall socket. Measre resistance from the ground prong to the chassis metal. Is ther continuity? A loss of chassis to earth connection could also eb involved.

[Chevy]

pdf64:
Hmmm.... on that schematic I see a hard connection from earth to electrical "gnd A". The 10 ohm R looks like it's in between the chassis ground and another ground they call "gnd", probably just for the preamp. I don't follow this... why 2 grounds?

Enzo:
Yes, there is continuity between the AC ground prong (amp power cord) and the chassis.... around .2 ohms.

I'll check out the tabs on the pots, see if that's an issue. Don't want this to happen again, due to some mechanical stress or whatever, so may just go with the 10 ohm resistor suggestion.

Thanks, guys!!! You're awesome.

[longshot]

Hi Chevy,
Did you manage to fix your amp? I've got Frontman 15G with the same problem and was wondering what to do with it.
Cheers,
Longshot

[dkelley]

same here - I know I'm reviving an old thread, but I just picked up one of these since it's tone is VASTLY superior to the little gorilla practice amp I was using before. I play professionally but don't have room or wife permission to keep anything but a little practice amp around the living part of the house (everything else has to remain in my music room/recording studio downstairs). :-)

So it buzzes/hums but sounds really quite good otherwise for clean stuff. the distortion channel shouldn't be there - it's a waste of space, althoguh again less bad than that of the gorilla this is replacing.

Did the stuff here help? and if so, can someone please post a quicky instruction for us 15g users who would like to do an easy fix/mod for our practice amps without having to study schematics ourselves or work out just where/how to solder some part (resistor I assume) to what part of what circuit board to what part of what else of the amp (hahaha you get my point I think :-)

cheers and thanks for people spending a bit of time with this little amp - aside from modelling amps this is my favorite practice amp - I can actually make music with, not just practice scales without decent tone like with some other practice amps.

[Chevy]

Hi guys,
Sorry for the very tardy response...
I did solder in a 10 ohm R from the jack to the chassis, just exactly as shown at the very front end of the amp (at the jack) in the amp schematic (link in a previous reply).
This did help, pretty much solved the problem, but these grounding issues are not my strong suit...
I'm not sure why they separated the jack ground with the 10 ohms from chassis ground... but you can trouble shoot this (see if it is actually the problem) by checking if the ground portion of your input jack is actually connected to chassis ground or not.

[longshot]

Hi there,

Given that there is some new activity on this old thread I decided to give you an update as well... I've fixed my issue by soldering a 100nF capacitor between the two grounds, actually I only soldered one side of the cap to the signal ground on the AUX jack and then used one of the heatsinnk screws to secure other side of the cap to the heatsink which is connected to the earth (electrical ground).

If you don't want to open up the case and solder, you can use the sleeve of the AUX jack which is connected to the signal ground and then connect it with a piece of wire (you can add a resistor and/or cap if you want) to the metal part of the handle on the top of the case (this is connected to the internal heatsink and the electrical ground). You can use the screw on the handle to secure your wire and obtain an electrical connection at the same time. Hope this helps.

Cheers,
Longshot

[jharris0221]

Hey everyone,

I know this is super old, however I just tried this and it worked great. Took a 100 ohm resistor (Brown Black Black or Brown Black Brown) and soldered it from the jacket of the input to a screw I added to the metal frame.

I took off the outside nut for the input jack, slid a wire from the inside of the amp out, and soldered the wire to the flat part of the threads. The nut went right back over no problems. Then put a 100 ohm resistor on the end, drilled a small hole and added a #8 screw/lockwasher/nut to solder the other end of the resistor to. See the pic.

The amp went from unplayable secondary to a 60hz hum to sounding amazing.

[Jazz P Bass]

Not too sure what the 100 ohms of resistance would do.

Did you try this with a simple wire?