Frontman 20G

Hello everyone.
This is my first post here, and I'm not so much of an audio guru, but I know a thing or two in electronics but alas not enough to solve my problem.
Recently bought a cheap but new Fender Frontman 20G, for which I do not have the schematics but I've already opened it and made some observations.
My problem is the following: the amp seems to work, but it generates huge amount of main (50Hz in my country) noise.
  • If I do not plug anything in, there is some hum but rather acceptable for the price of the amp
  • If I plug a guitar:
    • If I put the sound at zero on the guitar or on the amp then close to no hum
    • If I put normal volume on the guitar and the lowest amount of volume (1) on the amp then I'm having some massive humming. Very loud, not faint.
      • If I plug the cable without the guitar it's more or less the same, perhaps worse.
  • The only thing that mitigates this is that I touch any metal part with my hand, on the guitar, on the cable or on the amp itself then the humming is reduced to acceptable levels, though still hearable. This fix is actually annoying because when I'm holding the guitar normally I'm not touching metal parts but evey time I touch the strings there is a short pop noise.
Here's what I checked so far:
  • Trying another power outlet: it makes zero difference, even on the one closest to the electrical panel.
  • Checking the protective earth (PE) connection: there is no misconnection issue whatsoever on the amp+cable+guitar setup. I can measure a solid 0.4 Ohm from any metal part of the guitar to the PE of the power cable of the amp. Every metal part in the amp is connected to the PE, and one of the sound outputs (let's call it the negative one) is connected to the PE, which I assume is a design choice.
  • I checked the audio output on the oscilloscope. With just the cable plugged in, I can see the 50Hz sinewave 2V Pk-Pk, and some 300Hz superimposed with a much lower amplitude
  • The power supply appears to be a center tapped transformer + full bridge rectifier for a +/- 20V power supply. The output ripple seems acceptable, and I believe that if the supply was the guilty element the noise would be at 100Hz and not 50Hz.
I do not have any idea on how to fix this. I live on an appartment on the 14th floor of an old building and rewiring the whole appartment is not an option.
If you have any ides or tips to solve it, I'd be grateful.
Found the schematic of the Frontman 15G online, I’ve been looking at my 20G PCB and it looks very similar
Also the PE is connected to the chassis which then connects to the PCB trough some metal sheets. Total resistance from PE on the screw connected to the chassis to the negative connector of the speaker is 0.51 Ohm... but I just realized on the above schematic that there are two 0.68 Ohm resistors in parallel between the PE and the negative speaker terminal, if I measure between screw terrminal and the other end of those two resistors I measure 0.17 Ohm, so the PCB is well connected to the PE.
I also noticed there is no filtering whatsoever when it comes to the AC 230V power of the transformer.
Finally I’ve tried to disconnect the earth completely (wich I know, isn’t safe), and I was a bit surprised to see it didn’t help, it even made it a bit worse I think.
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Only 30mV Pk-Pk when at volume 0 and 1 (where I have almost 2V Pk-Pk of 50Hz with jack plugged in). Waveform looks like noise from the power supply, its period is 100Hz and it’s not a sinewave. If I increase the volume, I can see the 50Hz sinewave superimposing unto the signal with its amplitude increasing more and more until at max volume it reaches 56mV Pk-Pk, but it’s not very hard on the ears.
Also with the jack unplugged, I just realized that if I move my hand closer to the PCB (without actually touching it) with a volume of 1 or more I can make the 50 Hz sinewave appear onto the audio output, I can reach a Pk-Pk of more than 2V on the 50Hz sinewave if I bring it near components of the input stage, especially R2 and C2, but If I touch the metal enclosure then it goes away.
Just checked, and all the pots are screwed tightly.

I opened the cable ends, because I suspected a connection between plus and minus in the cable, I resoldered everything, I now measure 150 mOhm from one negative part of the jack to the other jack, 300 mOhm between the positives and a resistance high enough between positive and negative ends of the cable so that the multimeter cannot measure it. After plugging just the jack back in, it made zero difference.

I tried to connect the PE directly to the jack groud pin on the PCB solder joint of the jack, I also tried to add a wire between ground of supply and this pin, it didn’t change anything. I also measured the resistance between cable jack ground and solder joint of jack on the PCB, it’s very low which is coherent with my previous measurements.

Today I tried to measure the signal directly at the input, right before R1. With nothing plugged in there’s just some light white noise, coherent with what I hear. With jack plugged in I can see and measure the 50Hz sinewave with a 30mV Pk-Pk amplitude, and if I also plug the guitar at the end of the jack cable it goes to 20mV Pk-Pk. And, I measure the same values if I unplug the power cable from the amp... Also the node connecting R4, R5 and R6 together is oscillating at 494 kHz with an amplitude of nearly 0.6V, which seems strange to me....
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I am having a similar issue with the even cheaper version of the same: Frontman 10G. What helped a lot in my case was actually outside the amp: a bad or broken ground of the guitar bridge!

Are you by any chance using an inexpensive hardtail (as in no tremolo) bridge? If so, it is commonly grounded by sticking a wire under the bridge before screwing it in. And there is a good chance that that wire fell out. This was the case for me. If touching the guitar strings or bridge noticeably helps, this is pretty likely, I had the same symptoms.

Unscrew the bridge (loosen the strings first of course), and see if there is a stripped wire under it - there should be one. If not, trace if the bridge is grounded in any other way - it should be. If not, find a way to ground it, e.g. to the ground of the 1/4 socket.

P.S. I did all that, and it helped a lot. Not all the way - the Frontman still buzzes but it is better. My next step will be putting the screening metal foil on the inside of the guitar cavities.
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So this is an extremely late reply as I have let this aside for a while, but anyway to answer the last post I already measured the Protective Earth from the power plug to the bridge, cords, anything and I get a low resistivity so I don’t think the wire fell off. Also the guitar was tested at the store and it was fine, although maybe it displays a problem only with this amp.
Right now I’m really considering changing completely the audio circuit inside the FrontMan 20G, because to me it seems really outdated in the sense that it could be replaced with a modern low price circuit with easily better quality. Maybe before that I’ll take apart the guitar as you advise.
If I would start to repair, I would do as follows:
1. Check if al the AC connections are good (Wall plug, ground connection in the amp) Is your socket grounded?
Obvious do each step with the power off AND remove the plug from the all socket before touching the inners of the amp.
2. Isolate the power amp from the preamp (remove the supply voltage and break the connection between the pre- and power amp).
3. Check the power supply for excess hum on the dc voltages.
4. Short the input of the preamp
5. Switch to the normal channel, inject a sinus signal and measure with your scope on the point where it has to go to the power amp you isolated. With the controls low and the gain higher.
6. Switch to the Drive channel and do the same
7. The point where the power amp is connected to the preamp, the point towards the power amp you disconnected, short it to the ground and reconnect the supply supply.
8. Reconnect the preamp with the power amp. Lower the volume settings Settings. Connect the sinus source.
9. Check the continuity of your guitar cable.
If all is well then connect your guitar to the amp. It should work just fine.
If not, you had a problem on point 1 or 2 or further.
Fix that first before attacking the amp on different fronts.
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Its not unusual to see such problems. Its usually the guitar and or the cord causing the problem or most of the problem. If the guitar picks up hum, the amplifier can only amplify it.

A common symptom is the buzzing/hum is less when you touch the guitar strings (assuming the bridge is properly grounded). You can also rotate the guitar around in a circle to see if the hum depends on the direction the guitar is pointing. Often there is some effect with non-humbucking pickups.

Also, some Fender guitars have 5-way pickup switches where the intermediate positions connect to two pickups at once. In that case the in the two-pickup switch positions the pickups may be designed to be humbucking when used in pairs.

EDIT: Regarding hum in the Frontman itself, most likely its just the power supply and or a lack of shielding around the electronics. Does it use a wall wart power supply?
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I'll try. IIRC what I did was set the guitar volume and tone pots to 5. Then I set the bass and treble tone pots on the Frontman fully CCW (to minimum), and distortion switch turned on. From that starting point I used the guitar volume pot to set the amount of distortion, and the amp volume pot to adjust the overall speaker volume.

To get a rough idea of how much distortion is optimal my typical starting point is to adjust the distortion so that if one string is played, then no or very little distortion. If two strings are played at once, then some clearly audible distortion.

From those starting points, I first adjusted to tone pots on the amp to get an interesting tone, but still mostly midrange focus. Then used the guitar tone pot to fine tune the sound. Again, the volume pot on the guitar always remains a distortion control in this scenario. Adjust the amp volume to change the speaker volume level.

By making fine adjustments to amount of distortion and by fine adjustments to amp and guitar tone controls, I could set the tone for different styles of music. Similarly, the amount of distortion can be adjusted for different styles of music, too.

Hope that helps.

EDIT: It does help to get the intonation, string height, and neck relief all adjusted properly for best sound and for ease of playing. In the case of my brother's guitar, I went through it first before figuring out how to get good tones out of the amp. Otherwise, things like power cords played up near the 12th fret will sound a little out of tune, so it won't sound exactly right with or without distortion.
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Thank you! I will try this. As for the guitar tuning, I did a pass on all that: neck relief, string and pickup height, intonation, and the tremolo (which I don't know how to use yet but like to have and play with). I might do anohter pass - there is a slight string buzz around 15, perhaps a fret sticking out a bit, need to take another look.
BTW, what is more important than good tone, arguably even more important than playing in tune, is playing correctly in time. Its the thing most beginners get wrong. Don't know if you already have been trained to play some other instrument. If not, I would be happy to talk about time in another thread or by PM. Its really important to playing well, and there are things you can do to help accelerate you progress.
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