Fender Frontman 25R - hum / buzz on clean channel

[Guga Ramone]

Hello!

A friend of mine has a Fender Frontman 25R guitar amp and the clean channel is producing a low level hum, even when there is nothing plugged into the amp. When switched to the drive channel the buzzing goes away and the amp becomes very quiet. The power supply voltages are all ok. My guess is that the U1-B may be damaged, since there are no diodes to protect it's input. Am I right here or is there anything else I should check before disasemble the board and replace the IC?

The schematic is here: Frontman+25R+schematic.pdf

Thanks in advance!

[Randall]

Assuming the Normal volume effects the hum, I'd say U1 is a good place to start.

[vintagekiki]

Check voltages change to U5b (TP18, TP19) when switching (sw Gain select) from clean to the drive.
Check that the voltages at U1, U2, U3 at pin 1 and pin 7 are about 0 VDC.
If the voltages are OK check Q1, Q2, D5, D6.

[Guga Ramone]

Hi there!

The voltage changes on U5b are:
TP18: +13,8 (drive channel) / -14,1 (clean channel)
TP19: +0,03 (drive channel) / +1,29 (clean channel)

I guess the small differences I'm reading are ok, right?

Pins 1 and 7 of U1, U2 and U3 are reading 0 Vdc.
I removed Q1, Q2, D5 and D6 and tested them and they seem ok.
I removed U1 and installed a new IC and the buzz is still there.

The clean channel volume control doesn't affect the buzz. The bass control does amplifies it a bit.

I tried to put U3b pin 7 to ground and the buzzing stopped. Trying to ground pin 6 doesn't stop the buzz. I changed U3 for a new IC and the buzz is still there.

Any tips?

Thanks!!

[g1]

Is the buzz there with the reverb turned all the way down?

[Guga Ramone]

^ Yes. Actualy the buzz is present even when all the controls are on minimum.

Here is a video showing the buzz:

https://www.youtube.com/watch?v=n3y4S3lCdc0

Thanks!

[g1]

What stood out the most to me about that video was how much the hum was reduced when you turned the treble up. Is that what is happening, the hum gets worse if you turn down the treble?
Have you looked at the solder joints on the pots and jacks, and looked for any cracks on the pots themselves?

[Jazz P Bass]

I would remove Q1 jfet and see what the amp does.

Of coarse the Clean channel will always be On.

[Guga Ramone]

What stood out the most to me about that video was how much the hum was reduced when you turned the treble up. Is that what is happening, the hum gets worse if you turn down the treble? Have you looked at the solder joints on the pots and jacks, and looked for any cracks on the pots themselves?

I guess this has more to do with how my camera captures the audio and compresses it when there is other noises around. In the room the amp doesn't change the hum when the treble pot is turned. The bass pot yes, it makes the hum a tiny bit louder, but I guess it happens because it filters the noise when it is turned down and it is not the cause of the problem.

I would remove Q1 jfet and see what the amp does. Of coarse the Clean channel will always be On.

Removing the Q1 the amp stays on clean, regardless the position of the switch. When the switch is out (clean selected) the hum is present, as showed in the video. But when the switch is in (drive selected, but clean sound), the hum goes away. I put in new FETs in Q1 and Q2 and nothing changed.

Any ideas? Thanks!

[Jazz P Bass]

"Removing the Q1 the amp stays on clean, regardless the position of the switch. When the switch is out (clean selected) the hum is present, as showed in the video. But when the switch is in (drive selected, but clean sound), the hum goes away"

And this is with Q1 Removed???
That makes no sense if true.

[Guga Ramone]

What should I expect by removing Q1?
- Clean channel selected = clean sound
- Drive channel selected = mute?

Thanks!

[g1]

I think we better make sure we are looking at the right schematic. Seems to me this is the one where there are several versions.
Does your amp have the TDA1514 output IC, or the LM3876 ?
The Aux Input is not shown in the schematic in post #1, so that can't be the correct schematic.

Both versions with Aux.In are in this attachment:

[Guga Ramone]

This one has the LM3876 as the output IC.

[Jazz P Bass]

What should I expect by removing Q1?
- Clean channel selected = clean sound
- Drive channel selected = mute?

Thanks!

Q1 mutes the clean channel when On.
If you remove it, the switch position should not influence that channel.
There should no longer be any connection to the switch voltage.
So how can it make the clean channel hum?

[Guga Ramone]

Removing the Q1 the amp stays on clean, regardless the position of the switch. When the switch is out (clean selected) the hum is present, as showed in the video. But when the switch is in (drive selected, but clean sound), the hum goes away. I put in new FETs in Q1 and Q2 and nothing changed.

Sorry but I messed up a little here. Whe Q1 is removed the amp stays on clean when the switch is set to clean. When the switch is set to the drive channel, the distorted signal from this channel is mixed with the clean signal that is always present because of the absence of Q1.