Fender H.O.T. practice amp hum (50hz) noise even the guitar is un plugged.

[xaxxop]

Hello people.

I am trying to fix a Fender practice am with a loud hum noise.
the amp works but it has a hum noise (50hz) loud even when the guitar is unplugged.

- Volume, Contour, Reverb doesn't affect the noise.

I had attached the schematic for the amp Fender R.A.D. that is the same without the reverb circuit.

I tried to connect the preamp output to another amp and the noise still there. My first thought the issue is in the power supply I changed all the electrolytic capacitors in the circuit but the noise still there

Please any ideas or help would be much appreciated!

Leo

[Manfred]

I just now had the same problem with my Fender Frontman 15R.
I followed recommendations from the Internet and connected the DC ground via a parallel circuit of a 150R resistor and a 0.1uf capacitor to the ground screw in the chassis.
The hum has gone.

[xaxxop]

Thank you for getting back! I will try that tonight.

I show you a video so you can see and hear what I’m talking about.

Thanks

[Manfred]

To ensure safety, use the circuit with the bridge rectifier in this way.

Safety hum loop breaker circuit.jpg (18.15 KiB) Viewed 444 times

[Manfred]

I built in the safety hum loop breaker circuit with the parts I had in my stock.
That is a 16 Ohms resistor, a 47nF suppression capacitor and a 800V/4A bridge rectifier.
The black wire is soldered to the ground point of the board.
It works perfectly the hum is gone.

[Manfred]

Thank you for getting back! I will try that tonight.

I show you a video so you can see and hear what I’m talking about.

Thanks

Was the installation of the "Hum Breaker Circuit" in your amp also successful?

[Intripped]

Before modifying the amp, check all ground connections, solder joints and wires, expecially at the Input jacks

[Manfred]

Before modifying the amp, check all ground connections, solder joints and wires, expecially at the Input jacks

I had checked all these things on my Fender Frontman 15R and also changed all electrolytic capacitors, all without success.
Only after installing the Hum Breaker circuit, the hum was gone.
Immediately after that I installed the circuit in a Park G10R that had the same problem, also here the hum was then gone.
I think this is a design problem with the cheap amps that were produced in the East Asian countries.

[Intripped]

OK, but have these amps always had this problem, i mean since the first time you switched them ON, or the HUM has appeared later on?

If the latter, then something has gone bad; maybe also a poorly designed circuit, but i would like to understand what has happened.

[Manfred]

I bought both amps many years ago as used when I was repairing and selling used amps besides my job.
The amps had this hum from the beginning so these have remained until now.
The same problem I have with a Hohner HR10 amplifier there I have also checked everything and also replaced the electrolytic capacitors but also without success.
Since I have the PCB ground point test via a 16 Ohm resistor connected to the protective earth screw, the hum was also gone.
From the type of construction could be all cheap amps from that era amps of different brands from the same manufacturer.
I still have a Marathon M10 and a Hohner Merlin amplifier from that era, those are the next ones I need to fix.
Let's see if the loud basic hum is also present.
I am now aged and am in the process of repairing and selling my remaining stock of amps, effects pedals, guitars and more.

[Manfred]

I wired the rectifier incorrectly, no more safety in this case.
Do not follow this image but the schematic.
The picture with the correct wiring I post as soon as I have rewired.