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

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xaxxop
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Post by 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 :roll:

Please any ideas or help would be much appreciated!

Leo
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Post by 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.

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Post by 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
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Post by Manfred »

To ensure safety, use the circuit with the bridge rectifier in this way.
Safety hum loop breaker circuit.jpg
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Post by 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. :D
Safety hum loop breaker applied 1.jpg
Safety hum loop breaker applied 2.jpg

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Post by Manfred »

xaxxop wrote: 19 May 2023, 11:04 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?

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Post by Intripped »

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

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Post by Manfred »

Intripped wrote: 01 Jun 2023, 14:01 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.

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Post by 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.

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Post by 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.

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Post by 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.
Rectifier Wrong wired.jpg

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Post by xaxxop »

Hello, I am back after been away from home.
My amp still not fix :lol: but I am back to it :horsey:

I don't think its about design issue because when I plug a lead cable in "power amp input" noise disappear.
From "power amp input" to speaker there is not issue. I don't think there is a issue on the power supply neither as the power amp stage it's using +27, -27, +15 and -15Volts.

if anyone could give me an idea I would appreciate a lot :) thanks.
Leandro

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Post by xaxxop »

Hello people,

I still not have luck finding where the background noise come from.
As I said in the last post when I plug a lead cable in the "POWER AMP INPUT" the background noise disappear.

I attached a picture of the schematic where you can see the mixing stage, tape input, reverb and volume. I disengaged tape input, turn volumen and reverb pots to zero and still have the background noise :?

I assumed the op amp is ok as sharing half with the power amp stage.

I swapped c13, c14 and R27 and still have the background noise issue :roll:

any ideas? thanks a lot
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Post by Intripped »

is there a reverb tank?
What if you disconnect cables from it?

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Post by xaxxop »

Hi, thank you for replying
Yes i has a reverb tank, I tried to disconnect the cables but background noise still there.
Leandro

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Post by Manfred »

BTW.
Did you tried the Hum Breaker Circuit?

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Post by Manfred »

Rewired the ground loop breaker circuit, now the wiring is correct according to the wiring diagram.
The circuit works fine, the hum is gone.
hum loop breaker wiring  corrected.jpg

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Post by george giblet »

xaxxop wrote: 27 Oct 2023, 12:19 any ideas? thanks a lot
If the amp circuit ground is floating from the chassis then Manfred's scheme is the best option. If the Jacks are not isolating then maybe not.

If the inputs hot line are floating then it could cause noise. If all is working pin 4 or Jack J1 should short the input to ground when the input jacks are removed. You can check pin 4 is shorting to ground (when measuring turn amp down as DMM will cause a loud bang). Another trick is to plug in a dummy 6.5mm jack with a short circuit and see if the hum disappears or stays the same.

Does the hum increase with different control or switch setting, which one? That will give you an idea which stage is causing the noise.

In some cases moving the connection of C2 closer to the input jack ground can help.

Another possibility is bad caps: C27, C28, C29, C30.
- Volume, Contour, Reverb doesn't affect the noise.
So that's a bit weird because I would expect the noise to be closer to the power amp and perhaps caused by filter caps.

But then why is the input jack affect the hum? Perhaps you external connection are fixing/diverting the problem!


Here's another trick which seems to fit a lot of your symptom details,

https://elektrotanya.com/peavey_bandit- ... nload.html

Look at the top right corner. On pin 5 of U3B, the opamp +ve input there are two ground connections. This can help tap off the quietest ground point. On your schematic it's the opamp just before the pre-out jacks. May require some experimentation to get right. And be aware you might get a situation which has no hum with no connections to the amp then hum with the connections - ie. the reverse of what you have. In that case you need to put more time into the ground points. Not all of these types of problems are fixable without considerable re-jigging of ground wiring.

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Post by george giblet »

These problems often occur because of the multiple ground connections at the panel mounted 6.5mm jacks. Depending on the type of jacks used and if they are PCB mounted, a solution is often to replace the 6.5mm speaker jack with an isolated jack.

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Post by WALTER49 »

Put a resistorv 50 Ohm approximatly in series with speaker, so you cant damage anithing. Did you tried inserting a jack with shorted contacts into the amplifier's input? If the noise disappears the problem is in the preamp. If it's so, take off the two opamps one by one and see what happens then solder sockets on pcb and insert new opamps. Excuse my english, it's not my language

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