Radio Station coming out of my 1990 Red Knob Twin

[jalexquijano]

Surprise! Now my amp also plays a Salsa radio station either connected or not connected to the guitar cable but only on the clean channel (Channel 1) This 1990 Fender Red Knob Twin amp was sold to me stock so i will be replacing all the electrolytic capacitors (already swapped the 100uf 100v Bias cap) Should i also consider replacing the plastic stereo jacks at the front? I dont see them broken!

I have tried several guitar cables and with or without the guitar plugged in or not and it does reproduce the radio station at the background. Where should i start???? Anyone with this same problem and how was it solved?

[Tom Phillips]

...Should i also consider replacing the plastic stereo jacks at the front? I dont see them broken!...

Probably not. You say they are not broken. What makes you want to replace them?

Caution about the red knob twin. You have several threads going which give the impression that you are jumping into parts replacements without solid evidence that the parts need replacing. If you keep doing that with a Red Knob Twin you are at high risk of creating damage and more problems than you started with. That amp has construction flaws that make it tricky to flip over the circuit boards to get to the solder connections on the back side. Specifically, the flying wires are not properly strain relieved and the wire itself is 7 strand brittle conductor stuff.
Remember that troubleshooting must be logical and systematic. Not guessing.
Please consider these points.
Regards,
Tom

[glebert]

The Fender plastic PCB mount jacks have pretty delicate internal contacts. They can also break solder joints pretty easy if the nut is loose or missing. I had picked up a "junker" Fender keyboard amp, all 3 input jacks looked OK but not one of them worked. Replaced them all and it worked great. Did a a friend's Hot Rod Deville and he was getting all sorts of noise and radio, turned out one of the jacks had broken solder joints. Flowed some solder on both jacks and it worked great. Amp had been sitting unused for years because of this simple problem. Amazingly, this guy is an EET!

Might want to try putting a guitar cord in each input jack and wiggling it around to see if the radio station pickup gets better or worse. It could be worse, it could be picking up talk radio

[jalexquijano]

The Fender plastic PCB mount jacks have pretty delicate internal contacts. They can also break solder joints pretty easy if the nut is loose or missing. I had picked up a "junker" Fender keyboard amp, all 3 input jacks looked OK but not one of them worked. Replaced them all and it worked great. Did a a friend's Hot Rod Deville and he was getting all sorts of noise and radio, turned out one of the jacks had broken solder joints. Flowed some solder on both jacks and it worked great. Amp had been sitting unused for years because of this simple problem. Amazingly, this guy is an EET!

Might want to try putting a guitar cord in each input jack and wiggling it around to see if the radio station pickup gets better or worse. It could be worse, it could be picking up talk radio

so you thing i should just spray some deoxit contact cleaner inside and flow some new solder on both jacks instead of replacing the 4 input jacks? this amp looks like it was stored and not used for a long time!

[rjb]

Now my amp also plays a Salsa radio station either connected or not connected to the guitar cable....
Where should I start????

Try here.

-rb

[jalexquijano]

The Fender plastic PCB mount jacks have pretty delicate internal contacts. They can also break solder joints pretty easy if the nut is loose or missing. I had picked up a "junker" Fender keyboard amp, all 3 input jacks looked OK but not one of them worked. Replaced them all and it worked great. Did a a friend's Hot Rod Deville and he was getting all sorts of noise and radio, turned out one of the jacks had broken solder joints. Flowed some solder on both jacks and it worked great. Amp had been sitting unused for years because of this simple problem. Amazingly, this guy is an EET!

Might want to try putting a guitar cord in each input jack and wiggling it around to see if the radio station pickup gets better or worse. It could be worse, it could be picking up talk radio

Opening the amp today and will check solder. Will also spray some deoxit d5 on the four jacks. Guitar cable is not the issue as you can slighty hear the radio station when you max the volume at the background.

[jalexquijano]

Now it only reproduce the signal, if i plug the cable and lift it over me. Maybe acts as an antenna! Or even like that it should not pick any radio signal????

[Gnobuddy]

Now it only reproduce the signal, if i plug the cable and lift it over me. Maybe acts as an antenna!

When I was about seven or eight years old, I built my first simple radio. It worked exactly like your Fender amp - I had to attach a length of wire (antenna), and lift it up high, and then I could hear a radio station or two.

However, that was a radio. It was supposed to pick up radio stations. You, on the other hand, are speaking of a (guitar) audio amplifier...

Or even like that it should not pick any radio signal????

That's right, it should not! A guitar amp should only amplify audio frequencies, precisely to avoid the kind of problem you are experiencing. Does your amp have the usual 68k grid stopper resistor between the input jack and the grid of the first triode stage? That resistor is the first defense against radio frequencies entering your amp from the guitar cable. It works with the input capacitance (mainly Miller capacitance) of the first triode to remove radio frequencies from the signal before they ever enter your amp.

-Gnobuddy

[jalexquijano]

Well i just got all the stereo and mono input jacks from Antique electronics. THis amp was indeed sitting for a while. I could replace all the jacks to determine if the RF interference is eliminated completely or just desolder and resolder the stock ones to see if the problem is eliminated. I dont see any 68k grid stopper resistor in the schematic.

[Mick Bailey]

See if it's your cable. Sometimes a low-value ceramic cap (33pf - 68pf) soldered between hot and ground inside the plug fixes it. Make sure this is at the amp end.