vox pathfinder 15 problems

[plucky]

Hi every body, I own a vox Pathfinder 15 22watts limited edition, the problem I had is that It act like an SW radio, yes, I can hear an SW station and the level goes with the vol pot. It seems that the guitar cable acts like an antenna.
I opened it, and noticed that the back side isn't shielded. The chasis is open back too, and the wood of the back cover is on it.

Could anybody help me?

thanks

[R.G.]

First, the odds are that your particular amplifier has a loose ground somewhere. Loosen and retighten all the jacks and controls which contact the chassis. Also find and retighten the safety ground wire which comes in at the AC power entry. Needless to say, do this with the amp unplugged from the AC power.

If that does not help, you can lessen the sensitivity of the amp to RF by inserting a series 1K resistor and a 100pF ceramic capcitor to ground right at the control pin (i.e. grid, base, or gate) of the first amplifying element coming into the amp. If there is already a series resistor there, you can just use the ceramic cap to ground. In hard cases, a ferrite bead in series with the input jack helps as well.

There are at least a million RF decoupling tricks to do, but those are what to start with in the absense of any other data.

[plucky]

Thank you very much...
I was thinking that something like that was happening, beacuse the input jack is a bit broken, but the problem appear also when I plug only the foot switch without the guitar.
another tip???

Regards...

[R.G.]

Could also be that the amp is not making good contact with the AC power safety ground. What is the resistance between the safety ground third terminal on the AC power cord and the signal ground at the jack? Should be 0.000000 ohms.

[plucky]

In the country I live, there aren't AC ground connections... So I took out the third pin of the AC plug. Well it's time to borrow a multimeter and check everything.

Thanks a lot....

[R.G.]

I took out the third pin of the AC plug.

That will do it.

You have a problem. You cannot play that amp with any degree of safety. There are some conditions where it will shock or in the extreme kill you. That's what the third wire is for.

And it's probably at least one reason you are picking up radio.

There are fixes, but they require rewiring on the AC power section of the amp, and that process itself might get you killed. Do you have a tech who is experienced that can do the mods?

[plucky]

Do you have a tech who is experienced that can do the mods?

I'm afraid, I don't.

[R.G.]

That's too bad.

Seriously, you are at risk any time you play that amp and also touch anything that is well grounded. About all you can do is carry a voltmeter to test microphones, other guitars, anything metal you can touch.

Since the problem happens any time there is a long wire connected to the amp, simple things on the inputs of the amp probably won't work.

The answer is to connect the signal ground only to ONLY the neutral incoming power line. But that cannot be done safely, as the AC power plug could be plugged in backwards. The old Fenders used a line voltage rated capacitor which could be selectively connected to one or the other side of the AC line by a switch. This switch came to be known affectionately as "the death switch", which is what it became if there was any flaw in the power transformer.

There is one thing that will help. Get an isolation transformer. This converts, say, 240Vac to 240VAC isolated from the main power lines. That may cure the problem itself. If it doesn't, hook your signal ground to an earth ground rod.

It is truly difficult for you to fix this without a power grid that is well wired.

[plucky]

If I understand you well... could I use and AC regulator, like the ones for computers in order to get the 220VAC isolated?