Monday, July 02, 2012

Fixing an Amplifier 3

Fixing an Amplifier: 3

So that Fender Blues junior amplifier has come back under my nose, and it has a very weird problem.

Altering the volume knob makes it sound like it's tuning a long wave radio.

it whistles and hums, you can modulate the whistling sound bu altering the volume and tone.
something is definitely not right.

Trouble shooting.
So the first thing to do it try to figure out what's wrong.
Clearly there is something unstable and oscillating inside the amplifier, and this is causing random and weird noises to be picked up and generated.

So first, I take the back off.
have a cursory glance for anything loose or disconnected.
I can't find anything.

I plug the amplifier in and see if anything looks a little out of place, or weird.
the humming starts again, (notice the humming is worse because the main board is no longer completely shielded.)

Lightly touching the ribbon cables that connect the main board to the valve board produces interference, it seems that the low voltage signal going into these boards is particularly susceptible to interference.

In order to reduce this problem the ribbon that carries the instrument signal to the pre-amp valve, and the ribon cable to the valve used after the reverb recovery amplifier are shielded in aluminium foil.

The amplifier now has slightly less interference, but when adjusting the volume the weird noise (though not as pronounced) is still there.

Now I begin inspecting the circuit diagram in detail.
It seems that there is not really a lot to go wrong in these amplifiers, in fact the only thing that can go wrong is, Valves break, reverb recovery amp any break (but very unlikely without excessive abuse) and electrolytic capacitors leak.

I looked long and hard at every electrolytic capacity on the board, and found that one of the four large grey filtering capacitors was slightly deformed.

Searching on-line led me to find that what appears to have happened is, in some inane bid to save money, Fender bought the cheapest sub par quality components that they could find. there are loads of stories of these leaking, and bulging and stopping amplifiers working.

So I ordered some more capacitors and changed these for new (and better quality) ones.

the amplifier is now working perfectly again.

I have left the foil shielding on the ribbon cables, this adds a slight capacitance to the circuit and rolls off the brightness of the amplifier slightly.

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