Monday, October 17, 2011

Fixing an amplifier 1

The problem with this amplifier is that the input jack is a little hit and miss.

When you plug the input in, there is a 50:50 chance if the amp will make a noise, wiggling or holding the jack seems to make the circuit complete, so it seems that the amplifier is working, but the jack is not.

However, along the way, I managed to break this amplifier more than it already was, and had to replace part that were non-standard with standard parts that I had to customise.

The amplifier in question is a Peavey Backstage amp, a tiny little guitar practice amp thing.
The basic problem is that sometimes you plug the guitar in and it works, sometimes you plug the guitar is and it doesn't work.
sometimes you plug the guitar in and it works, but only if you wiggle the jack lead around in the socket.

Basically, the input jack has for one reason or another seen better days, and could do with repairing.

Start by disconnecting the speaker wires, to ensure that you re-attach them with the correct polarity, (and thus correct phase) write down which terminal the white wire comes from and which wire the blue one comes from.
Next, remove the four screws on the top of the amplifier, and sliding out the tray that holds the electronics, push from the front, to the back.

next remove the wiring for the transformer, and output power transistors as these are mounted on the chassis case, and remove the circuit board from the case.

In order to remove the circuit board from the case, you'll have to remove the plastic knobs from the front panel, these just pull off, or at least should just pull off.

This is where the first "challenge" of the build started, when removing the knobs the potentiometer came apart on two of the controls!

This is no trouble, all I needed to do was replace the potentiometers. I could see that they were the small type, (mini pot) as the body was about the size of a 1penny piece, (instead of the regular size of just smaller than a 2penny piece).
looking at the body of the pot I was able to determine that I needed a B47K (linear 47k pot) and an A10K (I think) (Analogue 10k pot)
So I nipped to the shops and bought 2 mini pots of the correct size.

the only trouble is, rather than receive pots with a stubby shaft with straight splines attached, I received a pot with a long smooth shaft. Alas, I found that I could buy small splined pots, if I were willing to wait a few weeks and pay twice as much, or I could use the pots that I could get and make them fit.

to make them fit, I need to shorten the shaft, and then put a texture into the shaft such that the plastic knob will have something to grip.

so, step 1,
Put the pot next to one of the existing ones on the board, use a marker to mark the length that the shaft needs to be.

step 2, cut the shaft.

step 3, hold the newly shortened shaft in a vice, and using a needle file score a spiral pattern into the shaft.

step 4, reverse the spiral pattern to give a knurled looking finish.

Here's a picture showing an unaltered pot next to my newly customised one.

Now remove the old broken pots from the amplifiers circuit board, (Don't do this at the start, else you may forget which is meant to be the analogue one and which is the linear one!)

now solder in your new pots.

Now it's time to replace that jack socket.
simply remove the jack socket by de-soldering it's 4 legs,  pull out the part and solder a new one in!

screw the circuit board, back to the chassis.
plug in the transistors and transformer again.

slide the amplifier tray into the back of the amplifier.
insert the four screws into the top that hold the tray to the amplifier body.
re-attach the speaker.
now test the amplifier.

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