Variax JTV59 External Power Supply

The Variax JT59 and other guitars in the series are well made instruments and sound pretty good on their own magnetic pick ups, but switch in the digital modelling and a whole raft of guitars are at your finger tips. This digital technology requires power which is normally supplied by the Li-Ion battery supplied with the guitar. The batteries are expensive and over time have proved unreliable. Many users have experienced issues with fully charged batteries not powering up the guitar. A power adaptor is available but at significant cost, so i set about looking into creating a cheap alternative method of powering up the digital axe.

Parts required:

Small Project Box

Stereo 1/4" Jack Plug ( Tip/Ring/Sleeve)

Std 1/4" Mono Jack socket

Adjustable DC-DC step down converter capable of supplying 1A

Mains to 12v plug adaptor 1A

Twin core and screen audio cable

Grommets to suit audio and power input cables.

Tools:

Small screw driver

Wire cutters

Soldering Iron and solder

Drill and bits

The supply required to fire up the electronics in the guitar is 7V @ 500mA via a Stereo (TRS) 1/4" jack.

The DC - DC step down module is readily available from Amazon @ £6.99 for two, the cheapest I've seen. The power adaptor I had lying around as I seem to collect them. It's rated at 12V @ 1A. The 1/4" jack socket is again one I had lying around, its just a standard mono socket. The cable is an audio grade twin core and screened microphone cable.

Operation is quite straight forward, the mains input is stepped down to 12V DC by the power adaptor. The output of the DC - DC step down convertor is set for 7 volts. This gets fed via the ring connection into the guitar's 1/4" socket. Meanwhile the tip connection is fed straight to the output jack socket in the unit. Note the extra connection from the 1/4" jack socket screen to the PCB output ground.

Firstly drill appropriate holes in the case to accept the jack socket, the power cables coming in and the audio cable. Rather than use a DC socket for the power adaptor I cut off the connector and fed the bare wires through the case using some heat shrink around it to bulk out its fit through the grommet. Once the wires are through tie them in a loose knot or fit a cable tie prevent them being pulled back out . Identify the polarity of the output wires from the power adaptor and strip and prepare the ends to be screwed into the relative connectors on the input side of the PCB.

Solder the audio cable to the stereo jack plug (Tip,Ring & Sleeve). Make a note of which core connects to the tip and which connects to the ring. Thread the bare wire ends of the audio cable through the hole in the case, again using a grommet if possible to secure it.

Use a length of heat shrink or apply tape to the bare screen wire to prevent any shorting incident. Now solder the Screen and the audio core from the Tip onto the appropriate jack socket terminals.

An extra length of wire now needs to be soldered to the screen terminal of the socket, ready to connect to the PCB.

Place the PCB (DC-DC step down Convertor) in situ and connect the two power leads to the input terminals ensuring the correct polarity. The PCB has clear legend on the underside.

Connect the extra ground wire to the negative terminal of the output, and the audio core from the Ring connection to the positive output terminal.

Use a multimeter set to DC volts. Monitor the output terminals of the DC-DC convertor and adjust the voltage pot to achieve 7 volts as shown. The project is now ready to rock 'n roll.

Once the PCB is wired at both ends, it is held quite securely without any need to fix it. If you want to secure it, a blob of hot melt glue would probably be the easiest way.

I cut off the connector from the power adaptor and wired it in directly. If you wanted to use the adaptor elsewhere it would be best to fit a DC input socket to match your adaptors output connector.

I've used a 1/4" jack socket for the output to allow the use of a standard guitar lead to be used to connect the unit to an amplifier or Recording Interface. I purposely did not use a socket for the input so as not to introduce any issues selecting the correct type of lead for the unit to function.

In use the heatsink does generate a bit of heat but is not excessive. You might want to drill a couple of vent holes? The unit seems to work well powering up the guitar electronics. No noticeable hum or interference from the supply. I recommend removing the battery from the guitar prior to using this unit so as not to cause any conflict regarding the internal electronics.

I slapped some naff labels on mine just because I could.

It could be improved with maybe an on/off switch and a LED that comes on but I couldn't be bothered, I just wanted my guitar to work so I could continue recording.

Let the good time roll .....