Dual Showman SR red knob footswitch

The amplifier

As a new owner of a Fender Dual Showman red knob reverb amplifier head, I set out to get a footswitch for my prize. The Fender “The Twin” of that period was almost exactly the same as the Dual Showman head (only MUCH heavier with its compliment of speakers.) After talking to about 20 sources including Fender and exhaustive searches on the Internet, I had gotten a handful of answers, but most were different; mostly debate about whether it called for a one or two switch foot-pedal or not. If you try looking it up, much of the data indicates the amp should have a single switch footswitch to just control the channels. There were not many made in this era that had reverb, but I happened to own one. So after getting nowhere asking around, I grabbed the schematics from a Fender “The Twin” amp and what I found did not surprise me in the least.

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JND 983A dispenser 120VAC conversion

 

The front of the JND Advanced 983A dispenser.

 

The rear panel of the JND Advanced 983A dispenser (note the IEC power connection.)

 

Factory supplied EU 220VAC power cable.

The conversion

Converting these units to operate on 120VAC as apposed to the 220VAC input they come shipped as is quite easy. Ironically once you disassemble the unit you will notice that the transformer is already made to accommodate either voltage. In fact, mine looked like they were converted to run on 220VAC as one of the ends of the transformer was cut off and had heatshrink tubing over one of the outer wires to keep it from shorting against anything and the board was tied into the center tap of the transformer instead. Start by dissembling the dispenser.

OE transformer wiring.

The transformer

 

Center tapped transformer.

Let’s take a look at an example of an ordinary center tapped transformer. I do not know the output voltage that the dispenser’s transformer actually maintains, but for this example it does not matter. Our primary winding is what plugs into your wall outlet. While it was wired to expect 220VAC, we will be halving that and inputting 120VAC. If you look at this example and compare it to our dispenser transformer, it becomes very clear how to rewire the transformer to output the proper secondary voltage with a primary winding with our lower input. Notice that between both NA and NB and the center tap, we see a potential of 12VAC as opposed to the full secondary winding which outputs twice that across it at 24VAC. We are going to half the primary voltage to 120VAC from 220VAC. So, in order for the secondary to output the same voltage, we need to use the entire secondary to double the output voltage. Halving the primary winding input mandates doubling the secondary output to produce the same secondary output.

Getting down to business

You will need a pair of wire cutters, a soldering iron, solder (preferably, rosin core 63/37 [eutectic] electrical solder) I use Kester solder mostly, but there are other manufacturers out there that make quality solder. NOTE: I do not recommend silver solder due to its higher melt and remelt temperatures… You should use rubber gloves to protect your hands from absorbing the lead from the solder. You will also need a couple of pieces of appropriately sized heatshrink tubing to make a safe and neat job of the conversion. Also a heatgun or hair dryer to shrink the heatshrink tubing. So now that we are ready and have a clear area to work, let’s get started.

Start by cutting the secondary’s center tap wire – the black one in the center of the transformer in the image below. Make sure you leave enough wire from the board for the wire to connect to the outside secondary wire that currently is being insulated by red heatshrink tubing. 

Cutting the center tap wire from the transformer as supplied by the manufacturer.

Preparing for soldering

First, we need to cut the heatshrink tubing off of the end secondary wire, strip and prepare it for soldering to the black wire coming from the board. Now prep the black wire you cut that is attached to the PC board to be soldered to the end secondary wire. Cut two adequate lengths of the properly sized heatshrink tubing and place one on the cut off secondary center tap wire on the transformer and shrink it down to keep the end from possibly contacting anything. Next we need to remember to push a piece of heatshrink tubing on the black wire to the PC board up toward the board in order for the heat from soldering does not shrink the tubing while we are soldering. Now solder the black wire from the board to the outside secondary wire and then push the heatshrink tubing over the solder connection of the wires and shrink it over the bare wires you just soldered.

Finishing up

Once the transformer wiring modification is complete, you can use some zip ties to organize the wiring some and keep it off of other components; the dispenser should have had one or two in place when you dissembled it. Now reassemble the case. The last component you need to acquire is a common IEC 120VAC power cable – just like is common to say a computer power cable.

Finally, plug in the 120VAC IEC cable to the back of the unit, plug the other end into your wall outlet and test for functionality. Now you are ready to start working on all of your surface mount electronic projects!