PUPco Research Group

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.

The problem

If you have a modern red knob Dual Showman, it could have been outfitted with either a damping selector or reverb. The reverb units were pretty low volume amps; that’s why we don’t see as many in the used market. From a cursory look at the schematics, I could tell first that any red knob unit not equipped with the damping option was going to have a two switch footswitch assignment. So, knowing the Dual Showman circuits and “The Twin” amp circuits were essentially identical, I homed in on the control circuit for the switch and finally decided on what switch to order.

As you can see from the box, (and the footswitch with the “right” P/N labeled on it, but the wrong internals) looks like it is going to be plug and play. So after unwrapping the footswitch (which are now not made like the originals, style-wise) I plugged it in and went a stomping. Something was amiss. The left switch marked “Channel” was actually controlling the reverb and the switch on the right was controlling the channel. Crap! So back to the schematic.�

I knew from prior work on this era of Fender amps that the footswitches only used two wires and used diodes and in some cases resistors to change the DC that was plugged into the footswitch. Sure enough inside was really simple with some diodes, LED’s and such so all that would need to be done is to reverse all of the diodes in the footswitch. Don’t forget an LED is a diode…

My switch called for some pretty high-brow diodes (IN4448) in the circuit and I did not have any laying about. They had done quite a nice job of soldering everything together and I suspected I might not be able to reverse the diodes already in place due to how short the leads had been trimmed, but since I had nothing to loose I went ahead and tried. What do you know? I was able to get everything reversed and soldered up properly. Plugged the pedal back in and all was as it should be. The left switch labeled “channel” controlled the channel and the right switch controlled the reverb (on/off) – just as it was meant to be.

The footswitch solution

Below I have attached a copy of the full sized red knob Fender “The Twin” schematic as well as a zoomed in shot of the footswitch where it is readily apparent all that needs done is have all of the components’ (less switches) polarity reversed. This should take you about 10 minutes. If you cannot reuse the diodes (you should have no trouble with the special long leaded LED’s…) Fender calls for IN4448 diodes. You can likely get away with lesser more available diodes for the footswitch.

…and here is the business end you will be working with to get the footswitch to behave properly.

Things you will need

A clean work surface.
#2 Phillips head screwdriver.
A 9/16″ nutdriver or socket and ratchet (Don’t over tighten if you must use a ratchet!)
Quality soldering iron.
Solder sucker/vacuum or desoldering braid and flux.
Rosin core 63/37 (eutectic) solder. (Silver solder not recommended here due to high melt temps.)
Flux cleaning solution to clean the PCB during and after it has been re-worked.
Two�IN4448 diodes if you have them as they are much easier to install with long leads on them.
Regardless of whether you can reuse the diodes, you pretty much need to reuse the LED’s.

In the footswitch schematic and PCB you will see the diodes’ polarity is marked by�their cathode.

Diode Polarity

Likewise, the stock cathode connections should be silk-screened on the PCB. You need to unsolder and remove the two diodes as well as the two LED’s (Light Emitting Diodes). The cathode sides (the end you will see the stripe on the diode) are silk-screened right to the board as well as present on the diodes themselves, so all you need to do is insert them and solder them in with their polarity reversed. The LED’s since the leads have already been cut off, look for a flat side nearest the pins. This will be the cathode (negative) side. This procedure is really simple and more time will be spent disassembling, reassembling cleaning and such of the footswitch than reversing the two diodes and two LED’s. The two LED’s will likely need reused due to their long lead length and unique square emitter design.

A word of caution – do not over tighten the jack, switches or screws on the pedal as you could ruin something, most likely the mono I/O jack as it is made mostly of plastic. Nothing needs to be overly tight and it is best to have to tighten them up some further off in the future than to buy replacement parts to fix the damaged ones. Happy jamming!

Addendum

If you can find on the used market a vintage footswitch ASSY P/N 0028122000, this is the original OEM footswitch supplied with the Dual Showman and The Twin amplifiers. It needs no modifications and is a drop in replacement (as this is what Fender supplied with the amps originally when new) you’re in business. It took me several years to come across one and since I liked the styling better (matches my Fender concert in aesthetics) and the fact it was original to the amp, and was priced right, I elected to purchase one. – You’ll be set as I am now; just plug it in and GO! As for the new switch outlined above that takes modifications, I can see it perhaps having a place on a pedal board due to its more compact design. For now I am happy to have the original replacement footswitch as I don’t use a pedal board with this amp – some/many of you might, so the modifications outlined above for P/N 0099-456-000 switch might afford you some more real-estate on your pedal board; it’ll just need modified…

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 N_{A and} N_B_{�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!

Dual Showman SR red knob footswitch