Fender "the twin" footswitch

[Teledeluxe]

Hi, my 1987 "the Twin" footswitch stopped working. I can't figure out which Fender part to buy. Looks like maybe the 0994063000? But the descriptions I see for that part say it is for the Pro 185. The footswitch is 3-button (Channel Select, Reverb, Boost). On the green pcb inside the footswitch it says p/n 028148 A. If anyone could point me to the right part # I would appreciate it!

On a separate note, if anyone does repairs in Central NJ (Basking Ridge area) or recommends someone, I would like to get this amp its first overhaul after 21 years.

[oc disorder]

The schematic shows a 2 button footswitch.
http://www.ampix.org/albums/fenderamps/the_twin.pdf
The parts list I have shows a 2 button footswitch
see attached image ..A new number has been added by hand
so perhaps the original has been discontinued ?

[Teledeluxe]

Thank you, the schematic sure does show a 2-button switch. No boost. Funny, I bought this as a floor demo from Guitar Center in San Francisco in 1989, it came with the 3-button footswitch. Maybe they gave me the wrong one! The boost button definitely worked.

[Enzo]

WHy do you need a whole new footswitch? Fix the old one. If it doesn;t work, there is not much to it - a cable, some switches, maybe some LEDs. Not hard to find out what is bad on it. And we have to make sure the problem in not in the amp itself.

[Teledeluxe]

Hmm. I guess I don't know its the switch. I took it apart and could not find any obvious problem. Switching all works fine using the knobs, so I assumed it was the footswitch. I do not have fix-it skills so either have to take the switch to a shop or try a new one. Amp is pretty old and has been dragged all over the place so might need a good pair of eyes on it anyway. Thank you.

[Enzo]

You checked the switches with an ohm meter? Just clicking on and off is no test. Furthermore, we could have a broken wire within the cable, so the real test is an ohm meter at the plug end. Does continuity switch on and off with the switches aall the way at the plug end? If not, then the switch has an issue. If the switches all go off and on at the plug ends, then the amp has the issue.

[bnwitt]

Co inky dink

What a coincidence, I'm about to build a couple of these foot pedals. It should be a 2 button not a 3 button. For a "The Twin" the switches control the Channel select and Reverb on/off. I used a generic two button foot switch last time and added the LED's and diodes but I can't remember which parts I used. Anyone know?

I'm curious as to how this circuit functions. With just a mono plug and a single circuit path it controls both functions. I see that 30 VAC is fed to the footswitch and that each switch has it's diodes opposite of the other so one is conducting on one half of the cycle while the other is conducting on the other half cycle. I also see that the output of each 1/2 of IC101 controls either function. What I don't understand is how the signal coming back from the pedal makes the comparator sections deliver the control voltages at their output. Can someone explain this? It would seem that half of the return AC cycle controls one function and the other half controls the other function due to the polarities of D101 and D105. The voltage drop on each half cycle caused by the footswitches LED and diode controls the two opposite polarity linear amps right? The LED/Diode assemblys are seeing 19 volts (64% of peak for each half cycle) forward voltage. Now what I don't understand is how this works with the 30VAC tied into R133 as well as being routed out to the footpedal. It seems like you'd get no voltage drop as long as that supply is tied there.

On a separate note, does anyone know the equivalent VTL number for the dual element Vactrol LDR's in this amp?

[Enzo]

How many dual resistor Vactrols are there? (5-leg types) 016282 seems to be the Fender part number.

This footswitch circuit is classic Fender encoding. They make many of their amps this way.


30VAC comes through R132 on its way to R133, so there is plenty of room for dropping.

D104,107 make a two drop voltage refernce for the two comparator ICs. There is 30VDC across zener D108 with R138 being the current path for them. So that looks to me like about a volt + on the input pins of IC101. D101 and D105 separate the negative and positive goings on from the FS jack. If the FS switches are open, then the pedal is invisible. The AC voltage, diminished in level by the voltage division or R133, 134, 140, feeds into those comparator ICs through the diodes. The comparator compares that voltage to the refernce, and sends the output pin high or low.

When a switch closes in the pedal, it places the LED/diode pair across the jack. Each button controls one polarity. (In more complex pedals, zeners are involved so it detects not only polarity but levels within.) That LED/diode crops off the AC waveform, reducing the voltage finding its way to the IC. That drops below the reference so the IC output toggles.

D201 at the jack thus defaults the reverb to ON if the jack is empty.

[bnwitt]

Enzo,
thanks again for the explanation. That helps a lot. So, if D201 were open or the footswitch jack were dirty and not making contact with no foot pedal in place there'd be no reverb. So it looks like I can use plain old red or green LED's and silicon diodes for the footswitches.

There are only 3 dual element vactrols:

http://optoelectronics.perkinelmer.c...lisolators.pdf

The do all have different dark resistances so I can probably take a reading on one of the good ones in the amp to figure out which one I need. I'll have to figure out the equivalent resistance between the element and other resistors which are parallel to ground to calculate the vactrol's resistance.

[Enzo]

Or look at its job in the circuit. Imagine you had to replace it with a pot - the pot would either be all the way up or all the way down. What value pot would you chose?

LDRs generally serve as pass elements or shunt elements - so they are either passing the signal along or they are shunting it to ground (or completing a path to ground for a cap maybe)

So if you have one across a grid of a 12AX7, chances are you don;t want one that goes from 100 ohms to 5k ohms. And likewise maybe not one that goes from 10meg down to 500k

[Kazooman]

Resurrecting an old thread:

I have a Fender Dual Showman Reverb head that I brought back from the grave. This amp head is based on the schematic for "The Twin".

I would like to build a footswitch (circuitry as shown in bnwitt's note above). Can anyone suggest good values for the LEDs and the diodes? I can't find a free parts list on the Internet and don't want to pay someone $10.00 to identify a buck's worth of parts.

Note to Enzo: I know you have this one down pat. Thanks from across the state for any suggestion you can make.

[Enzo]

I am unsure what you have. Your amp is The Twin inside? Or you have a Dual SHowman and you want to add all that switching circuitry to it?

The The Twin schematic says right on it "all diodes are 1N4448" (unless otherwise noted). Common 1N4148 would probably also work,but I would use the 1N4448 just because.

LEDs have characteristic voltage drops, and color makes a difference. These are red LEDs and probably ought to stay that way.

The parts list just says "LED" with Fender part number, no particular description. Same with the LDRs, they are just listd as LDR with 016282 as Fender part number.

[Kazooman]

Arrrrrrgggggghhhhh,

I wrote a long reply only to find that I must have "timed out". It is history, lost to the subtle changes in "the force". I am SORRY, I just can't think and type fast enough to keep up with many websites.

Anyway........ Thanks for the reply, Enzo. I am certain that what I have is "The Twin" circuit (often called the evil twin). I have had this apart and I believe that "The Twin" schematic is correct.

What I would like to do (for no other reason than a lark) is to make a foot switch. The "30 volts AC" sort of threw me and I didn't want to make a simple mistake with the parts and I couldn't access a parts list. It sounds like garden variety red LEDs should be fine and I will try the 1N4448 diodes.

Thanks again, and thanks for all you do to make this forum a great place for advice.

[52 Bill]

I seem to remember that one led was red and the other was green, but I can't remember which was which. I think the channel switch was red.