Help with Footswitch

[Joe Garfield]

I'm trying to make or find an inexpensive footswitch for the Fender Princeton 112 Plus amp. However this one seems to be a little different. The two button switch (channel and reverb) operates on a mono cable. It works as follows:

When I plug into the footswitch jack the reverb goes off. If the amp is on the clean channel, then shorting the plug will operate both the channel select and the reverb. If the amp is on the drive channel, than the short only operates the reverb.

I don't understand how this is supposed to work. The two switches operate in parallel on the circuit, however the schematic says nothing about resistance. Are they special switches or potentiometers? You can see the schematic here in the middle of the first page, on the left:

http://www.fender.com/support/amp_schematics/pdfs/Princeton_112_Plus_Sche3BB.pdf

Just so you know, I know very little about circuitry, other than the month I've been in my electrical engineering class. So please keep it simple.

I would like to find or build a switch for $20 or less. Unfortunately there are several versions of Fender two button footswitches and they are all wired differently.

[compuwade]

I'm not sure because I'm kind of a newbe, however it seems that all the switch does is ground the diodes that are connected to each switch. When the diodes are grounded it seems to change the amount of voltage to the circuit thus causing the the 4053 to change the channels. So if the switches on the amp itself work with no problems, then either the switches or the diodes in the footswitch are bad. You can test this by taking the footswitch apart and connect the switch terminals together with a jumper wire. This should cause the channels to work if the switch is bad. If the diodes are bad you can replace them easily enough. They are standard 1n4448 types.

I hope this works.

-Wade

[Joe Garfield]

Sounds like it's something to do with a diode then. Haven't tried a switch, just shorting wires.

Do most stompbox switches have diodes?

[compuwade]

Honestly I don't know. Im just going by the schematic you posted. I have a Fender Power Chorus that uses transitors to switch the channels when you depress the footswitch. I do know that usually the footswitch is the problem and not the amp itself. Sometimes all the parts themselves are fine, but the solder joints on the footswitch PCB are broken caused by loose switch mounting nuts. This makes the board bow when you step on the switch, causing the solder joints to crack.
At least yours is simple enough. Some of them are very complicated.

Good luck!

Wade

[Paul Marossy]

Sounds like it's something to do with a diode then. Haven't tried a switch, just shorting wires.

Do most stompbox switches have diodes?

It depends on how the channel switcher is designed, and how many switches there are on it. Two switches using one mono cable would have to use a diode somewhere for it to work.

[pgcorky]

Did we ever get this figured out? I know this is an old post - but when I followed along and looked at the linked schematic, the 2 button footswitch appears only to have a mono jack, and two switches and nothing else! Okay, okay, it has wires, too. Now I'll admit to having zero experience reading schematics - so if any experts catch this old post, I'd appreciate a quick thought or two on what we're seeing here (PDF attached). Is it simply two switches wired to the single mono output jack? I don't know what the pairs of dashed lines mean, so maybe I'm missing something! Many thanks in advance for the guidance.
http://www.flickr.com/photos/pgcorky/4726796933/

[zombiwoof]

Did we ever get this figured out? I know this is an old post - but when I followed along and looked at the linked schematic, the 2 button footswitch appears only to have a mono jack, and two switches and nothing else! Okay, okay, it has wires, too. Now I'll admit to having zero experience reading schematics - so if any experts catch this old post, I'd appreciate a quick thought or two on what we're seeing here (PDF attached). Is it simply two switches wired to the single mono output jack? I don't know what the pairs of dashed lines mean, so maybe I'm missing something! Many thanks in advance for the guidance.
http://www.flickr.com/photos/pgcorky/4726796933/

There are four diodes in that schematic  (the little symbols that look like arrows).

Al

[pgcorky]

Thanks, Al. Before I ask any other stupid questions, is there any suggestion on getting up to speed on reading schematics? I guess the only other question is about the diodes. I looked at small bear and there are 20 different diodes - any thoughts on what I need to use for this footswitch? Many thanks again!

[zombiwoof]

Thanks, Al. Before I ask any other stupid questions, is there any suggestion on getting up to speed on reading schematics? I guess the only other question is about the diodes. I looked at small bear and there are 20 different diodes - any thoughts on what I need to use for this footswitch? Many thanks again!

On the first large schematic you showed in the link, it says unless otherwise specified, the diodes are 1N4448, so I would guess that's what is used in the footswitch.

As for learning about the symbols, a good place to start is the DIY FAQ, which you can find a link to at the top of the forum pages.  It has a lot of info on circuits, symbols, etc. as well as answers to a lot of DIY questions.

Al

[pgcorky]

Hey, Al. You are a prince among men! I appreciate the guidance and patience!

[zombiwoof]

Hey, Al. You are a prince among men! I appreciate the guidance and patience!

Wow, I don't hear that very often! (or ever)

Al

[pgcorky]

Hey, Al. You are a prince among men! I appreciate the guidance and patience!

Wow, I don't hear that very often! (or ever)

Al

Seriously, you obviously take time to help out others - and even somebody who's clearly new and more than a little slow on this stuff - that's just great and what makes the forum worth being a part of. I hope to be able to pay it forward some day!

[greaser_au]

Looks like the diodes with labels (and little arrows indicating light emission) in the footswitch are 'otherwise specified' by the labels or something  Those appear to be indicator LEDs in the footswitch itself.

This is actually a fairly clever idea- though some would say nasty!  It uses AC to share one wire (plus ground) between the 2 control functions.   the +ve swings control the normal/drive channel switch & the -ve swings control the reverb.  An interesting application of analogue devices doing digital time division multiplexing...!    

I'm new around here,  so I'm asking first: is it worth working up & posting a service-level analysis? or is this old news or off-topic?

david

[pgcorky]

I'm new, too - but can tell you that a ton of Fender solid state amps used this footswitch and I had a devil of a time finding anything decent on the web that explained how to do a build. So this thread is a bit old, but a new thread that does a decent job of working through this is sure to come in handy for folks looking for a)something fun to build that's not too complex and b)something a bit less expensive than the $45 Fender footswitch that's being sold today.

[zombiwoof]

Looks like the diodes with labels (and little arrows indicating light emission) in the footswitch are 'otherwise specified' by the labels or something  Those appear to be indicator LEDs in the footswitch itself.

This is actually a fairly clever idea- though some would say nasty!  It uses AC to share one wire (plus ground) between the 2 control functions.   the +ve swings control the normal/drive channel switch & the -ve swings control the reverb.  An interesting application of analogue devices doing digital time division multiplexing...!    

I'm new around here,  so I'm asking first: is it worth working up & posting a service-level analysis? or is this old news or off-topic?

david

Thanks for pointing that out, I missed the LED symbols there.  So two of them are the 1N4448's, and the two with the different symbols (with the little arrows) are the LED's for the pedal functions.  I guess I should look closer next time!

Al

[greaser_au]

Thanks for pointing that out, I missed the LED symbols there.  So two of them are the 1N4448's, and the two with the different symbols (with the little arrows) are the LED's for the pedal functions.  I guess I should look closer next time!

Al

Al,
I wasn't picking, I saw the 'otherwise specified' note, too...

Your last sentence reminded me of the day I went for a job interview with a local supplier of pinball & video games, like 18 years ago...  we were wading through the techy bits & he showed me a circuit....  the plug on the columns of the switch matrix had a keyway pin, the row plug didn't.  he said: 'how many switches in the matrix?'  i said: 'umm...  8 times ...umm... 7,   56?' He pointed out to me that the plug with the keyway had NINE pins including the key...  what an IDIOT!!!     I still got the job, though, so I had a good day all the same!

david

[zombiwoof]

I didn't think you were "picking" at all!.  Actually, since the OP said he is not very experienced in this stuff, it's a good thing you pointed out what I missed (that two of the "diodes" were actually LED's).  I think you should share in the gratitude he aimed at me!.

Al

[pgcorky]

I don't need to be stingy with my thanks - good catch, David! Between you and Al, I'll get this built! Thanks to you both! Do we all agree that an SPST switch will do the job?

[greaser_au]

here is my interpretation of a layout of the circuit  in your image "slide1" -  is this what you wanted?            http://www.aronnelson.com/DIYFiles/up/topic79656.pdf

the top one is a literal layout directly from your picture, the second one is how I would build it to perhaps make it a little more reliable (the diode, led & switch are a series connected string - so the order in the string is unimportant, as long as the diode & led polarity remains the same relative to the socket).  

As the retail item is fairly inexpensive, by the time you add a couple of quality arrow or carling stompswitches & a decent cliff or switchcraft jack, some nice LED bezels,  a robust enclosure  & purchase (or even make) a cable, you have probably exceeded the retail cost, never mind the investment of your own time... so I'd still recommend buying the 'real thing' for something simple like this...  But, that value judgement is for you to make!!!  

Years ago I built a switch like this (but TRS plug and no diodes as per the other designs discussed in the 2 threads on this topic) in a 5"x2.5"x2"  ABS box (like #270-1803) with low-cost switches (like #275-617) to keep me going while I waited for the the real thing to become available at the right price*,   you sure as H-E-double-hockey-sticks wouldn't want to take one of those on the road though  

*this sort of stuff was always soooo expensive in australia - even these days it's STILL usually cheaper to buy online overseas...

david

[pgcorky]

That's  sweet diagram, David. Life is good! Re build cost vs buy price, I've seen the current generation of this footswitch from Fender at retail for about $40 plus shipping - so a total price just shy of $50. So I think this can be built for less, but if you factor in the research time and build time - well - I'll be discounting my labor to zero, but adding the priceless fun factor!
Bill of Materials - Equivalent to Fender P/N 028122         
or Fender P/N 099-4056-000         
Description            Qty   Price           Extended
Carling SPST 110-P   2    $6.00     $12.00
LEDs                          2   0.25             0.50
LED Bezel                  2   0.25            0.50
1590 Enclosure          1   6.75             6.75
Switchcraft Mono Jack   1   1.70             1.70
Diode - 1N4148          2   0.15             0.30
Wire          0.00
Shipping                                   5.25
Total                                        $27.00