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Thread: PS-100 and FC-3B footswitch info needed

  1. #1
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    Exclamation PS-100 and FC-3B footswitch info needed. STOP PRESS - The solution is here!

    Hi guys. I'm a complete newbie here, this is my first post. I have a PS-100 which is an amazing piece of thinking. I'm looking for a footswitch with which to control it. I know the manual says that any "standard footswitch" will do and I have a Line 6 double switch unit which can be made to control the Fryette perfectly. However...

    The PS-100 only has front panel LEDs to indicate which channel is selected and in a live situation this can be awkward, footswitch LEDs would be so much better. It actually has no indication at all as to the state of the Fx Loop. I want to add LEDs to the switching setup which should be a done deal.

    I'm an electronics design engineer so this should be a piece of cake - should! There are two basic ways of doing this. 1) LED in parallel with the switch so it lights up when the switch is open, (they usually have a current limit resistor in series with the LED in the footswitch though this may be internal to the PS-100). 2) LED in series with the switch with cathode towards switch and ground, this will lift the lower voltage threshold by about 1.5V-3.3V depending on the LED. The sensing circuitry thresholds are usually designed to cope with that.

    I set my footswitch up to the second series option and it works perfectly with my Marshall and H&K amps, both the channel and Fx Loops switch and their LEDs indicate as they should. But The PS-100 which behaves correctly with bare switches stubbornly refuses to do anything when the LEDs are in place. So there is something not quite so standard about the PS-100 switch sensing circuitry. Perhaps its threshold levels cannot cope with the LED forward voltage drop.

    One thing I did wonder is whether the sensing voltage polarity may be reversed. If the amp switches by grounding or leaving open a negative line voltage then the LED would be reverse biased when the switch was closed. Does anyone have any experience of how Fryette wires their own FC-3B footswitch?

    Incidentally that would be the easy option to solve this issue, (a 1000+ pound unit is NOT supplied with one - eh?). Try getting one on the right hand side of the water! Hen's teeth and unicorn droppings are easier to source.

    Thanks in advance for any info you can offer.
    Last edited by Bordonbert; 06-10-2022 at 11:27 AM.

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    Default The Question is answered!

    I have now looked into this with the live unit back at home and the answer is ridiculously simple. The PS-100 switches a NEGATIVE voltage on its control lines for Channel selection and Fx Loop switching. This means the LED must be fitted in reverse to the normal way round.

    Simply put it in series with the switch but with its cathode towards the socket tip or ring terminal and its anode to the switch, through it and on to the socket sleeve terminal. The Channel selection is connected to the tip and the Fx Loop to the ring, and they both share the sleeve ground connection of course.

    It wasn't rocket science and I don't understand the element of secrecy at all. It took 5seconds to spot where the problem was, I suspected that might be the case in the first place but didn't have the unit at hand to test. So there you go. It only takes any old PS2 footswitch type, just as Fryette advised. The thing to spot was the direction of the LED.

    (I've tried attaching a correctly size limited bitmap of the schematic for that but the Attachment Manager doesn't seem to like me. It just selects the file, briefly whirs around, then displays nothing in my list. Am I allowed to upload graphics at my newbie stage?)

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    thanks for following up! I'm sure somebody else will find this very very useful at some point

    I don't know if attachment upload is currently working, but you can always upload the picture to an image website like imgur.com and copy the link here

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    Default

    I thought I would post a little more technical info here for anyone who may have more demanding needs. It will round off the topic and leave it accessible for the future.

    As we have said, the Fryette switching system is based on a -32V control line. Note the "-" sign, that is crucial. With the footswitch open you will find -32V on the tip and the ring. When the switch closes this drops to 0V of course. Running the footswitch with a LED in the line as an indicator is easy. As I wrote above, the LED is simply put in series with the switch but with its anode (+ve) terminal connected towards ground on the sleeve and its cathode (-ve) terminal towards the tip which is -ve. This is the reverse of the more common amps where the voltage being controlled is positive. So effectively there are two equally valid ways you can set this up:

    Tip->Switch Terminal 1->Switch Terminal 2->LED Cathode->LED Anode->Sleeve

    Tip->LED Cathode->LED Anode->Switch Terminal 1->Switch Terminal 2->Sleeve

    Now a couple of new bits. Can you use any colour of LED you want? The answer is Yes. LEDs are usually specified with a Forward Voltage at a highish current, commonly 20mA. This gives an idea of how the LED will perform if you want to go for maximum brightness. Rough specified Forward Voltages @20mA are in the ballpark of Red 1.8V, Green 2.2V, Blue 3.2V. Don't quote me on that, it is very rule of thumb. However, they will work perfectly well at much lower currents than 20mA and the Vf is then significantly lower in practical circuits. Red about 1.3V, Green 1.8V, Blue 2.5V. For my own pedals I prefer a much lower light level as I think having a set of upwards pointing spot lights at your feet on stage is not always a good idea. In the home environment this might not seem terribly bright but in the dark it can be intrusive and distracting. I design to run at anything from 4mA to about 8mA as the LED dictates. Well, how is that relevant to setting up your own footswitch with LEDs?

    I checked the Fryette PS-100 control action and it seems that the -32V is supplied at an impedance of very close to 1k. That means the control line looks like -32V coming to us through a 1k resistor. With just the plain switch it means the switching levels are -32V/0V. Adding LEDs in the line will affect the "switch closed" voltage level at the footswitch socket preventing it from being pulled to 0V. So what is the limit of reliable switching there? I added in additional resistance into the line until the switching action became visibly affected. Everything was fine until I reached 1.8k when the channel switched and fully lit the amp Red LED but the Green LED now stayed on dimly. Switching was fine but indication on the PS-100 is becoming less definite so I considered that the limit of how far it should be pushed. There will be no damage but it isn't much further beyond that that the switching may also stop.

    That resistance meant about 19.3V is lost across the added resistor leaving -21.3V at the amp footswitch socket when it should ideally be 0V. The "switch open" level of -32V is the same of course as no current then flows. This shows that even the 3.2V of a Blue LED will have no effect on the switching action of the Channel or Fx Loop, it will alter the voltage from -32V to only -28.8V, well within the safe area of use. Fryette's design is very flexible in that way, and I do mean that as entirely a good thing. You could even add a number of LEDs there if you ever needed to and the action would still be rock solid.

    If you find your footswitch LED is too bright you have the leeway, thanks to Fryette's design, to add in a degree of resistance to reduce the current through it without compromising the switching action. A bare Red LED with Vf of say 2V will draw about 30mA. Adding a 1k resistor in series with the LED will drop the current to around 15mA which will help reduce intensity while it leaves around -17V at the control socket. A 1.5k resistor means 12mA and -20V control voltage, just about on the edge of "not quite right". Anything up to about 1.5k should be ok. Of course you could just be pragmatic and put a dab of semi-transparent red paint on your LED to dim it down.

    Hope this helps with anyone wanting to add or trim their footswitch indicator setup.

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