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 Post subject: Zener diode question
PostPosted: Mon Feb 06, 2023 7:33 am 
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I have a Fender Stage 100H DSP amp that I bought used without the 4 button footswitch. I want to build the footswitch, well half of it for the Reverb on/off and Effects on/off and exclude the Ch Select and More Drive switches. The schematic shows a 2.7v zener but the closest I have is a 3.3v. I’m wondering if I should give it a try with the 3.3v or order a few of the 2.7v. Schematic of the footswitch section is attached (a little blurry but hopefully useful). Thanks.


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 Post subject: Re: Zener diode question
PostPosted: Mon Feb 06, 2023 10:21 am 
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If it was me, I'd try it with the 3.3V Zener, and if it doesn't work well, order some 2.7's.

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 Post subject: Re: Zener diode question
PostPosted: Mon Feb 06, 2023 11:27 am 
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Thanks DVM. That's where I was heading but good to get a second opinion!


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 Post subject: Re: Zener diode question
PostPosted: Tue Feb 07, 2023 11:05 am 
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So I built the switch only including the parts circled in green with a 3.3v zener in place of the 2.7v, and leaving out everything circled in red. Here is what happens:

1) The amp defaults to the Drive Channel with More Drive engaged.
2) The upper switch in the diagram turns the Effects on and off.
3) The lower switch turns the Reverb and Effects on and off simultaneously.

Items 1 and 3 are not what I’m wanting. I’m not sure if changing to a 2.7v zener will fix those or if the full four switch setup is needed. I’m guessing that a 2.7v zener may fix item 3 but all four switches may be needed to access the clean channel. Any thoughts?


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 Post subject: Re: Zener diode question
PostPosted: Tue Feb 07, 2023 11:29 am 
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I'm not really sure how this switching system works. This is for a DSP based amp? It looks like the foot switches form some sort of voltage divide. Various switch combinations will produce various voltages. The DSP or another microcontroller probably reads these voltages and understands that 1v = default; 1.5v = reverb on; 2v = reverb + distortion, etc...

That's the only thing I can figure. Usually it's just a ground signal via a stereo cable that turns things on or off. Here you have a single hot signal that would change voltage depending upon which switches you have pressed.

Anyhow...if I am correct in this assumption, then you would need to A use the correct zener diodes AND green/red LED, and B build the entire switching unit, not just cherry pick the features you want to control.

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 Post subject: Re: Zener diode question
PostPosted: Tue Feb 07, 2023 11:34 am 
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Yes, I think I'm correct in my assumption. If you look at the larger piece of the schematic, it tells you the voltages for the various switch combos for test points 40 - 47. So you would need to build the entire switching unit with the exact parts.

edit: or see what the voltage is for the combo you want and just build a single voltage divide to produce that exact voltage. Then all you would need is a single resistor. Probably would be easier (and more interesting) to use a pot for a variable voltage divide. Then you could just use a knob to turn it to the setting you want. Probably more practical if this were to be used in a "desktop" setting.

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 Post subject: Re: Zener diode question
PostPosted: Tue Feb 07, 2023 12:09 pm 
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Thanks for that. Yes, a DSP SS amp. I think you are correct that it is designed to work as a complete unit.


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 Post subject: Re: Zener diode question
PostPosted: Tue Feb 07, 2023 2:42 pm 
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g-man wrote:
Thanks for that. Yes, a DSP SS amp. I think you are correct that it is designed to work as a complete unit.



Sorta. The components in the switching box form a voltage divide along with the R158 750ohm/1W resistor inside the amp. So you don't actually need to do all the elaborate diodes and LEDs in series. You could actually just use a single resistor. Or have a switch that switches between single resistor values for multiple settings. You'd just need to know what the voltage for the particular setting(s) you want and calculate the resistor value(s).

So in theory, this would be easier. But in practice, it might just be easier to build the foot switch unit "as is".

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 Post subject: Re: Zener diode question
PostPosted: Tue Feb 07, 2023 4:48 pm 
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So, if you only wanted to control the effects on/off and reverb on/off and not have the channel switching, you would measure the voltage at the "top" of R158 (labeled VAC on the schematic) and determine the resistor values needed to get these at TP40?

0.52 vdc (efx off, rev off)
3.13 vdc (efx on, rev off)
4.99 vdc (efx off, rev on)
7.45 vdc (efx on, rev on)


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 Post subject: Re: Zener diode question
PostPosted: Tue Feb 07, 2023 5:01 pm 
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g-man wrote:
So, if you only wanted to control the effects on/off and reverb on/off and not have the channel switching, you would measure the voltage at the "top" of R158 (labeled VAC on the schematic) and determine the resistor values needed to get these at TP40?

0.52 vdc (efx off, rev off)
3.13 vdc (efx on, rev off)
4.99 vdc (efx off, rev on)
7.45 vdc (efx on, rev on)


Well, I cannot say for certain. But, yes. That is how I'm reading the schematic.

And I would also assume that the MCU is wanting to see very precise voltages. As an armchair engineer, I would assume they programmed the MCU to recognize the various voltages created by the series diode array, rather than selecting very specific diodes and LEDs to produce extremely specific, yet arbitrary voltages. So if you're going to go that route, I'd suggest using trimpots rather than actual resistors. Then you don't have to calculate anything or worry about finding some odd ball precision resistor value. You can just dial in the trimpot till the amp selects the setting you want for that switch position.

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 Post subject: Re: Zener diode question
PostPosted: Wed Feb 08, 2023 12:16 pm 
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After thinking about it for a bit, I think you are probably best off just building the entire switching unit as it is in the schematic. I realized that the LEDs are "phantom" powered and that they play a part in the voltage divide. If you use resistors to create the various voltages, it's going to be impossible to have status LEDs. You would need to use a secondary power supply like a battery or a DC adapter. And so you might as well just build it as it is in the schematic. If you don't want the other foot switches, still add the components to the PCB, but omit the foot switches.

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 Post subject: Re: Zener diode question
PostPosted: Thu Feb 09, 2023 6:15 am 
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Appreciate the comments/ideas. I was kind of thinking it would be easier to build the diode setup for all four functions and only use two switches and make the channel/gain sections “fixed”. Just need to wait until my next parts order to get the 2.7v zener.


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 Post subject: Re: Zener diode question
PostPosted: Wed Feb 15, 2023 10:22 am 
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Just to wrap this up, works great with the 2.7v zener.


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