# DPDT switch...



## ezcomes (Jul 28, 2008)

so...my chorus pedal quit working, but, the LED turns on, the signal still passes through the pedal, but it is not affected...i thought it was the switch but i'm not sure...
my question is, i don't fully know how the switch works, and with the circuit board being double sided, and fairly complex circuit i can't quite follow it... to figure out where the circuits are leading...
so
my question is...as the DPDT switch has two columns of three contacts on each side...do the contacts on each side correspond with each other?
i tried jumpering straight across, but the pedal still isn't working...
any ideas?


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## mhammer (Nov 30, 2007)

What chorus? I'm guessing a Small Clone from your description of the switch and inclusion of an LED.

As for the switch, you are correct in your inferences about the contacts. The switch has two "commons" in the middle, each of which can be connected to the outside lug on one side or the other side. So, whether a DPDT or 3PDT, it is arranged in columns of 3.

Stompswitches can be "rehabilitated". I've done this successfully many times. However, if the problem is with a Small Clone, or similar, it may be the JFET used for electronic switching. Since the chorus effect disappears if the wet signal is left out, many manufacturers over the years have used one set of contacts to simultaneously enable a status LED and a second set of contacts to lift/enable the wet signal for "the effect". The input and output connections of the circuit's buffer stages always remain in circuit.


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## ezcomes (Jul 28, 2008)

its a Rocktron Deep Blue chorus...


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## mhammer (Nov 30, 2007)

Thanks for the info. Looks like the switching arrangement I described. This image from the beavisaudio site indicates they used an "X-wing" DPDT for the switch.








My experience with those switches is that, while generally reliable, they can sometimes come with too much grease inside. When too much heat is applied to the solder lugs, that can cause the grease to liquify and coat the contacts inside, effectively insulating them and preventing contact. I've taken switches like that apart, cleaned off the grease, and re-assembled them to restore functioning. So, it can be done. The question is whether that is worth the trouble, since the problem might be located elsewhere. 

Is there any sign/hint/omen/sense of occasional or intermittent/unreliable contact? or did you go through a stage where stomping sometimes got you the effect and other times you had to press a couple of times to get it to "stick"? If that is the case, then rehabilitating the switch may be worth doing. If we end up going down that road, drop me a PM, and I'll talk you through it.

This image below suggests that it might be possible to identify the JFET involved in switching the wet signal. The transistor in question is likely to have either a K or a J, followed by a 2-3-digit number, and it is likely to be the only one of that type. The shadows make it hard, but I see one contender in the exact middle of the board, and another over by the MN3102 chip.


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## ezcomes (Jul 28, 2008)

i don't know...it just doesn't work, except for the LED, i don't even know if i know when the last time it worked was, i don't use it often, but never had a problem with it...the push button still "feels" ok...i can try to take it apart...but, i've jumpered the leads, and it didn't change, so maybe it is the JFET...
i'll see if i can track one down to replace it and see where it goes...
thx for the help!


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## mhammer (Nov 30, 2007)

Well, if it uses a JFET for enabling the wet signal, the way I suspect it does, then you can jumper that too and see if that's the problem. But you need to let me know what transistor it is and the part number.


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## ezcomes (Jul 28, 2008)

so...tracing the board it looks like they are PNP's as the middle leads appear to go to ground...so i jumpered the outside legs, like you suggested...still nothing...


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## mhammer (Nov 30, 2007)

You wouldn't use a bipolar transistor to do switching, whether PNP or NPN. What are the part numbers of all the trannies?


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## ezcomes (Jul 28, 2008)

i replaced the LM339 chip...theres nothing on the schematic shownig a JFET with a J or K...they are listed as C1815...


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## mhammer (Nov 30, 2007)

The 2SC1815s are more than likely used for either the input buffer, the filtering before and after the delay chip, or both. The 339 is likely used for the LFO section, as it is on a number of other delay-based modulation pedals (e.g., the Washburn Chorus I have).


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## ezcomes (Jul 28, 2008)

i think i am lost then...here is the schematic...


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## mhammer (Nov 30, 2007)

Oh man, that is one UGLY circuit. What they tried to do was have a stereo output (sum and difference from Outpu A and B), while using the cheapest switch they could find. So, the signal runs through everything, all the time, but the switch does two things when it "bypasses". First, it removes power to the LED so that it goes dark. Second, it grounds the delay signal at the "Intensity" pot, so that only clean signal reaches the two mixer stages at the output. It works, I suppose, but the signal always goes through 3 op-amps. 

The image is kind of fuzzy, so I can't clearly see what type they are, but I'm guessing some sort of 4558 variant. You can see that the signal goes through two parallel paths between the first and second op-amps. The capacitor/resistor path provides some treble bopost, as part of a pre-emphasis/de-emphasis network. The de-emphasis can be found in the capacitor/resistor pair in each of the two output op-amps. This is sort of the "poor man's Dolby", and is a time-honoured tradition for delay-based pedals that aren't expected to generate too much noise. Boss uses this in almost all BBD-based pedals.

While it is fine as a means to reduce any noise coming from clock whine or aliasing, you're relying on the relevant components having tight tolerances, and that's not typical of caps, or likely to be found in resistors where the manufacturer uses an X-wing. So there is the risk that the bypass output is not absolutely identical to the input. It could be fine, but it could also be a little brighter, or a little duller, depending on where the component values fall. Hence, not a preferred method unless one picks components very carefully.

What we have NOT discussed so far is the biasing. All delay chips, like the 3207 used in the Deep Blue, need to have the input signal ride on a DC bias voltage. So, if the actual audio signal was, say 300mv, it might hit the delay chip on top of a DC voltage, such that it varies between 7.8v and 7.2V (I'm pulling numbers out of thin air here). If the bias is not set right, the chip will NOT pass delay signal at all. If the bias is set close to right, you'll hear a distorted delay signal, and if the bias is spot on, you'll get cleanish delay.

Luckily for us, there is only one trimpot on the board, and you will NOT damage the delay chip by playing with it. Misbiased BBDs is a common problem, particularly when using inexpensive trimpots like the one on that board. Though using a scope would help in getting flawless sound, biasing by ear will get you 97% of the way there in most circumstances. Using a small screwdriver (plastic preferred, but metal permissible), rotate the trimpot all the way to one side, and slowly turn it while providing an audio signal. If everything but the bias is in order, you should hear no effect, a crappy version of the effect, a nice version of the effect, a crappy version again, and finally no effect, as you rotate the trimpot from one end to the other. Your goal is to find the sweet spot, where the bias is just right.

Try that with the pedal in the "on" setting, and report back. We'll take it from there.


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## ezcomes (Jul 28, 2008)

ok...interesting...would that have moved itself? b/c it was working...then just wasn't

i'll take it apart again this week and try this...see what happens...will check back in...

thanks mark!


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## mhammer (Nov 30, 2007)

In many instances, companies will secure the setting of a trimpot with some goop. Sometimes dirt can get in to the trimpot, and interfere with the contact the wiper makes. As well, generally speaking, I find the smaller the pot, the poorer-designed the wipers tend to be. Tale apart one of those pots that are an inch and a half wide, and the wipers are a thing of beauty, making contact with the resistive strip at many points. Take apart a 16mm or 12mm pot, and the wipers start to get cheesier-looking. The style of trimpot shown in the picture holds the wiper in contact with the resistive strip on the basis of the rivet in the middle of it. If a person opened the unit and bumped the trimpot or the metal piece on top that forms the wiper, the contact between wiper and resistive strip could be interrupted. I'm not saying that is definitely what has taken place in your unit, but it IS something that happens, and at the very least you need to verify that's not it.


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## ezcomes (Jul 28, 2008)

actually i just got thinking too...if the signal is too great...the chip won't work right, and thats where the trim pot comes in? to reduce the 'load'...

so...the last i remember it working, i was running it in front of the amp...(i have tested it since) and when i moved it to my effects loop, i noticed it wasn't working...and since still isn't...so...

would this potentially mean that, if i wanted to move the pedal from in front to in the loop i would have to 're-bias' this trim pot?


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## mhammer (Nov 30, 2007)

No, the biasing is independent of what you describe. The trimpot has no effect on the signal itself. What it does is introduce a steady DC voltage that is _combined _with the input signal. Think of it like putting on platform shoes so that you can go on a roller coaster with a sign that says "You must be THIS tall to go on this ride." The DC voltage makes the signal tall enough.

Brief digression.

The bias voltage for the BBD is derived from the supply voltage. So you take the 9v or whatever, and divide it down by a given amount, like 14/15. Trouble is, when you have a pedal powered by a 9v battery, that battery changes voltage over time, and simply dividing down the sbattery voltage by a fixed proportion is not necessarily going to yield the optimal bias voltage. It's like child support payments: if your spouse pays you 40% of their income AND they have a good-paying job, then you're doing okay. But iof they become unemployed, 40% of EI doesn't really amount to enough. Clearly, a pedal owner can't be opening the chassis and monkeying with the bias trimpot to compensate for the change in battery voltage, and they aren't going to be buying a new Duracell every other day either.

Enter the 32xx series of chips from Panasonic. Unlike the 30xx series (MN3005, MN3007, etc.), the MN32xx series could run off much lower voltages. So here is what happens. The 9v battery powers the other parts of the circuit, but for the BBD alone is dropped down to 5V by a little 3-pin regulator (that looks like a transistor). That regulator needs to see 2vdc more at its input than it provides at its output. It will be able to provide a stable 5vdc to feed the delay chip until such time as the battery itself has sagged down to around 7v. This takes a while, and by the time the battery has hit 7v, it probably doesn't have enough juice left in it to power anything, so using a 5v regulator is a sensible approach. The circuit then derives the bias voltage from that rock-steady 5v. And of course, once it is set, it is set for life and always valid, whether you have a new battery, tired battery, wallwart that provides 8.5vdc, wallwart that provides 12vdc, or whatever.

Long story short, the 32xx series were developed to provide more reliable performance, following factory setup. Some folks will swear by the MN30xx series, but those tend to be used in designs where a number of other aspects of the circuit are also different, so I have no way of knowing whether they offer any audible advantage, indepenedent of the circuit they're part of.


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## ezcomes (Jul 28, 2008)

Mark...

you sir are a god amongst men...i twiddled with the trim pot...on my second time back around...i started getting something...and have gotten it working again...i think there was definitely some dirt in there...

you definitely know what you are talking about...and thank you for helping me through this...

i will say that i hope i never have to take that pedal apart again...i think if i have it on my bench again, it may have to go under a hammer...no pun intended...hahaha

thanks again!


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## mhammer (Nov 30, 2007)

I don't know if I'm a god among men, but I do aspire to being a worthy peer. If you only knew how many thousands I've spent on magazines, books, photocopying, experimentation, etc., how many thousands of hours, and literally millions of words too (I hit 18,000 posts on the stompbox forum the other day, and that's just the _current_ incarnation), then you'd realize that every time I help suss out a problem like this, it's *vindication*.

That, and a dollar, will get you a cup of coffee downstairs at work if you bring your own mug, but it still makes me happy. Bias drift IS a regular problem with a wide array of BBD-based pedals. Tuck that factoid away for safekeeping the next time a buddy finds their flanger or analog delay pedal "just stopped working", and you too can be the hero.


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## ezcomes (Jul 28, 2008)

well...god among men for sure...you definitely are a wealth of knowledge...thx again!

and this is why i come here...if i can learn stuff from others it'll help me...plus the knowledge i've learned cam ehelp others...i think that this is a graet place to be!


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