Testing Switches Before Installation

[sta63bmx]

Is there a way to test switches before installation?  Some of the blue 3PDT's I get pop badly and I'd like to check them BEFORE installing them in a circuit.  Could you just hook them up to an amp with a cable and switch them on and off and listen for the pop?

[markm]

I've never had a problem with any of mine, maybe the circuit you are using them in is doesn't have a pull down resistor on the input?

[sta63bmx]

The pulldowns are there, but the pop is still hanging around.   The circuit in question is the tonepad Microamp layout.  I built a pair of them for friends, and both popped.  There's less than 1mV DC at the input and output.  In one of them, I did not filter the power to the LED as described in some of the other threads, but I've built several other pedals the same way (no filtering on the LED power) without any popping problems.  I put a "known good" switch in one, and it went dead quiet.  I haven't changed out the switch in the other one yet, since the pop is pretty mild.  But I want it gone.

If a switch is popping, is it because it's losing contact with one pole before making it with the other one?  ANd if so, is there any way to fix that by prying apart the switch and bending contacts or adding conductive grease?

[sta63bmx]

I may have probed on the wrong side of the input and output coupling caps.  I'll look at them again and make sure there's no DC potential there.  Should I be probing inboard or outboard on those caps?

[Mark Hammer]

It's a bit like trying to know if the light in the refrigerator stays on after you close the door, but....

Switches that can "test good" before installation can operate poorly after installation.  My pet theory is that the small dab of grease used to hold the rocker contact in place during assembly can be made liquid when too much soldering heat is applied for too long and the grease starts to flow.  If you pull a misbehaving stompswitch apart, you'll often see a thick layer covering the rocker contact.

Here is how I think one can get around that problem.

The rocker contact pivot on solder lugs that represent the "common" or center contact point.  They work a bit like a see-saw, tilting in one direction or the other.  When the rocker contact is in electronic contact with a side lug, it is also in thermal contact with it, so that whatever heat is applied to one solder lug istransferred along the rocker contact to the other.  What that means is that if you use your meter to confirm that no electronic contact is being made between side lugs A/B and their respective common (i.e., open circuit on that "side" of the switch), then you can solder your little heart out attaching the wires to those lugs.  Okay, now push the switch so that the other side breaks contact (and is physically/thermally decoupled from the common), and solder THOSE wires on.  Then do the common lugs.

Obviously at some point you WILL have to apply heat to the commons, which will transfer to the rocker contacts and the grease around them, but the less cumulative heat is applied to the grease, the less the risk of it flowing along the rocker contacts.  Keeping that cumulative heat away from the rocker contact for as long as you can, and making each solder joint a quick, intervention (cleaning the lugs and applying a bit of liquid flux helps), maximizes the likelihood that the switch will function exactly as the manufacturer intended.

[sta63bmx]

Ok, is this what you're thinking?

Your Hypothesis: The grease melts and then runs down (or "up", since we solder the switch upside down) and gets on the contacts (and maybe gets hot enough to turn to varnish), and since it's not conductive grease, it causes enough resistance that during the switching operation, the switch effectively loses contact with one pole before making it with the other one.

To Confirm: Mechanical popping is caused by losing contact with one pole before making it with another

True?

I will probably pull a known bad switch apart and have myself a little look then.  I could always solder the switches "upside down" somehow.  Directly over my eyes.  With no safety glasses.  And huge blobs of solder.  Or not.

[markm]

I think generally speaking, if you were to clean the lugs of the switch prior to soldering,
maybe even tinning the lugs, chances are you could get a good solder joint and not have
to worry about overheating?

[Processaurus]

Ok, is this what you're thinking?

To Confirm: Mechanical popping is caused by losing contact with one pole before making it with another

True?

If I understand what you mean right, then no, thats just how these switches work, they are break before make, so there is normally a short time between when one pole is disconnected and the other is connected.  Mechanical popping could be from the contact bouncing repeatedly on the contact before settling down, or corrosion (from melted grease?) making a crummy contact, or both. 

With your circuit, for educational purposes, you could always test an unsoldered switch if you have 9 electrical leads that have the little clips (smaller than alligator clips, I forget the name but they hold onto little loops and wires in tight spaces real well). 

Marks suggestion about avoiding heating the common is interesting, also you can wet a solder joint with a damp sponge or rag or something if you want to cool it down faster immediately after soldering it, I've had to do that with cheapo plugs that would melt if you gave them a hard look.

To see if there is DC potential between poles, a good way to do it is put each lead of your meter on the lugs for each pole, that will tell the difference in dc level between them.

heres a couple questions about your particular build, to see if the LED situation is causing the trouble, if theres an option for DC power, does it make a difference in pop how its being powered (LED current not running through the input's ground line)?  Also is the LED taking a lot of current?  Hell, just desolder one wire from the LED to disable it and see if that makes any difference.  Also did you use a wiring scheme that grounds the effects input in bypass? 

[JimRayden]

It's a bit like trying to know if the light in the refrigerator stays on after you close the door, but....

I solved this by putting a video camera in my fridge. Gosh that recording sure was interesting. All it needed was a golf commentator electrifying things up a bit.

And another myth proved to be correct - the light does not stay on.

----------
Jimbo

[Mark Hammer]

You're a pioneer, Jim.  You should hook up with these guys: http://dsc.discovery.com/fansites/mythbusters/mythbusters.html 

Just to clarify some earlier points, the flowing grease is a source of intermittent contact, but NOT pops.  The pops are a product of a huge rush of current caused by a switching action.  One of the ways that can happen is for a cap on either the input or output to have one end "floating" such that any charge stored in the cap has nowhere to drain off until you flick the switch.  When a path for draining is suddenly provided, the cap discharges and you hear that as a pop.

As we've learned here recently, after tons of folks started building with 3PDT switches and installing status LEDs, the second way that one can get switch popping is for there to be a sudden current drain resulting from the LED being used commanding a significant (in the sense of enough to cause a noticeable effect) current draw when switched on.  In the same way that your car will shudder whether you go over a speed bump OR a pothole at high enough speed, rapid transistions in current being dumped OR being drawn will result in audible pops.

The solutions for "hanging" caps are to have terminating resistors that provide a perpetual path to ground so that the cap is always able to drain, even when nothing is plugged in.  The solutions for status LED-based popping are varied, but certainly one of them is to use super-bright LEDs that can still provide usable brightness even when the current draw is turned way down(via the fixed resistor in series with them), and another is to slow down the turn-on time of the LED a bit via strategic placement of power-filtering caps.  Again, just like driving over a speed bump, how fast you do it determines the degree of shock to the system.  Slowing down the turn-on of an LED distributes that "sudden" current draw over a longer period of time.

[Paul Perry (Frostwave)]

A good test to see whether it is the LED causing the pop, is to disconect the led.