Mach 1 Overdrive mods?

[Chuck D. Bones]

For opamps to bias properly, they all need a DC path from their inputs to somewhere. If an input is left floating, it will cause the input circuit to saturate. Some dual and quad opamps share internal bias circuitry and if you upset one opamp, you can upset them all. Also, floating inputs make good antennas and will pick up noise or RF and might spread that around in the circuit. The LM386 is an exception, it's not really an opamp and it has internal 50K bias resistors to ground.

 

[Devoureddeth]

Wow well now I know. I don't think I could google an answer that good, thank you.

 

[HamishR]

So if I want to "fix" my Animal or 45 Caliber pedals where should I connect pin 5? I understand that it's bad form to leave it disconnected but the pedals sound as good as I could ever want and I have never had any issues with noise or anything other than they sound great! I'm reluctant to mess with them but if you think that connecting pin 5 to something will help then I can try it.

 

[Chuck D. Bones]

Best place would be Vref.

 

[HamishR]

Hey Chuck! I just built myself another Caliber 45 to put in a box with a Majestic. I have changed my vero layout so that I can connect pin 5 to Vref and it works perfectly and sounds exactly the same as without, so I guess that's a win. So the pedal is potentially more stable now? Whatever, if you recommend I build this way that's good enough for me, especially as it doesn't appear to affect how great this pedal sounds.

 

[Chuck D. Bones]

It can only help to connect pin 5 to Vref. When can we see your new creation in Build Reports?

 

[HamishR]

Well it's all vero but I guess I can post it...

 

[phi1]

Best place would be Vref.

The tagboard layouts for blue note and animal have pin5 connected to ground (on the advice of the fsb blue note trace). It’s really handy on vero since you can just leave pin4 and 5 connected. Is vref significantly better than ground? I only ask because I’ve used the same method on a few other circuits where an op amp was unused.

 

[Chuck D. Bones]

The advantage to using Vref is is doesn't violate the datasheet's common-mode voltage range requirements. In layman's terms, it means that some opamps go ape-shit if their inputs are driven to the rails.

 

[HousTom]

You can use film or electrolytic for C5. The DC voltage across C5 is close to zero, so polarity won't matter if you use electrolytic.

You could use an electrolytic cap for C2, but then you would have to observe the correct polarity because there is 4.5VDC across C2.

I deal with this frequently -- for a specific cap (say 220n or 470n) I'll have on hand only electrolytic which is supposed to be polarized but the PCB/schematic expects a ceramic/film/box cap so polarity is unspecified. I stress out trying to make sure my caps gets installed with the positive lead facing signal/power input and the negative lead facing ground but always wonder what would happen if I get it backwards? Would the cap explode? Would the circuit just not work? Or would everything work fine because even electrolytic caps are somewhat "reversible" as long as we are talking about minimal voltages (i.e. ~ <1v )? I realize this is a newbie question and also that I ought to just invest in $100 worth of box caps. Any response from any point of view appreciated.

 

[Chuck D. Bones]

If you're serious about pedal building, then you really should invest in the right parts. The likelihood of success depends on a good design, good parts and good workmanship. Compromise any of those and the chances are good you will end up troubleshooting or shit-canning your pedal.

Now, about determining the correct polarity for a cap when the build docs don't tell you (through no fault of their own). The easiest and most reliable way to determine the correct polarity is to measure it in-circuit. Build you board, minus the unpolarized caps that you're subbing with polarized electrolytics. Power it up and measure the DC voltage from one capacitor pad to the other. There's your answer. Don't be surprised if the DC voltage is close to zero. In that case polarity doesn't matter. In some rare cases, the polarity can go either direction depending on signal level. In those situations, I refer you back to the notion of using the right parts.

Polarized capacitors will leak current if enough reverse voltage is present. That voltage is somewhere between 1V and 5V, depending on the cap. If enough current is available, the capacitor will overheat and either explode or catch fire. This is usually not a problem in guitar pedals because the currents are so small. The circuit might not work right with a backwards cap, but there is no safety hazard. The only cap that is likely to explode if installed backwards is the one between +9V and ground. I have seen some pedal schematics where a polarized caps is backwards. Because the DC voltage on the cap is low enough to not upset the circuit, the designer never caught the mistake.

 

[Markus Kersius]

I have both original pedals, LS & SL and degooped them both.
The SL is literally build on the LS pcb.

Differences are:

LS:
a100 vol, a10k tone, a500k gain pots, 5 1n914 diodes and a opa2134 ic

SL:
a 250k vol, a10k tone, a1meg gain, 5 1n914 plus 2 extra Bat41's diodes in the loop and a tl072 ic.

Resistor and cap values are the same in both pedals

 

[music6000]

I have both original pedals, LS & SL and degooped them both.
The SL is literally build on the LS pcb.

Differences are:

LS:
a100 vol, a10k tone, a500k gain pots, 5 1n914 diodes and a opa2134 ic

SL:
a 250k vol, a10k tone, a1meg gain, 5 1n914 plus 2 extra Bat41's diodes in the loop and a tl072 ic.

Resistor and cap values are the same in both pedals

Where are the 2 BAT 41's in the Diagram :

 

[Markus Kersius]

Where are the 2 BAT 41's in the Diagram :
View attachment 6673

Haven't traced it yet, just visual identifying on my part.
But it seems to be a pair of asymmetric bat41's in the 430k, 3.9k resitor and 220nf cap loop.
If that makes any sense..

 

[music6000]

Does R4 - 4K7 still remain or has it been replaced by the Bat 41's.
Continuity test would tell you if you are in the Ballpark.

 

[Robert]

But it seems to be a pair of asymmetric bat41's in the 430k, 3.9k resitor and 220nf cap loop.

Those are all in different parts of the circuit going to Vref, so it sounds like the BAT41s might be hard clipping to Vref, possibly after R10?

Could you post a pic of the board?

 

[Markus Kersius]

All resistors remain as in the lightspeed, i have some pics of both pedals though only component side.
Never thought of taking pictures of the back.. doh..
A piece of doublesided grey tape is on the bottom to protect from shorting and fix the pcb to the pots.
The southland is a "first batch" november 2014 with the 1st rev lightspeed pcb "v3.0" on it, i replaced the poly caps with wima cause i like them better.
The degooped lighspeed is 2019 model 2nd rev.
I also have an unmolested Lightspeed from 2017 but both Lightspeeds share the same pcb.

 

[Markus Kersius]

 

[Markus Kersius]

And the Lightspeed:



You see the 2 extra spots for the bat41 on the lightspeed pcb.
I put sockets in just for experimenting with diodes types and configuration.

 

[Robert]

Very cool, thank you!

Which cap is this? 10n?



I suspect you'll have continuity between pin 5 and the two BAT41 diodes, like this: