Noise sources in old modulation pedals

Vesperado

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This week has been a doozie. Got through reconditioning a handful of my old treasures. I confess myself a novice when it comes to pedal maintenance, but am slowly learning the ropes. I figured I'd share a few pointers which I have picked up on in getting my pedals performing their optissimus.

Reflow:

Reset all major solder connections by heating up the old solder; or go a step further and desolder, suck out the old, and re-solder with new solder. This fixes the majority of noisy and/or inoperable pedals.

Clean moving parts:

Get some 90% Isopropyl Alcohol and hit up your knobs and switches. Cycle the switch while wet, turn the knobs, too. Clears up the gunk.

Recondition parts:

Change out Aluminum Electrolytic caps with equivalents, or better, as time wears down the electrolyte, which degrades the cap and alters the value.

Bad caps are easy to spot. Some visual clues are:

- bulging cases
- discoloration on the leads
- corroded solder joint on solder-side of the board

You could measure the cap, too, with a capacitance tester, to verify its performance.

There are some technical blogs out there which recommend upgrading all electrolytic caps which are considerably old, say 20+ years or so. For values above 10u, use low-esr aluminum electrolytics which meet or exceed the voltage rating of what needs replaced. Anything 10u and below, replace with tantalums. Aluminum typically label the negative lead, whereas tantalum: the positive, so be mindful of annotating what way the polarity is soldered BEFORE replacing it, as being careless can cost you your BBD depending on the circuit.

Change out opamps with equivalents or better. Check the data sheet for pinouts, voltages, and other parameters. Typically you can find equivalents for cheap, but buyer beware, many discontinued opamps are pulled parts, which may or may not perform as expected. Finding better (upgrade) parts might require some research, so figure out what it is you need. It is good practice to install a socket for your desired opamp. This makes maintenance easy in event you happen to come upon a bad opamp, you can simply change it out, or try out other opamps for performance. When you purchase opamps, always get an extra one or two in case you get duds, or you will have to delay your repair/modification.

BBD performance issues:

The biggie, especially now seeing that no one seems to have NOS lying around anymore. Out of all my pedals on hand, only one required the BBD replaced, which I happened to have a spare on hand. Call me fortunate.

If your pedal is clipping heavily, or only the rise is heard and not the fall of the wave, or if you get no effect and only a dry signal, chances are your BBD has bit the dust.

In our day and age it is paramount to monitor your pedal's BBD on occasion. Take a multimeter and get a data sheet, connect the negative lead to the chassis and use the positive to measure each pin. Before powering up the unit, ensure you have the correct power supply voltage feeding your pedal. Once you have the right published voltage, insert a plug into the Input to close the contact of the switch which engages the power to the circuit. With the indication of power, measure the pins on the BBD and clock. Annotate the measurements.

For MN30xx, 15V max is good rule of thumb.

For MN32xx, 9V max is good rule of thumb.

Some circuits drive the 3000 series a volt or two over the data sheet's specified drain voltage. This might have been ok back in the day when parts were plentiful. But we want to maintain our equipment well. If your BBD is getting anything over 15V, consider using less voltage for the supply. This will ensure longer life. I have a pedal which has a published 18V for power by a guy here stateside who makes pedals. His circuit doesn't clamp down the voltage, the voltage goes strait to the BBD. I never have fed it 18V, 15V is what it gets. This is why measuring drain voltage is so important these days. You might not know what the circuit does and next thing "hmm, my pedal isn't working".

The inverse is true. I have a pedal which has the 3000 series BBD and clamps down hard-core on the juice, leaving only 8.7 V for the BBD, no matter if you feed it 9V or 18V! This beloved pedal of mine clips somewhat when strumming hard on my ASAT, not because of the input or some worn transistor, but because the BBD is starved of its juice. Oddly enough I also have a 3200 series pedal which clams down hard, leaving only 5.6 V for the BBD. Luckily it doesn't clip, but it does produce noise due to the robbed headroom.

Noise:

And now we segue onto potential sources of noise. Bear in mind I am no guru, only a novice attempting to maintain his pedals the best he can.

- Shielding from RFI is key. If your pedal is missing a battery door, or a back plate, or cover of some kind, noise will bombard the circuit in the form of radio frequency interference. Get that opening shielded somehow and your good to go.

- Filtering is next. Replace your polarized caps (and perhaps diodes) at the circuit's supply entry with new ones. 25V ratings are good here, but pay attention to the cap parameters, especially esr. Getting low esr spec caps will help keep noise low. Some blogs suggest building values buy wiring caps in parallel to keep the esr down if you cannot find a suitable spec'd cap.

- BBD drain voltage operating at good headroom keeps the noise floor down.

- IC upgrades for opamps. Low-noise, low-current equivalents do wonders.

- Grounds lifted or use of an isolated power-pack, such as a Li-ion capable of driving your pedal board. Ground hum is no fun.

- Redesign/rebuild your pedal so as to consume less current and maximize headroom. You can always send you pedal into a tech or pedal builder to have this done.

- Use good shielded instrument cables.

- Shield your guitar pick-up cavities and tone control cavity with foil tape.

- Move away from LED lighting, or LED lighting away from you.

- Store your pedals in a dry place. Humidity corrodes metal surfaces, and corrosion degrades your pedal's performance.

- Power off your pedal board when not in use. Saves the life-expectancy of the BBD somewhat.

Update on noise:

- Speaking of minimizing the noise, raising the voltage for the circuit is preferable. To do this, locate every polarized cap in the circuit and note their voltage rating. Replace these with 16V or greater ratings if you are planning on running the pedal at 12 or 15 V. Be mindful of your polarities!

- Trace out the power jack entry on the PCB.

(1) If there is a diode bridging the negative pins on the jack, consider removing it and linking the pads. Pedals from the 80s and 90s are notorious in this, as isolated power-packs did not exist (or at least were not common) then, everyone used a wall-wart plugged into the wall where all sorts of transients were lurking. This was done in effort to combat harmful spikes. I have found that these can rob anywhere from .75 V to 1.8 V of power from your barrel plug, turning what should be 9V into 7 and some change--not good if there is a low-voltage BBD under the hood!

(2) If there is a transistor array, consider trying different gate voltages to alter the clamp on power. Same effort as above, to combat harmful spikes and deviations from lesser-spec power supplies of yesteryear. Find the resistor responsible for setting the gate voltage and simply clip on a pre-determined value in parallel to that very resistor, either raising or lowering the value of total resistance, and with a multimeter read the new voltage pass-through off the power-entry diode. Once you have arrived at the voltage you want your circuit to operate at, either piggyback the resistor to the other, or replace the old one with a new value altogether. Consider perhaps even using a trim-pot (ok, rheostat) in its place if you really need to hone in on that voltage.

(3) Power-up procedure for certain circuits: CONSIDER THIS. I am at fault for frying a BBD on my PSE FL-01. Removing and inserting the Instrument Cable while feeding power to the pedal (15V) caused surges to harm the MN3007. So now I hook all patch cables FIRST then after everything is hooked up, I connect the barrel jacks. Once that is done I power up the Li-ion power-pack. Some pedals automatically "light up", those with electronic switching relays/chips do at least. Another reason why it is tantamount to be mindful of power-up procedure, as well as power-down. FIRST turn off the Li-ion power-pack, then disconnect all barrel plugs. Now I am free to disassemble my board.

Like they say, learn from another's mistake so as not to make it your own :hmm:...
 
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