How to Fix Hum in Your Wurlitzer Electronic Piano (Or Other Vintage Amp): Part I

Before we start, a disclaimer: hum should be addressed on a case-by-case basis, because every vintage amp is special and degrades in its own way. What cures one amp may not work for another. That said, reading this guide should give you a good starting point on how to address your own hum problems. This guide is pretty basic and going to assume that the only piece of test equipment that you have is a multimeter.

Part I of this guide addresses hum that can be addressed without any prior electronics experience. Suggestions in this part do not require removing the amplifier or testing components in the chassis. In Part II, we’ve collected some more advanced suggestions that require testing and replacing parts in the chassis. Note that if you’re not able to replace components yourself, you can send us your amplifier for repair (or, if you’re in the New York metro area, you can drop it off at our Tuckahoe location).

Wurlitzer amps are very similar to guitar amps, and the tips we’ll give can be applied to both. The main difference between a Wurlitzer amp and a guitar amp, for the purpose of this guide, is that a Wurlitzer amp sits inside of the piano’s case in close proximity to the electrostatic reed bar. This in itself can be a source of hum.


Safety Notes

Audio amplifiers, and tube amplifiers in particular, contain high voltages. Do not attempt to repair an amplifier without following the appropriate safety protocols, including (but not limited to) discharging the electrolytic capacitors. Note that electrolytic capacitors can pose a shock hazard even in the amp is turned off and unplugged.

Even if you don’t plan on opening up the amp chassis, always unplug the Wurlitzer before removing its lid. The terminals of the switch always have mains voltage across them when the amplifier is plugged in, even when it isn’t turned on! Furthermore, student model Wurlitzers may have mains voltage in unexpected places, such as ports for the umbilical cord that once connected the various classroom keyboards to each other.

No amplifier is worth compromising your health and safety over, and getting shocked by a marginally-functional hum-bucket only adds insult to injury. So be careful and make safe choices! If you’re not sure how to safely proceed at any point, there’s no shame in asking a tech (like us!) to take over.


Steps to Eliminating Hum from Your Vintage Amplifier

Before you begin…

Watch out for hazards both electronic and non-electronic. Note that neglected vintage gear can be dangerous even if you don’t plan on going near the amplifier. If your Wurlitzer hasn’t been opened in a very long time, watch out for dust, animal droppings, and other hazardous particles. Watch where you put your hands: there’s nothing worse than reaching into the depths of an old Wurlitzer and coming out with a handful of dead spiders and a used bandaid from circa 1979. If it’s obvious that your Wurlitzer hasn’t been cleaned since the Reagan administration, this would be a great time to get that out of the way. More on how to clean your Wurlitzer here and here.

Determine whether the sound you’re hearing is actually hum. Hum, as we’re defining it here, is a continuous and uniform droning sound. Other undesirable sounds, including crackling, popping, wheezing, squealing, or motorboating, are outside the scope of this guide.

Step One: Assess the condition of your Wurlitzer.

In this guide, we assume that your Wurlitzer is otherwise in decent working condition, and that your goal is simply to bring the noise floor within your standards. If you have loud hum and no signal, or if the signal is present but unrecognizably distorted, or if the hum is so loud you’re not even sure whether you have signal or not, this is not the guide for you. You can, however, get in touch with us and we may be able to point you in a better direction.

Hum can affect any Wurlitzer, whether tube or solid state.

Determine whether your Wurlitzer has issues other than hum. If your Wurlitzer has an issue other than excessive hum, no matter how minor, consider addressing that first. Investigating a more specific problem with your piano can unveil problems that may also be the cause of your hum. For instance, consider a noisy amp that also has nonfunctioning tremolo. Sure, it’s unlikely that the absence of tremolo is directly causing the hum, and, okay, maybe fixing it wasn’t your priority because you don’t even like using tremolo in your music. But what if the tremolo isn’t working because there’s a problem in the power supply that is incidentally introducing hum? What if a defective component brought down the tremolo, and there’s nineteen more identical poor-quality components throughout the circuit, introducing hum through their deterioration? What if the tremolo is being strangled by a bad mod or failed repair that, if reversed, would clean up the sound of the amp?

There’s another benefit to saving your hum problem for last. Hum is one of the most nonspecific issues that an amplifier can have. It could lurk anywhere in the circuit. It could be caused by shielding. It could be caused by component failure. It could be caused by a design flaw. It could have one source or it could have many. On the other hand, many non-hum problems are much less elusive. Tremolo, to return to our example, only occupies a small portion of the circuit. Thus, when troubleshooting tremolo, you can limit your attention to just a handful of components. Hum rarely gives you this luxury.

Assume the correct state of mind. Chasing down hum is an exercise in patience and restraint. Do not replace components at random. Anytime you make a change to the amplifier, turn the Wurlitzer on and test your work before moving on to your next idea. That way, if you’ve made a wiring error and the amp stops working, you’ll know exactly what changes should be reversed.

Write down everything you’ve tried and all of the components that you’ve changed. This will be helpful if you take a long break from troubleshooting, because it’s easy to forget things that you’ve already tried. If you end up hiring a tech, it will also be helpful to know what you’ve done so that the tech can check your work and move on to other ideas. Finally, if you buy an amp in the future with the same problem, you’ll be thankful to refer to your own notes on how you fixed it in the past.

Step Two: Make sure that the hum is actually coming from the keyboard.

Just because the Wurlitzer is humming doesn’t mean that it is malfunctioning. Sometimes, an amplifier can pick up hum from an outside source, such as mains wiring or another device in the room. Although some of the following suggestions may sound like wishful thinking (fluorescent lights, really?), it’s better to rule these things out first, before embarking on some epic descent into the belly of the keyboard and only discovering, days or weeks later, that it actually was the fluorescent lights all along. Trust us: we once spent two hours rooting around the aux circuit of a keyboard amp, only to realize that the hum was coming from a noisy wall outlet (that had never given us problems before, so go figure). It kind of makes us want to show the keyboard what a hum problem is actually like. And then light it on fire.

Anyway.

Mosts of these tests require removing the Wurlitzer’s lid. If you're unsure how to do that, visit our guide to removing the lid. Note that, in models where the lid provides shielding, such as the Wurlitzer 120, 700, and 140-series, your keyboard will probably hum louder without the lid on. This is normal.

Hum induced from external devices

Is the noise coming from the wall outlet? Some wall outlets are noisier than others and can induce hum into audio electronics. If you’re in a place where the wall outlets seem suspect, you should consider trying a different outlet. You can also try plugging into a power conditioner: any kind that provides not just surge protection, but noise filtering as well.

As a rule, you should always avoid plugging your Wurlitzer straight into a wall outlet anyway. Vintage electronics are delicate and should be protected from power surges and other anomalies. Ideally, the power conditioner should also be switched - particularly if your Wurlitzer has a vintage power cord. In an emergency, you want to be able to remove the Wurlitzer from power in an instant. (This is true for any piece of electronics, vintage or otherwise.) If you have to gently tease a frayed two-prong cord out of the wall from that one angle that puts you in least contact with frayed wire - well, that’s obviously not optimal.

As an aside, if you have that aux output noise problem that we just mentioned, you should use a DI box between your keyboard and whatever device you’re connecting it to. This will to convert the signal into a balanced output and clean up any mains-induced noise.

Hum often originates in the amplifier, but it could be caused by improper shielding or some external device.

Are any other electronics interfering with my Wurlitzer? Unplug any other electronics that happen to be connected to the same power strip as your Wurlitzer. Unplug anything nonessential that’s connected to the same circuit. Turn the lights in the room, and neighboring rooms, off. Is the noise still there? If so, keep reading.

Are there pedals or other equipment plugged into my Wurlitzer? If the Wurlitzer only hums when an external piece of gear is plugged into it, you have a ground loop. This may be a problem with the Wurlitzer, or with the external piece of gear. It can be hard to tell which one is the true culprit. Try plugging a similar piece of gear into the Wurlitzer, and check if the noise floor is the same. For instance, if you hear excessive noise when you plug your Wurlitzer’s aux out into a mixer, try plugging it into a guitar amp.

If the noise is the same no matter what the Wurlitzer is connected to, it’s probably the Wurlitzer. But if the noise floor varies depending on what is connected to your Wurlitzer, the external piece of gear is probably to blame. Try connecting some intermediary piece of equipment between them, such as a DI box.

Non-isolated aux output. If your Wurlitzer has an aux output, and the jack is not isolated (or if it is isolated, but making an unintended ground connection for some reason, for instance because it is damaged), the hum is probably originating in your Wurlitzer. Install an isolated jack and try again.

Note that this is true for all models of Wurlitzer, whether the jack is installed in the amp chassis or the wooden case. The wooden case 200-series Wurlitzers and other models is covered in conductive paint, so the jack still has to be isolated.

Induced power supply hum from the external device. If the external device has a power supply that isn’t aggressively filtered, mains noise can enter the Wurlitzer. We see this a lot when we install an fx loop into the Wurlitzer 112. When pedals are connected to a mains supply, such as a wall wart, the Wurlitzer develops a slight hum. In a functional 112, this isn’t a big deal: the noise floor on a working keyboard should be so low that it can handle a small amount of additional noise. Where quiet operation is required, we use pedals that can be powered with a battery.

Hum originating in the Wurlitzer

If the Wurlitzer hums regardless of the outlet it is plugged into, the environment it is in, and what is connected to its inputs and outputs, the noise is probably originating in the Wurlitzer itself. We’ll move on to the next section.

Step three: check all mechanical connections.

This Wurlitzer already has a hum shield, but for the lowest noise floor a Wurlitzer needs a reed bar shield as well. You would have to remove the hum shield to check whether there is a reed bar shield as well.

We’re now ready to examine the Wurlitzer itself for noise. We’ll start by ensuring that all necessary mechanical connections are in place, including mechanical grounds and the Wurlitzer’s shielding.

Is every screw tightly screwed in place? If a screw is supposed to be completing a mechanical connection to chassis ground, and it becomes loose, the amplifier may hum. This is because the loose connection is interrupting the amplifier’s shielding. It’s not always obvious which screws are contributing to shielding, so make sure that everything is tightly screwed in place.

Is the noise coming from my reed bar? In a Wurlitzer, the reed bar is a rarely-seen electrostatic pickup. If the reed bar is causing excessive hum, the problem is insufficient shielding. To determine whether the noise is coming from the amplifier or the reed bar, unplug your Wurlitzer’s input and then turn on your amplifier. If the amp is a lot quieter, the reed bar is a source of hum. For next steps, see our guide to Wurlitzer shielding.

If the amp is equally noisy regardless of whether the reed bar is plugged in, the noise is coming from the electronics.

Step four: examine the electronics.

At this point, we can perform some simple tests to check for issues with the electronics. Even if you don’t have much experience working with electronics, it’s easy to change the tubes or visually inspect the wiring. Just make sure that you unplug the Wurlitzer before you begin, and don’t touch any exposed parts of the amplifier. The capacitors may still hold a charge even when the keyboard is off and unplugged.

Change the tubes. Tubes that have deteriorated can introduce hum into the circuit for many reasons. For instance, sometimes the insulation between the heater filament and the cathode can break down, which introduces heater hum into the signal path. Just because a tube is vintage doesn’t mean that it’s bad (and, unfortunately, just because a tube looks new doesn’t mean that it’s good), but since tube swapping is a non-invasive fix it’s a good place to start.

Replace all the tubes with known good tubes. If that fixes the problem, you can put the old tubes back in one by one to narrow down which one is bad. Keep the good ones as backups, or even put them back in the amp. If it doesn’t fix the problem and you want to return to using vintage tubes, you may want to keep the new tubes in for now. Troubleshooting can sometimes be stressful on tubes, particularly if you make an error and mis-wire something.

The wiring harness in 700-series Wurlitzer is particularly long.

Examine the wiring harness. If your Wurlitzer is a 120, 700-series or 140-series, it has a wiring harness. This connects the amplifier to the controls, which are mounted on the cheek block or the lid. Over the years, the wiring harness can degrade and become a source of noise. As a safety precaution, turn the amp off and unplug the harness from the chassis before you examine it.

Next, look at the wires. Is the insulation cracked or missing? This could cause a number of problems. If the harness contains bare wire, it could be shorting against some object inside of the Wurlitzer’s case, or even to another wire in the harness. Or, exposed to the environment, the wire itself could have deteriorated.

Then, look at the connections. Are any solder joints loose or missing? Are all of the wires attached where they should be? Are any of the connections bridged? For instance, a piece of debris may be connecting two lugs of a potentiometer, or the lugs themselves may have been bent at some point and could be resting against another object.

If you’re unsure, you can use a multimeter on continuity mode to check if the pots are adequately connected to their corresponding point on the chassis plug. You can also use the multimeter to check if the pots still have the correct value, or if they have drifted. Like any other resistor, deteriorated pots can be the source of noise in a vintage amplifier.

When examining the wiring harness, pay particular attention to the wires connecting the switch. On all Wurlitzer models, the switch is composed of two leads sticking out the back of the potentiometer. When you turn the knob, the switch closes, powering the amp. The rest of the knob rotation is connected to the potentiometer. As you continue to turn it, the resistance of the wafer increases, as does the volume.

Having a switched potentiometer is convenient, but it isn’t the greatest design from a noise perspective. Potentiometer wires can be very sensitive to induced signals, and the switch is powered by straight, unfiltered AC mains voltage. If these wires start to interact, which can happen if the pot becomes damaged or even if the wires are situated too closely together, mains hum will enter the signal.

In the Wurlitzer 200, the power transformer is mounted on the right side of the amp rail, close to the rectifier. However, the mains wiring and the power switch are mounted on the left side of the amp rail. For this reason, the black transformer wires are extremely long in this amp, and must travel past the preamp area. Make sure they’re not too close to any sensitive preamp components.

In a 200-model Wurlitzer, examine the black transformer wires. The above analysis applies to 200-model Wurlitzers, because they also have a switched potentiometer. The good news is that these Wurlitzers generally have a better layout, and all the wires are shorter. It’s easy to move them apart, and you shouldn’t get too much crosstalk.

However, the power transformer in Wurlitzer 200 and 200a models is mounted on the opposite end of the amp rail from the mains power supply. In order for the proper connections to be made, the transformer leads are absurdly long. Like the mains wiring, the black leads on the transformer contain unfiltered ac. If they get too close to the preamp or the speaker wires, they can introduce hum into the signal. Make sure these wires are physically separate by pushing them away from the circuit board while the amp is off and unplugged (but also ensure that they’re not in a position where they might get tangled in the mechanical action, of course).

Furthermore, in some 200-model consoles - in particular the Wurlitzer 206 - mains wiring originates at the left corner of the rear of the cabinet, then enters the amplifier via a hole in the center of the base. This is a gratuitously poor design because it allows these noisy wires to run rampant around the keyboard for absolutely no reason, because they could have just as easily come up through a hole at the left side of the base. They often flop around and end up next to the speaker wires, inducing noise.

With the Wurlitzer off and unplugged, move the mains wiring further into the mechanical action, as far away from the speaker wires and the amp as possible. Then, check the amp again. Is the noise floor improved? If so, the mains wires are the culprit. If you’re particularly handy, you could create a new hole somewhere in the left side of the base, giving the wires a better path to the power supply area. In fact, some Wurlitzer 206 even have a suitable hole on the left side of the key bed where the mains wires could be re-run through.

Next steps for mitigating hum

That’s pretty much it for the easiest fixes. If your amp still hums, proceed to Part II of our guide, where we give more advanced suggestions that may require unsoldering and replacing components. If you’re not comfortable working with electronics, but your amp still has a hum, contact us and we may be able to help.


Further Reading

Browse all of our articles on restoring vintage gear. Or, click on an image below.