Popping and Crackling Sounds in Wurlitzer Electronic Pianos: Is It the Amp, Or Is It the Reeds?
Popping and crackling sounds are a very common problem in Wurlitzer keyboards. Although they may sound dramatic, they’re most often caused by debris in the reed bar. This is very common and mostly harmless to the amplifier.
Reed bar noise is more likely to happen in a dirty, neglected Wurlitzer. But, because even a tiny particle of dust can be enough to bridge the pickup, even recently cleaned Wurlitzers can suffer from it as well. It is more likely to occur after a keyboard has been moved, so it is something to watch out for any time a Wurlitzer is transported from one place to another. Apparently, this was a problem even in brand-new Wurlitzers, because it is mentioned in the service manual in the context of troubleshooting new keyboards.
However, low-level crackling, popping, or sizzling sounds can also originate in the amplifier. It is most commonly caused by a deteriorated carbon composition resistor. In this article, we’ll describe how to address this problem.
Popping and Crackling in the Reed Bar
Although the pops and crackles might sound electronic, they are usually caused by a piece of debris shorting the reed bar. The Wurlitzer has an electrostatic reed bar, which requires a polarizing voltage to work. It’s basically a capacitor: the reed is one conductive plate, the pickup is the other, and the air gap between them acts as an insulator. The vibration in the reed induces a signal voltage in the pickup, which is ultimately amplified by the onboard amplifier. if the reed bar becomes shorted, you’ll hear loud pops and crackles. Occasionally, reed bar noise manifests as a low-level whine.
How can the reed bar become shorted? Usually, the cause of the short is a small piece of debris that is connecting the reed to the pickup. The debris can be anywhere across the reed bar, and it can be very difficult to spot without a magnifying glass. Or, occasionally, the reed screw can be loose, causing the reed itself to rotate and touch the pickup. If the short is severe enough, the Wurlitzer won’t make any sound at all.
How do I fix a shorted reed bar? If you haven’t already, you should remove everything from the interior of your Wurlitzer and clean it very well. If the inside of your keyboard is dirty, every time you move or even play it, you risk kicking up dust. Any dust that is floating around the inside of the keyboard can land on the reed bar and induce noise.
Next, make sure that every reed is straight and securely screwed into place. If the reed is loose, it can easily become bumped out of place when the hammer strikes and become bridged to the pickup. Removing the dampers may help you examine the reeds more clearly.
Finally, clean the reed bar with a soft brush. Sometimes, if the noise problem is persistent, we take a small piece of high-grit sandpaper (such as 600 grit) and slip it into the space between each reed and the pickup. In order to avoid damaging the pickup, make sure that the grit side is facing the reed. Sanding the pickup will negatively affect the timbre of the piano and is irreversible.
Popping and Crackling in the Amplifier
Amp-induced pops and crackles can happen because of a failing carbon composition resistor. Carbon composition resistors are inherently noisy, and as they degrade they become even noisier. In this case, you’ll need to locate the deteriorated resistor(s) and replace them, preferably with modern, low-noise metal film resistors.
Carbon composition resistors are found in all models of Wurlitzer except for the 200a, which had carbon film resistors. Carbon film are less susceptible to problems involving resistor noise.
Electrolytic capacitors fount in all models of Wurlitzer amplifiers are a common source of noise as well. Bad electrolytic caps in the filter stage will not do their job, filtering out hum and noise. We have also found failing components (resistors, capacitors and transistors) to make intermittent crackling noise (as opposed to a constant hum).
After years of experience repairing 200, 200a, 145, 140B, 120 & 112 amplifiers we have found that there are some key components in high stress areas of the circuit that are most likely to be the cause of noise. If you would like to try replacing these components yourself we have designed kits with these very parts and offer them in our shop.
Here are some links to repair kits you might find useful:
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