I recently acquired a 700 Series Wurlitzer Piano, since the baby boomers and their parents bought enough stuff to fulfill not only their wants but those of the generations to come. It’s one of the spinet models from 1959-1963,which means it is totally heavy, totally inconvenient, and totally awesome. I’ve decided to document the restoration process right here for your viewing pleasure. Thanks in advance to my dad who is both a professional engineer and a professional packrat. He has been helping me with the electrical aspects of the process since I don’t have top notch mechanical intuition.

The first order of business was to figure out why the internal amp was humming and crackling. We first removed the back, top, and front of the Wurlitzer to clean out all of the dust and locate the wires that connected to the volume knob and the 1/4″ output. The volume knob (or “potentiator” as I learned) and output were really dirty, so we cleaned it with an air duster and electrical switch and contact cleaner. That got rid of a lot of the crackling, but eventually the potentiator will have to be replaced because cleaning it will only hold off this problem temporarily. My dad says this is a classic problem with potentiators and dirt can permanently damage them, so we are going to order a replacement.

There was still a significant amount of humming, but it didn’t seem like there was a problem with the grounding. It wasn’t until I accidentally leaned on the dimmer switch for the living room light and turned it off that the humming stopped and we realized what was going on. The piano can’t be played anywhere near a light bulb with a dimmer switch because it will interfere with the input. Covering the input with the wood top won’t make a difference because wood isn’t a good shield. Regular light switches don’t cause the same problem.

Instead of strings, the hammers hit metal reeds which vibrate like tuning forks. They are sandwiched between a receiver and an insulator and the sound goes from there to an input wire which goes to the amp. I also learned that the Wurlitzer has three different outputs for external speakers – a 1/4″ inch jack with a switch that allows you to bypass the internal amp, a regular 1/4″ jack, and an RCA output. The Wurlitzer is somewhat like the electric guitar of pianos, except a guitar is electromagnetic and the Wurli is electrostatic.

The next task was to do a better job grounding the Wurlitzer, since it was built before three-prong grounded power cables existed. We removed the whole power unit…thing… (pictured to the right) and noticed that the transformer was taped on. My dad thought someone had used paper between the case and the transformer to ground the unit and just taped it back on instead of bolting it down. So, he removed the tape and paper, bolted it down, and soldered on a three-prong input so now the piano can be plugged in using a modern computer cable.

Unfortunately, once the unit was plugged back in, a very loud humming sound occurred and the fuse blew. So, apparently the paper did serve a purpose other than grounding and may have been put there by the manufacturer once the design flaw was discovered. If anybody has an alternate opinion about this issue, leave it in the comments. Meanwhile, he replaced the fuse and insulated the transformer with carpet tape where the paper used to be . He also took off the adapter for the computer cable and soldered back on a 2-prong cord which is what the system was built for. Further modifications might be needed before it can handle a 3 prong grounded cable.

The damper felts also need to be replaced. After the hammer strikes a reed, a wooden arm with a foam pad on the end drops down onto the reed to halt the sustain. After several decades of neglect, the felts disintegrated and are now not doing their job of dampening the sound after a key is struck. Therefore, it sounds like the sustain pedal is stuck “ON” and all of the notes bleed together. In the picture to the left you can see how one felt is chewed up and the other is totally gone. Hopefully I can find a restoration kit that contains these things that I need. I’m also considering buying this repair and restoration DVD.

All of the pictures can be enlarged for a more detailed view by clicking on them.

Tags: 120 series, 700 series, amplifier, damper felts, hum, restoration, wurlitzer

This entry was posted on Sunday, June 20th, 2010 at 4:33 pm and is filed under Updates. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.