The Free Wurli project is an alternative to refurbishing or buying a ready made replacement board for all Wurlitzer e-pianos.
Please remeber that Wurlitzer e-pianos are not manufactured anymore. I think that Wurlis will soon be considered historic instruments. So please, please, please don't drill holes, loose the original volume knobs or change anything that can't be reversed and thus reduces the value of your instrument. "Chopping" your piano (cutting off the legs) is another thing that I don't appreciate at all.
After all the things I have tried to get rid of the anoying hum, my conclusion is that hum is in he nature of the Wurlitzer pickup. This means I could not completely remove the source of hum - since shielding the whole pickup unit is impossible due to the piano action. However I recommend to line the whole cover with a conductive material and ground this shield. Another measure in the circuit was to reduce the amplification factor of the preamp to get a better proportion between hum and the actual audio signal. Moreover you should use a shielded cable as connection between the two boards and connect the shield with the ground.
That way the remaining hum is absolutely acceptable. If you want absolute silence you could be filter it out using equalizers or special hum filters in the effect loop.
I strongly recommend to order the PCB rather than try to create it yourself - the quality is just much better. Using http://www.pcb-pool.com was a good experience for me. Let me know if you can recommend other service providers around PCBs. The service provider must accept EAGLE files. The project file you can download was created with EAGLE version 5.2. In case there's a new version of EAGLE available, you can download version 5.2 here.
Soldering the components is straight forward. Start with the resistors, continue with the capacitors and solder the ICs last. The power amp IC needs a heat sink - with a Rth of 4.2°C/W you are on the safe side.
The really annoing thing in PCB kits is not the components but soldering all the cables and the hardware. It took me twice as long as soldering the components. I advocate pre-crimped leads and connector terminal blocks for connecting the two boards with all the external hardware.
One very important hint: Use the best quality you can get for hardware parts such as resistor potentiometers and jacks. Cheap hardware means trouble sooner or later.
After you have assembled everything it is time to mount the boards and the hardware into your piano. In my Wurlitzer 300 there is plenty of space for the boards and the amp. In other models you should think about mounting the two boards and the heat sink on a wooden board which has exactly the same size and mountig holes as the original board. Since the heat sink has a much lower height than the original heatsink, you can then easily mount the whole unit instead of the original board.
I know about the following vendors of replacement amps: Vintage Vibe (US), Speakeasy (US), Taste und Technik (DE), Electronic Piano Service (NL), Vintage Music Parts (PT) - let me know if you know more. To provide a replacement board these professional vendors must obtain all the parts, manufacture the PCB, shipp it all to the customers, provide warranty and mess around with customers that are not willing to pay - not to mention the cost of developing such a circuit. So I guess nobody is getting rich with replacement boards considering the prices I have seen so far.
At this point I would like to state that a professional replacement board is definitely worth its money. It took me a lot of time and money to analyze the original circuit, experiment with a number of alternative circuits, develop prototypes (and believe me, I had plenty of unsuccessful trials), design the PCB for the final circuit and publish this report. However, the Free Wurli way will cost you about 120 Euro (60 for the two PCBs, 40 for the parts and 20 for the transformer) plus: time.
Another question is how experienced you are with a soldering iron and electronics. You might find a friend who can assist you (or a youngster in the neigbourhood that will do that for a new skateboard). So it's up to you whether you purchase a board from a professional vendor or better try the do-it-yourself solution described here.
Please help the Wurli community by sharing your comments, your ideas for improvements, your experience and your pictures around Free Wurli. I will publish them here. Please send it to me: sascha.kerschhofer@googlemail.com - thanks.
And here it comes: Please consider that since this amp is powered electrically by high voltage it should only be handled by a qualified person. I take no liability for damages caused by these instructions or the circuit. Whatever you do with these instructions, you do it at your own risk.
© Alexander S. Kerschhofer