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distortion from a solid state Wurlitzer elec piano

Started by electricfunk, April 14, 2014, 11:31:54 AM

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electricfunk

So I've got this solid state Wurlitzer electric piano that has started to distort. It's a model 200 which recently had the amp re-built (not by me).

The problem started when I accidentally left the piano on overnight. The next day, I turned it on only to discover a horrible distortion coming through the speakers (and the aux out) when notes were played. There was also considerable hum coming through the speakers that did not change volume with the volume knob (and the hum was almost in tune with G). Also, the volume of the notes was very low even when the volume knob was turned all the way up. Funny thing is, when I switched it off and left it for a good few hours, everything went back to normal - distortion and hum was gone and volume was back. So, now this happens every time I leave it on for too long (many hours).

Does anyone know what this might be? Bad transformer? Bad resistor? Or perhaps it is normal for 200's to do this when left on overnight?

The distortion is definitely NOT bits of dirt/fluff between pickup and reeds. I've had that plenty of times before and I know the sound.

Roly

Hi @electricfunk and welcome.

I have a Pianette in my collection and have done a bit of keyboard service over the years.



{Jobsheet draft 140412 - we're trying to work out a way of streamlining problems such as yours.}


For best results...

So we can better help you,
you can help us,
and
follow this checklist;


Make: Wurlitzer electric piano
Equipt:  electric piano
Model:   model 200
Your local mains
voltage:
frequency:

Your skills/resources (0-9);
workspace (kitchen table - research lab)
soldering
electrical
electronic
Test equipment (y/n);
Multimeter
Oscilloscope
Signal Generator
Dummy load

Please try to find/obtain a circuit diagramme/schematic and post it here.

Take pictures of the general layout of the problem area, and under chassis as well if possible.

Please post here.

We discourage "shotgunning" repairs by randomly guessing and replacing parts - it never works out well.

We diagnose from the symptoms and test you may be asked to run until the precise cause of the problem is located,
then the proper repair procedure for best and quickest overall results.

Please disconnect the loudspeaker from the amplifier.

You will need to make up a Limiting Lamp to conduct tests without causing further damage.





As luck would have it I found the service manual and some other sources right off the bat.
http://www.bustedgear.com/images/schematics/schematic_Wurlitzer_200A.pdf


Troubleshooting the Wurlitzer 200 Series Amplifiers
http://www.ep-service.nl/joomla/downloads/Wurlitzer%20EP%20Manuals/wurli_pb_200.pdf#page=1&zoom=auto,0,772

http://www.youtube.com/watch?v=I6NCQQG1yRM

So it looks like we've got a real flying start on this one -
lmgtfy

If you say theory and practice don't agree you haven't applied enough theory.