Extremely distorted suitcase Rhodes output

[Granf]

I'm running out of ideas as to why the output of the suitcase Rhodes piano I'm trying to restore is so distorted.

My initial suspicion was the mismatched speaker impedance. A previous owner had replaced a few of the stock 32 ohm drivers with 8 ohm (still wired in parallel). I replaced the whole set with new 8 ohm speakers wired in series and it didn't make much of a difference.

I was then advised to try playing audio from some other source through the suitcase (thereby testing the power amps, power supply, and speakers). I did so and didn't notice any distortion at all.

Continuing my process of elimination, I tried playing the harp directly into a studio monitor. The sound, though thoroughly lacking in character, was not distorted.

At this point I concluded that the preamp must be the source of the distortion, so I swapped it out for a Vintage Vibe Stereo Vibe, sure that my problem would be gone.

The output is as distorted as ever.

My only remaining thought is that the harp must be operating at far too high a voltage. So my question for the forum is, what voltage should I read coming out of the harp, and how can I control the harps output? Or am I on the wrong track?

Thanks in advance.

[pianotuner steveo]

Wrong track, sort of... The harp appears to be fine. Your amp(s) appear to be fine if you say they sound ok with an external input. Speaker impedance wouldn't cause that distortion, (blown speakers could) but speaker impedance can make a noticeable difference in volume.

It sounds like a pre amp issue to me. What are the odds that the VV preamp is bad too? Probably slim, but it can happen.

Did you try plugging the harp into its own amp without a preamp? It should work, but sound similar to when you plugged into a monitor.

The harp is passive, no voltage other than millivolts. Do not be concerned with volts or millivolts. The number of millivolts here can't be the problem.

[vanceinatlance]

Maybe the cable connecting the preamp to the amp or its connectors is bad. Could also be the preamp  power supply portion of the amp that powers the preamp has issues.

[pnoboy]

It shouldn't be hard to find the problem, but you need the know-how and some test equipment to test both the amp and preamp methodically.  To do the job quickly and thoroughly, I would use a function generator, an oscilloscope, a multimeter, and a complete set of schematics. 

I would start by feeding a sine wave of an appropriate amplitude into the same cable that connects to the harp.  I would listen for distortion.  I can almost guarantee it will be there.  Then, I would probe the output of the preamp with the scope to look for distortion there.  Given the brand new preamp you have, I would suspect you won't see any.  If there is distortion, I would check the supply voltages to the preamp.  then, based on what I find...

I think you get the general idea.  Not doing the testing in a methodical way will just result in frustration and the purchase of items you don't need--as you've already found out.  If you can't do this, find a friend of pay someone to do it--in the long run it will be cheaper than taking a shot-gun approach to fixing the problem.

[Ben Bove]

Great advice. 

If you're interested in seeing if the harp is the problem, take the Gray RCA cord from the preamp and slap an adaptor on it, and plug a guitar, ipod or phone into it.  If the piano has an accessory jack loop, you can just plug in to one of the jacks there as well.