SUPPORT: Amplifier stuttering and not in a tremolo / vibrato like fashion?

[RetroLinear]

Symptom: Amplifier stuttering with sound cutting in and out thru speakers only. Lineout is clean and operating.

Notes:
What you are experiencing sounds like the power amplifier device cutting in and out due to a perceived overload or short situation on its output.

The power amplifier chip on our Warneck Amplifiers has a built in protection circuit inside that will cut off the output if it detects an overload or fault condition.  It will then wait a short period of time (usually a little less than 1 second) and then "try again" to operate normally.  If it senses the fault again, it will again cut out.  This cycle will then repeat infinitely as long as the fault is present, producing the 'stuttering' you describe.  This 'stuttering' action prevents the amplifier chip from overheating and burning out. The line out signal will be unaffected since its signal is taken ahead of the power amp stage on our amplifiers.

The issue here is whether or not there is actually a fault such as a bad speaker, pinched wire, or short somewhere in the speaker wiring. Sometimes a static discharge can cause the power device chip to fail prematurely, although this is extremely rare.

We suggest the following:
Hook a known good 8 ohm speaker (any will do, as long as it can handle 20W of power) *directly* to the amplifier's speaker output terminal, bypassing all of the internal speaker and headphone jack wiring in the piano.  Then, test the piano. 

If all operates normally using the external speaker, then one of your pianos speakers may have a short in it or your speaker wiring/headphone jack may be incorrectly connected or shorted.    Verify that you see the appropriate load with a meter across the speaker wires (8 ohms, likely a bit less with a DC resistance measurement).  Also check for shorts from the speaker leads to chassis- there should be NO continuity between either speaker lead and chassis ground in the piano when the speaker wires are disconnected from the amp.