Technical Note #101 - Fixing the H3000 Knob (optical encoder)

An occasional problem with the H3000 is a failure of the Knob.  One symptom of the failure is the inability to recognize rotation in one direction, so that whichever way it is turned, it appears to be turning only one way.  Other symptoms are failure to recognize rotation at all, and erratic operation.  Note that these are all electrical issues.  If there is a mechanical problem, such as the knob being hard to turn or noisy, try some oil.

The normal (but not exclusive) reason for these problems is that the "optical encoder" drifts or goes bad with age.  Referring to the photo of the rear of the front panel PC board, you can see that a clear wheel with radial markings is inserted in a plastic block screwed into the PC board.  The block has an LED and photocells on opposing sides, and the radial markings on the wheel interrupt the beam from the LED.  If all is working, electronic circuitry senses the direction and speed of the wheel.  If there's anything flaky about the encoder, you get the symptoms referred to above.

What to do?  The encoder device is obsolete; we have no more and neither does anyone else so far as we know.  However, we have found that varying the value of the small resistor next to the block (2.2K in this photo) can frequently solve the problem, at least for a usably long period.  Values are typically between 220 ohms and 2.2K ohms.  So if you have the problem and want to tackle it, vary that resistor!  If that doesn't work, we recommend sending the unit to us.