Help This Old Stomp Box

[barronvibrato]

This is a ross distortion pedal. 2 knobs, dist & output.
It has power but no distortion to the signal. Volume
pot works but there is no difference to the signal when turning
the distortion knob. Any suggestions would be appreciated.
Also is there a difference in pot quality, say between radio shack
and others?

[petemoore]

I havent noticed Any diff in different pots other than their size, value, and taper.  The actual R value can be measured between lugs 1 and3 of individual pots [for the finicky], but the marked values always work..
 I would firs t look for a loose offboard wire, although the symptoms don't indicate...I always like to suspect switches.
 After that I would proceed to the debugging stages, checking voltages and following along with the debugging page^and tracing methods.

[Greg Moss]

I think runoff grove has a schematic for th ross distortion, but if it helps the Ross Distortion is very very simular to the MXR distortion + and DOD 250, so any info on those would probably help you with the Ross unit.  what is the distortion settinglike?  it  it stuck in "clean" or "distorted."  Or partially one or the other?

[barronvibrato]

The box is stuck in a clean signal. The output knob works but the
dist knob seems to have no effect at all. I have heard before
about checking the switch, it works to engage the pedal and it
also works in bypassing the signal. Check off board wiring and
it seems to be ok. Having the output knob working, it has power
to atleast some of the board though.

[Mark Hammer]

The pot is probably not behaving well.  The Ross and MXR and DOD units all have the same basic topography, which is a non-inverting op-amp gain stage with a variable resistance to ground to adjust gain.  In a non-inverting mode, the gain is equal to the ratio of (R1+R2)/R2 where R2 is the resistance to ground and R1 is the feedback resistance between the output and inverting pin (a 1M resistor in this instance).  If R2 is non-existent or dramatically greater than R1 (and non-existent usually makes for a high value resistance :wink: ), the device has a gain of 1, and that seems to be what you have.

There are a few fixes.

First, check the appropriate solder connections and pads.  Is it possible that the pots are soldered directly to a pad on the board and a trace is cracked?

Alternatively, you can carefully pry off the back of the pot and check it.  It may be dirty, the wiper may be gummed up, or the wiper may simply need retensioning to make contact with the resistive element.

Worst case scenario, you replace the pot.  Just about any new pot of the same value will give you years of decent service.