~ 1948 Supro Lap Steel Cleanup and Repairs ~
Page 4; Electronics Cleaned Up, Troubleshooting & Repair:
Upon assembling and playing this Steel, I wasn't satisfied with some intermittence in the tone pot and in one side of the pickup. It's not unusual at all for electronic problems to surface once an old guitar is opened up and things moved around to detect faults on the verge of occuring. It passed the meter readings and tapping tests when opened up, but only playing will reveal full functionality. Time to take a look to find out what's intermittent.
This Steel's specific wiring chart; Factory original:
I have seen both 500K and 1 Meg tone pots in Valcos / Supros.
Schematic of fault readings. See fault and correct readings below this schematic.
Readings are in ohms:
CORRECT READINGS SHOULD HAVE BEEN:
Between red points:
Reading across coil on ground side of pickup = 2.5K
Between blue points:
From pot wiper to output ground = 0 to 250K.
From pot wiper to pickup input into pot = 5K.
From pickup input to pot, to pickup ground = 5K.
Between Green points:
From pickup input to ground tap = 0 to 1 Meg.
From ground tap to empty lug = 0 to 1 Meg.
FAULT READINGS:
Between red points:
Reading across coil on ground side of pickup = Incorrect intermittant readings up to 1 Meg+.
Between blue points:
From pot wiper to output ground = Correct 0 to 250K.
From pot wiper to pickup input into pot = Correct 0 to 250K.
From pickup input to pot, to pickup ground = Incorrect 1 Meg+.
Between Green points:
From pickup input to ground tap = Incorrect intermittant 0 readings.
From ground tap to empty lug = Incorrect intermittent 0 readings.
Internal readings determined that the tone pot's value is 1 Meg.
SYNOPSIS:
(1) 1 Meg reveals intermittent open coil; The ground side 1 of 2 coils. (reading aged pickup coil casing conductance / resistance to ground when open; Amost infinity but not quite).
(2) Intermittent 0 / infinity at tone pot knob stops reveals wiper resistance-race worn out; Confirmed and not rebuildable.
See work confirming faults on the next page.
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TECH SCHEMATIC:
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