The R-390A Frequently Asked Questions Page

Compensating for High Line Voltages

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This document is a compilation of the wisdom shared by members of the online community of R-390A enthusiasts. As such, it is advice to those who endeavor to maintain and restore the R-390A family of receivers. The information is shared without warranty, expressed or implied. This information is in the public domain and can be freely copied or shared. Appropriate attribution is appreciated.

A simple way to reduce the line voltage present at the receiver is to connect a filament transformer in series with the 'hot' side of AC line to 'buck' a portion of the incoming voltage. First, carefully measure the line voltage present. Then, subtract the desired line voltage from the line voltage present. That is the secondary voltage of the 'bucking transformer' you want to select . Usually, an easily obtained 6 or 12 volt filament transformer is sufficient to bring the higher input voltage down to an acceptable level. Select a suitable filament transformer with a secondary current rating sufficient to power the equipment. Experiment with the connections to get the polarity correct in the following manner. Connect a lead from the transformer secondary to the high side of the transformer primary and apply voltage to the transformer primary. Check the voltage from the open end of the secondary to the low side of the transformer primary. If it is less than the applied voltage, disconnect the power and swap the secondary leads. If it is greater, then the wiring is correct. Then, for use, apply the high line voltage (127 or so) to the series connection of the two windings and connect the receiver load across the original primary alone.

NOTE: NEVER NEVER put an unswitched and unfused device between the power source (the wall socket, etc) and your receiver. Failure can be very dangerous and disastrous!!!! Put a switch and a fuse in the incoming hot lead EVERY TIME.


  • Jerry Johnson, KØCQ
  • Roy Morgan, K1LKY

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Version: 4 - - Last revision: 2006-Oct-01