[R-390] Ballastube

[Drew P.]

Mark McNulty wrote:

> I wrote in a couple of months ago about a R-391 that a
> found at a yard sale, today I powered up the radio and it
> looks like three tubes did not light up.V508 beat frequency
> osc 5749, RT512 voltage regulator 3TF7 and V701 vfo tube
> 5749. I tried to tune the radio to an am station but only
> got static. does anyone have any ideas? Thanks.

Most likely, as others have suggested, the problem is the 3TF7 ballast tube.

Another substitution modification which works well is to plug into the 3TF7 socket a 12v tube having the correct heater current drain.  12V of the 24V heater line is then dropped by the substitute, and the remaining 12V by the BFO and VFO tube heaters.

Add a jumper on the 3TF7 socket from pin 7 to pin 5, and another jumper from pin 2 to pin 4.  You can then use a 3TF7, or a 12BH7, 12BY7, or 12BV7.

This is not the only ballast substitution scheme. Replacement schemes for the ballastube are myriad and controversy-generating. For a wealth of
information on the topic, go to r-390a.net Click on references, Pearls of Wisdom. There you will find a vast wealth of collective wisdom on the topic.

Drew