Fix for the 3TF7 problem in the R-390/390A/391

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w3jn:
Anyone who's had one of these receivers knows that the RT510 ballast tube - a 3TF7 - is prone to burnout.  Although readily available, a new one will set you back $25+.  

The purpose of this ballast tube is to stabilize the filament voltage of the 6BA6 PTO oscillator and the 6BA6 BFO tube.  This was important when the radio was used in a military field situation when run off a generator, but with relatively stable AC line power as found in your house, it's pretty much useless.  The circuit arrangement places the two afore-mentioned 6BA6s tubes' filaments in series with the 3TF7.

There are two easily-reversible ways to get your R-390 working again when the ballast craps out.  First, you can plug in a 12BY7 (one with poor emission is fine so long as the filament is OK) and jumper pins 2 to 4 and pins 5 to 7.  You need to remove the IF deck to do this.

Easier still is to jumper pins 2 and 7 of the 3TF7 socket (a piece of hookup wire inserted into the socket from the top will suffice) and replace the PTO and BFO tubes with 12BA6s.  I've done this in my R-390, R-390A, and R-391 with no noticeable effects on stability.

K1JJ:
If I do the mod, can I tune in my buddies on 6M?

KE1GF:
JN, I did the 12BA6 mod for awhile... I switched back to 6BA6's and put an 1N4007 diode between pins 2 and 7 of the 3TF7 socket, works great. I had a nice 3TF7 in there at one time but the filiment on the BFO 6BA6 shorted and killed it.

When I used 12BA6's the PTO was off a few graduations, when I switched back to 6BA6 with the diode it came right back on freq...

w3jn:
That pre-supposes you have any buddies to begin with.

That said, I'm sure you can get on that gay 6 meter band regardless of which modification you do.  But this mod would probably provide an extra 10 dB for you, which you obviously need, if the receiver in question covered 6 meters - which they don't.  Too bad - maybe you should try HF instead - there's a whole 'nother whirl out there beyond 6 meters - Irb, Boy Baxter, Johnny.... the list just goes on and one.

73 John

K1JJ:
Could you lend me, say five or ten of your best receivers so I could listen to each of the guys you mentioned at the same time [including the gay 6M band] ... so I don't miss anything?

T

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