The AM Forum

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.

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

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...

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

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

Shore thang.  Come an' get 'em!

OK.

I'll lend you my two HF logs in exchange... come and get 'em!   :lol:

Maybe we can both get a package deal on hernia insurance too.

T

That guy in your avatar should be able to carry ALL that stuff. Don't sweat the hernia there OM.

The Shriners at Hosstraders pay big bux rentals for Fez wearing dogs -
security duty, caw mawn.  Get him a tassle ands you're golden.
(http://amfone.net/phpBB2/images/avatars/178016232942adce90f1ed9.jpg)

My 75A4 and T-368 PTO that I use as a VFO both drift noticeably with even a fraction of a volt change in line voltage.  The problem was so bad with the T-368 unit that it would completely drift out of the passband when I tried to work 40 cw with it.

I run the PTO off a  regulated DC power supply.  Use the 12 volt supply that I originally used to run my 2m CB.  Put a dropping resistor in series with the line.  Now the PTO does not drift once it is warmed up.  I do not shut it off during standby.  Just shielded it well enough that I can't hear it in my rcvr.

Solved the 75A4 problem with a small Sola constant voltage transformer.  I found it at Dayton one year.  It is rated for only about the power level the rcvr draws.   It is quiet.  Most of those Sola things sound like a chainsaw running, but this one is of recent manufacture, designed to regulate the a.c. line voltage to a personal computer, and makes very little noise.

If you jumper pins 2 and 7 of the ballast tube socket with a 45-50 ohm, 10 watt resistor, you don't even need to swap out tubes. You can just bend the leads to keep them from touching anything, or even sleeve them if you're paranoid, then stuff it into the socket. I set mine up like this. Works FB OM and easily reversible when the collectors come a-callin'.

No pins to cut, no wires to slobber.  :)

The 12BA6 tube swap works but, the input voltage to the 3TF7 is something like 26 volts.

I have a solid state replacement I build.  Rick Mish puts a 10W, 40 ohm Dale resistor (after removing the print showing the value; how would anyone ever guess what the "secret resistor" is?  :o ) in line.  That goes in place of the tube socket which is removed.

HUZ needs glasses dats no guy on the vu avatar.

Yeah Frank, I was wondering if HUZ was looking at Redi-Kilowatt's cigar log or something by mistake.

Heh, heh. Ya never know these days, what with plastic surgery, hormone therapy and all. But when JJ posted, his avatar was a muscle bound Irb. This board doesn't retain the avatar used at the time of the post, but the most recent one. Good necro though.