Subj: [R-390] R-390A Ballast replacement ideas Date: Wed, Jan 13, 1999 7:06 AM EDT From: SBJohnston@aol.com Sender: owner-r-390@qth.net Reply-to: SBJohnston@aol.com To: WBH3@daimlerchrysler.com, r-390@qth.net Bill wrote: >Hi, just got into this reflector a short time ago. >I am very much interested in how you hooked up >the I/C regulator to replace the ballast tube. >I think I'm using a resistor or something. >Haven't looked for a while. Could u pass the pin >numbers and what goes where along to me. Well, I'm in the same boat - but in my case I haven't thought much about it for ten years! Hold on and I'll go see if I documented the change in the manual... No, I don't see any of my notes on it... but let's figure it out... The ballast resistor tube RT-510 provides some measure of current regulation (and limits the initial inrush current as a byproduct) for the filaments of two tubes: V505 (BFO) on the IF subchassis and V701 (VFO). These tubes each expect 6.3 V on their filaments. Since they are wired in series, they need 12.6 v. The RT-510 is fed from the 25.2 vac line, so there must be a 12.6 v drop across RT-510. I've read here on the list that ballast tubes are not so rare as they once seemed to be, so it may make sense to replace the bad ballast. If not... I see two main ways to operate with no ballast tube: Plan 1. No special filament regulation. In my experience, houses and businesses with modern electrical service have very stable primary AC voltage. If you are going to use the R-390A in such an environment, then special regulation is not required. In this case, I would change the tubes from series wiring to parallel, and feed them from the 6.3 VAC line used for the other tubes. To do this, you could remove RT-510, disconnect the wire from pin 7 of the socket for RT 510 and move it to chassis ground. This puts the BFO tube V505 and VFO tube V701 in parallel. Then connect a new wire from pin 3 of the BFO tube V505 to the 6.3 VAC line which is available on any of the other IF tubes' pin 4 or J512-pin20. I'd get it from pin 4 of a nearby tube. RT-510 socket BFO VFO 2 7 4 3 J512-19 J709-H 4 3 25.2vac >---X X ----X V505 X-----X-----------X-----X V701 X---Gnd J512-8 | | | | Gnd | X V50X X 4 disconnect wire on add new wire pin 7 from pin 3 of V505 and connect to 6.3 v on pin 4 it to gnd. of another tube V50X on IF chassis Plan 2. Solid-state filament regulation using 7812 IC regulator. If you feel you need extra-stable R-390A performance, or will be using it on an unstable primary AC power source, you can build a three-terminal regulator into the IF chassis in place of the ballast tube RT-510. The first step is to rectify the 25.vac to DC... connect the anode of a diode such as a 1N4007 to pin 2 of the RT-510 socket. Connect the cathode of the diode to an unused pin of the socket. Connect an electrolytic capacitor with a value something like 100 uF at 50v between the cathode of the diode and chassis ground to smooth the pulsing DC. Mount the 7812 three-terminal regulator on a nearby chassis surface (no need to insulate it - the tab can go to ground) and connect the left pin (1) to the junction of the new diode and cap. Connect right lead (pin 3 of the 7812 - the output lead) to pin 7 of the RT-510 socket. Connect the middle lead of the regulator to chassis ground (or just use the tba mount to make the connection for you). For added reliability, connect a 1 uf electrolytic or tantalum cap between pin 7 and ground to surpress any tendency for the regulator to oscillate. For a less intrusive mod, I suppose if you added a ground to an unused pin of the RT-510 socket you could build the regulator on an old tube base and make it a removable module. Be sure to heat sink the regulator appropriately. pin 2 of RT-510 pin 7 of RT-510 socket socket 2 ___________ 7 25.2vac >---X---Diode---X---| 7812 | -------X------->to V505 BFO from | | | | pin 4 J512-8 +| ----------- +| 100 uF - | - 1 uF @ 50v - | - @ 25v | Gnd | Gnd Gnd In both cases, I would bend the shield ring around the RT-510 socket inward on the upper side of the chassis so that you can't accidentally plug a tube into the socket later on. And perhaps note the change using an extra-fine "sharpie" marker on the chassis surface nearby. Again, it may be wiser to replace the ballast tube, but this gives you some options...I'd probably do Plan 1. Good Luck! Steve WD8DAS sbjohnston@aol.com http://www.qsl.net/wd8das ------------------------------------ Radio is your best entertainment value. ------------------------------------