[R-390] 3TF7 Substitutes

[mikea]

On Fri, Sep 24, 2004 at 08:36:01AM -0500, Dallas Lankford wrote:
> The standard 3TF7 substitutes, (1) using a 42 or 43 ohm 10 watt
> resistor in place of the 3TF7, and (2) using an appropriate tube, like
> a 12BY7A, with a 12.6 volt filament in place of the 3TF7 aree both
> acceptable substitutes. Using a 10 MHz rubidium standard I determined,
> somewhat to my surprise, that the power resistor is generally a more
> stable substitute than a 12BY7A. Recently while examining the long
> term frequency stability of one of my R-390A's with the BFO turned
> ON (for SSB, ECSS, or CW) using a rubidium standard, I found that
> the 3TF7 does not do a very good job of stabilizing the BFO and PTO
> frequencies when the AC line is varied. A change of only 2 or 3 VAC
> in the line voltage (I used a VARIAC to vary the AC input voltage to
> the R-390A) causes a substantial (4 or 5 or 6 Hz or more) departure
> from zero beat. Next, I removed the 3TF7, inserted a 9 pin tube test
> extender into the 3TF7 socket, and powered the BFO and PTO filaments
> with an external regulated 12 VDC supply (12 VDC was found to give
> almost exactly 300 mA filament current). With this arrangement, no
> change in zero beat was observed as the AC line voltage was varied
> from 120 VAC nominal down to 100 VAC and back up to 120 VAC. WOW.
> Whoever designed the original BFO and PTO filament stabilizition
> circuit was on the right track. They just used the wrong method to
> stabilize it. Current regulation is the wrong approach; voltage
> regulation is the correct approach.

> Rather than rewire the 3TF7 socket, I opted to make the mod
> "plug-in" in so far as it was possible. I cut the metal flange off a
> miniature 9 pin ceramic tube socket, pushed 9 pieces of #18 tinned
> solid copper wire in each receptacle, soldered them, cut off the ends
> to the appropriaste length for a 9 pin tube, deburred and polished
> the tips, drilled out the cylindrical center piece of metal and
> removed it. I ground off most of the head of a 6-32 brass screw of
> the appropriate length and attached an inch long (or somewhat longer)
> insulated spacer, and mounted an insulated standoff on the threaded
> end. This provided me with a home made tube socket extender on which I
> could build most of a 12 VDC regulator. There is a nut on the front of
> the IF deck where I added a ground lug. I ran a diode from the #2 pin
> lug of the adapter to the standoff, and a 1000 mF 50 volt electrolytic
> from the standoff to the ground lug. The ground tab of a 3 pin 12
> voilt 1 amp regulator was attached to the RF deck corner nearest the
> IF deck using one of the green screws that hold the oscillator deck
> to the RF deck plate; the regulator pins stick up above the top edge
> of the RF deck plate. The input and output pins of the regulator were
> bypassed to the ground pin with 0.1 mF 50 volt capacitors, and the
> regulator ground pin was wired to the added ground lug on the IF deck.
> An insulated wire from the standoff to the regulator input pin and an
> insulated wire from the regulator output pin to lug 7 of the plug-in
> adapter completed the modification. Well almost... three (3) complete
> wraps around the lugs of the home made adapter with Scotch Glass Cloth
> Electrical Tape and heat shrink tubing on the standoff protected the
> plug-in adapter from shorts.

> This mod is not 100% plug-in because to remove it you have to (1)
> remove the nut on the front of the IF deck to remove the ground lug,
> and (2) remove the green screw on the RF deck to remove the 3 pin
> regulator.

> I have now had the mod running continuously for about 48 hours. No
> problems were expected and no problems have been experienced. Not
> only does this mod give you improved frequency stability for ECSS,
> SSB, and CW, it should also provide a permanent solution for the 3TF7
> replacement problem. There is still some very slow frequency drift,
> as much as 1 Hz per hour, sometimes more. I currently do not know the
> cause of this drift.

Thanks _very_ much for your research results. I have had the benefit 
of your expertise for some years now, and think it is appropriate to 
give you your due in public. 

I feel some construction coming on.

-- 
Mike Andrews
mikea at mikea.ath.cx
Tired old sysadmin