[R-390] 3TF7 Substitutes

[Bob Camp]

Hi

The crystals in the crystal oscillator deck probably have a 0.25 to 0.5 
ppm per degree C temperature coefficient when the radio is at normal 
room temperature. On the 10 MHz band the second crystal oscillator is 
running at 13 MHz. A one hertz drift on a 10 MHz signal would equate to 
roughly 0.08 ppm of drift on the second crystal oscillator. That would 
equate to something in the range of a degree C change every three to 
six hours. That's not a bad temperature change for a room with good 
temperature control.

	Take Care!

		Bob Camp
		KB8TQ

On Sep 24, 2004, at 9:36 AM, 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.
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