A solid state replacement for the 3TF7 current regulator of the R

A solid state replacement for the 3TF7 current regulator
of the R-390 and R-390A receiver
Dr. Kurt Schmid, DH3PJ, [email protected]
There are two VFOs in the R-390(A), i.e. the BFO and the PTO. Frequency stability among other things is heavily
influenced by supply voltages. Collins engineers spent a regulated power supply for B+ voltages (+150 VDC) for
both VFO tubes. For the stabilization of the tube heating current regulator 3TF7 is used.
Whereas voltage stabilization of B+ in both receivers is very effective regulation of the filament current using
the 3TF7 shows poor results. Experiments of Dallas Lankford demonstrated that stabilization of the filament
voltage by far is the better alternative. The 3TF7 substitute of Dallas required the 12 Volt regulator to be
mounted on a large heat sink because considerable heat (comparable to the amount the 3TF7 dissipates) is
generated. This fact did not allow the construction of a full plug-in stand-alone device.
Switching DC-DC down converters can reduce heat generation drastically. The most recent devices are tiny
enough to build a regulator fitting into a housing of a size comparable to the 3TF7. In contrast to the original
3TF7 (dissipating around 10 W) the replacement device runs very cool (around 1 Watt, only). It is tested to
work in the R-390 non-A as well as in the R-390A.
Fig. 1 3TF7 and 3TF7 replacement side by side
It’s already visually evident that the all silicon 3TF7 module is more robust than the fragile original 3TF7.
Fig. 2
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Internal wiring diagram of the base of the 3TF7 replacement
Pin 1, 9, Top Stud & Case
Pin 2 & 3
Pin 5
Pin 7 & 8
GND
25.2 VAC
Internal test point: + 36-39 VDC
+12VDC ( 300 mA)
Fig. 3
There is one important thing to mention. In contrast to the original 3TF7 the 3TF7 replacement needs ground
potential. Grounding the device can be achieved in three different ways:
a) Using a grounding tube shield, e.g. IERC, W.P.M. or nickel plated with top spring (e.g. EBY)
b) Preferred method: connect pin 1 or 9 (or both) of the socket via a short wire to a nearby grounding tab
under the IF-chassis
c) Grounding via the threaded stud on top of the device (last resort)
Despite simple, method a) works very reliable because there is no current flow from the
device to ground.
Replacement procedure

Switch the receiver OFF
 Replace the 3TF7 with the replacement module
 Mount the tube shield (to get ground potential from the chassis)
 DONE
In case you run the module without proper grounding there is an internal circuitry that powers the device
safely down.
Warning: Always switch the receiver OFF before inserting or
retracting the device!
Caution: Recently a customer reported that in his IF chassis the wiring of the 3TF7 tube
socket had been modified by a former owner. Depending on the change a
catastrophic failure may occur. In an unmodified chassis only pin 2 & 7 of the
socket are wired.
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Fig. 4 IF chassis equipped with the 3TF7 replacement using a medium height IERC
tube shield (TRN6-6020B)
If for any reason you have removed the IF-chassis from the receiver take the opportunity to solder a wire from
pin 1 or pin 9 of the tube socket (RT510) to nearby ground (= method b). This is the preferred method. In this
case you are no longer obliged to mount a tube shield because the 3TF7 replacement needs no cooling.
The following two photos show the modification procedure.
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Fig. 5 Underneath of an unmodified IF-chassis. The bellow of the BFO (cylinder on the
left-hand side) is temporarily removed.
Fig. 6 Modified IF-chassis. A wire connects pin #1 and pin #9 of the 3TF7 socket with a
nearby ground lug.
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Measurements of power line voltage variation on frequency output of the R-390A
PTO (70H-12)
The effects of power supply variations on frequency output of the Collins PTO (70H-12) using either the 3TF7
or the 3TF7 replacement are compared.
The line input of the R-390A receiver was connected to a VARIAC that could be switched from 230 VAC (normal
European line voltage) to 210 VAC. With a chart recorder (Gould TA 240) the line voltage, the regulated +150
VDC, and the output frequency of the PTO were registered. The latter one was measured with a frequency
counter connected to a digital-to-analogue converter (1 Hz resolution). PTO frequency was set to about
midrange.
Measurements confirmed that the regulated +150 VDC is stabilized perfectly. In the R-390A where all
measurements were taken B+ remained constant at +149.6 VDC independent of line voltage variations from
230 VAC to 210 VAC and back to 230 VAC. Therefore this trace is not included in the following chart.
Fig. 7
Effects of power line voltage changes on PTO frequency
Traces A & B: using original 3TF7
Traces C & D: with 3TF7 replacement module
Traces A & B show results with the original 3TF7 current regulator. Traces C & D show results with the 3TF7
replacement inserted into socket RT510 of the IF chassis. Traces B & D show the line voltage (lower part partly
suppressed). The VARIAC was switched to low voltage (210 VAC) for 1.25 min.
When the original 3TF7 was in place decrease of the line voltage was followed by a delayed increase in PTO
frequency (A). Maximum increase was about 12 Hz. When the 3TF7 was replaced by the module (C) and the
line voltage was decreased no frequency change was observable.
Conclusion
Power line voltage variations induced changes in output frequency of the 70H-12 PTO do not
result from B+ changes but from heater current changes. These results confirm the early findings
of Dallas Lankford (and others).
A COSMOS manufactured PTO (type 136-1) showed similar results. Result of the effects on the BFO on request.
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