Sparton Equasonne Radios

930 Equasonne RF Amplifier

The RF amplifier of the Sparton Equasonne is contained in a black metal box located in front of the power converter and beside the selector unit.  It houses six 484 or 485 tubes.  Five of the six tubes serve as low gain amplifiers for the radio signal, while the final tube serves as the detector.  The Equasonne differs from more 'conventional' TRF radios, in that the 5 stages of amplification used are 'un-tuned'.  All tuning is performed beforehand by 4 ganged tuned circuits inside the selector unit.

Of the 3 modules of which this radio is comprised, the RF Amplifier will most likely require the most effort to restore.

The RF Amplifier of most Sparton Equasonne models appear to be very similar.  However that of the 930 does differ slightly from the more common type found in the 301, 589, 931 etc.

930 RF amplifier circuit board (Top view)

RF amplifier circuit board removed from its shield can.  The tubes have been removed from sockets for clarity.  The connector cable to the terminal strip of the power supply extends off to the right.  See schematics for values of labeled components.  The four black paper capacitors (C5-C8) have been emptied out and rebuilt using modern metalized polypropylene capacitors.

 

 

 

930 RF amplifier circuit board (Bottom view)

The underside of the RF Amplifier circuit board.  The main features are numerous coils on bakelite or wood spools and 4 capacitors and a resistor.  The paper capacitors (C9 and C10) have been hollowed out with a drill, and modern replacements have been inserted inside them.  This repair is virtually invisible.  The two picofarad value capacitors (C1, C3) are more durable than their paper companions, and are original.  The resistance of the single resistor (R1) should be checked with an ohmmeter, this can be done without removing it from the circuit board.  Its value should be 2800 ohms.  The value of this resistor seems particularly prone to drift with age.  Coil L8 is also prone to failure.  Measure the resistance between its two ends (about 260 ohms) to ensure continuity.

931 Equasonne RF Amplifier

Of the 3 modules of which this radio is comprised, the RF Amplifier will most likely require the most effort to restore.  A 931 Equasonne chassis is shown below, the RF amplifier module is located at the front right, it has a removable lid with 8 holes for ventilation.

The RF Amplifier of most Sparton Equasonne models are very similar.  Models 103, 111A, 235, 301, 301A, 564, 570, 578, 589, 600, 610, 620, 737, 740, 750, 870 and 931 have identical amplifiers.  Models 930 and 89A have a slightly different RF Amp. circuit, but is interchangable/compatible with other Models.  Some models have the modules painted gold, in others, they are unpainted, and are made of copper or steel.

931 RF amplifier circuit board (Top view)

RF amplifier circuit board removed from its shield can.  The tubes have been removed from the sockets for clarity.  The connector cable to the terminal strip of the power supply extends off to the right.  See schematics for values of labeled components.  The four black paper capacitors (C5-C8) have been emptied out and rebuilt using modern capacitors.

 

Testing coils

The coils in the RF amp are prone to opening up. It is unusual to encounter a set that does not have at least one bad coil. Test each winding with an ohmmeter. Open windings are normally due to issues at the coil terminals. About 50% of the time, carefully heating up the terminal with a soldering iron will reattach the wire and fix the issue. If the issue is not fixed by this procedure, carefully remove the coil and inspect it with a magnifying glass. Often the broken wire connection to the terminal can be located, and then fixed.  Coil L8 is especially prone to opening. If L8 ohms out bad, remove It from the board, then very carefully unwrap the black cloth electrical tape from the coil. You will likely find both ends of the coil winding have actually broken loose.  Unwind about 1.5 inches of wire at each end, scrape it clean, then carefully tin it, and then resolder them back to the metal ends of the coil form, then glue the cloth tape back in place. As an aid I have created a diagram showing the resistances that should be measured on all the coil windings of a 931 type RF amplifier board. Coils L2 and L6 are located on the upper side of the board, all other coils are on the lower side. Ohmmeter readings off-scale or infinite, instead of near to the values shown below indicate an open winding. Expect your coils to measure somewhere within the vicinity of what is shown.

Positioning tubes in the RF Amplifier

Sparton recommended positioning the tubes in a particular order (strongest to weakest) in order to maximize performance.

 

 

930 RF amplifier circuit board (Bottom view)

The underside of the RF Amplifier circuit board.  The main features are numerous coils on bakelite or wood spools and 4 capacitors and a resistor.  The paper capacitors (C9 and C10) have been hollowed out with a drill, and modern replacements have been inserted inside them.  This repair is virtually invisible.  The two picofarad value capacitors (C1, C3) are more durable than their paper companions, and are original.  The resistance of the single resistor (R1) should be checked with an ohmmeter, this can be done without removing it from the circuit board.  Its value should be 2800 ohms.  The value of this resistor seems particularly prone to drift with age.  Coil L8 is also prone to failure.  Measure the resistance between its two ends to ensure continuity.