Schematic Diagram of Boonton 260 Q Meter. The instrument is constructed in modular form with Synch-Jones and coaxial cables joining the modules. The schematic is drawn in accordance with this method of construction. It will be described in this same manner.

The power supply.

A three wire power cord is shown connected to J403. The bottom pin of J403 connects to pin 1 of J402. The middle pin of J403 connects to chassis ground. The top pin of J403 connects to one end of a fuse, F401. A note says for 96 to 130 volts use 1 amp, for 190 to 260 volts use 1/2 amp. The other end of the fuse connects to the on/off switch, S401 ganged with R405, single pole single throw. The other end of S401 goes to pin 2 of J402. Pins 1 and 2 of P402 go to the input, pins 1 and 2, of V R 401, line voltage stabilizer. The output of V R 401, pins 3 and 4, go to pins 3 and 4 of P402. The power transformer has a dual primary for 115/230 volt connection. It is shown wired for 115 volts. Pin 4 of J402 goes to the bottom ends of both primaries. Pin 3 connects to the top of both primaries. The power transformer, T401, has 4 secondary windings. Winding voltages are given but currents are not given. The top most winding is 6.0 volts and the wire colors are both green. The bottom end goes to chassis ground. The top end goes to pin 3 of J401. The next winding is 2.25 volts and the wire colors are both brown. The bottom end goes to chassis ground. The top goes through a shielded wiring harness to the RF voltmeter. The voltmeter circuit will be described in a section of its own. The next winding is the B plus winding. It is a center tapped winding of 372 - 0 - 372 volts. The center tap, red-yellow, is connected to chassis ground. The top end, red, goes to pin 1, plate, of V401, 6X4. The bottom end, red, goes to pin 6, plate, of V401. The final winding is 6.3 V and the colors are yellow. The top goes to one end of the power indicator, I401 and to pin 4, heater, of V401. The bottom end goes to the other end of I401 and to pin 3, heater, of V401. There is no connection shown between the V401 heater supply and chassis or any other part of the circuit. The cathode of V401, pin 7 goes to one end of L401, 10 henry filter choke. The other end goes to the positive side of C401 40 u f capacitor. This point has many connections so we will give it the name B plus one. The other end of the capacitor is grounded. B plus one connects to pin 1 of J401. B plus one goes to one end of a 4.7 k ohm resistor R 401. The other end goes to pin pins 1 and 5 of V402, zero B two. This point is labeled 258 volts. The 258 volt point goes into the shielded wiring harness to the RF voltmeter. Pins 2, 4, and 7, of the zero B two go to pins 1 and 5 of v 403 zero EH 2. This point is labeled 150 volts. Pins 2, 4, and 7, of the zero eh two are grounded. The 150 volt point goes to R305 in the RF voltmeter circuit. The 150 volt point also goes to one end of a resistor made up of two resistors in parallel. The other end of this resistor goes to B plus one. The two resistors in parallel are R402 and R403 both 33 k ohm. This completes the power supply circuit. In order to help you understand the circuitry when other portions of the Q meter are described the voltages that are being applied from various pins of the plugs will be identified by more than just pin numbers.

RF Voltmeter and Measuring Circuit.

Unlike the simplified circuits above the measurement terminals are drawn upside down. The two coil terminals are on the left and the two capacitor terminals are on the right. The coil low terminal is above the high terminal. The capacitor low terminal is above the high terminal. The signal from the oscillator comes in through a coaxial cable through P201 and J201. The shield is grounded and the center conductor connects to one end of the 0.3 ohm hot wire resistor. The other end of the 0.3 ohm resistor goes through the 0.02 ohm resistor to ground. The junction of the 0.3 and 0.02 ohm resistors connects to the coil low terminal. The thermocouple which is in contact with the hot wire resistor has its upper end connected to one end of a resistor labeled R203. The other end of the resistor goes to ground through a 0.01 u f capacitor. The other end is grounded. The junction of the resistor and capacitor go to the positive side of a meter movement labeled multiply Q by. Similarly the lower end of the thermocouple goes through R204 then a 0.01 u f capacitor to ground. The R and C junction goes to the other side of the meter. The values of R203 and R204 are factory selected to match the meter and thermocouple. The capacitor low terminal is grounded. The coil high and capacitor high terminals are connected together and go to grid cap of V301. This tube is labeled on the schematic as 535-eh but you need to see the text about this tube. The grid goes through a resistor R201 100 meg ohm to ground. The grid also goes through two variable capacitors in parallel to ground. They are C201eh and C201B. they are not ganged but each is rotated by its own knob. The plate of the tube, pin 2, is bypassed to ground by a 0.1 u f capacitor. Pin 2 also goes to one end of a 1 k ohm resistor. The other end goes into the shielded wiring harness to the 258 volt point. Pin 1 of the heater is grounded and pin 6 goes into the wiring harness to the 2.25 volt winding of the power transformer. The cathode, pin 5 goes through the parallel combination of a 22 k ohm resistor and 0.1 u f capacitor to ground. The cathode also goes through a 3500 ohm resistor to the positive terminal of a meter movement labeled circuit Q.

The meter mode switch.

The negative terminal of the meter goes into a rather complicated circuit of switches, pots and fixed resistors. There are two switches, Delta Q and low Q. The delta Q switch is shown in the down position and the low Q switch is shown in the up position. These two switches are not independent but are controlled by a single spring loaded lever on the front panel. It is assumed that the position shown is with the lever in the natural position. Each position of the switches will be described as if it were a standalone circuit.

Normal Q position.

The negative terminal of the meter goes to the swinger of the delta Q switch. This switch is in the lower position, touching the lower contact. The lower contact is connected to the swinger of the low Q switch. This swinger is in the up position touching the upper contact. This contact is connected to one end of a 39 k ohm resistor. The other end of this resistor goes through a 10 k ohm rheostat, R 310, to the wiper of the zero adjust pot 200 ohm. The bottom of the zero pot goes through a 750 ohm rheostat R312 to ground. The top of the zero pot goes through two ganged counter posing rheostats which will have no effect then through a 22 k ohm resistor to the 150 volt point. The wiper of the zero pot also goes through a couple of resistors to the upper contact of the delta Q switch which is in the open position. So this is a dead end path. Note: R 310 and R312 are calibration adjustments.

Low Q position.

The delta Q switch is still down but the low Q switch is now also down. The negative end of the meter goes to the swinger of the delta Q switch which is touching the lower contact. This contact is connected to the swinger of the low Q switch. The swinger is touching the lower contact. This contact is connected to one end of a 3 k ohm rheostat, R308. The other end goes to the wiper of the zero pot, 200 ohm. The top of the zero pot goes through two ganged counter posing rheostats which will have no effect then through a 22 k ohm resistor to the 150 volt point. The bottom of the zero pot goes through a 750 ohm rheostat R312 to ground. The zero pot is wired exactly as in the normal Q position above. Note: R308, and R312, are calibration adjustments.

Delta Q position.

This position is intended for detecting small changes in the Q of a coil for example a change brought on by sliding a plastic coil form into an air wound coil. The delta Q switch is now in the up position touching the upper contact. The low Q switch is still in the up position. This presents open circuits on both paths from the wiper to the low Q switch. The negative terminal of the meter goes to the swinger of the delta Q switch. The swinger is touching the upper contact. This contact connects to one end of a 1300 ohm resistor. The other end goes to a 3 k ohm rheostat, R306, to the wiper of the delta Q balance control 1500 ohms. The upper end of the balance pot goes to the wiper of a 50 ohm pot. The lower end of the balance pot goes to the wiper of another 50 ohm pot. The two 50 ohm pots are ganged. The lower end of the upper 50 ohm pot and the upper end of the lower 50 ohm pot are not, that's right, not, connected together or to anything else. The upper end of the upper 50 ohm pot connects through a 22 k ohm resistor to the 150 volt point. The lower end of the lower 50 ohm pot goes through the 200 ohm Q zero pot then the 750 ohm rheostat, R312 to ground. The wiper of the zero pot has been disconnected by the delta Q switch so it is acting only as a resistor. The two ganged 50 ohm pots are the two counter posing rheostats mentioned under the heading normal Q position.

Oscillator and Band Switch.

This is without a doubt the most complex part of the Q meter due to the band switching turret. P401 plugs into J401 to send power to the oscillator circuit. Pin 1 is B plus. Pin 2 of J401 goes to the top pot in the XQ coarse control, dual ganged 40 k ohm. The bottom of the top pot goes to the top of the bottom pot. The bottom of the bottom pot goes to ground. The wiper of the top pot goes to the top of the XQ fine pot, 3 k ohm. The bottom of the XQ fine pot goes to the wiper of the bottom pot in the XQ coarse control. The wiper of the XQ fine control goes to pin 6 of j401. Pin 3 of J401 goes to the 6.0 volt heater winding on the power transformer. The other side of this winding is grounded. Pin 4 is not used. Pin 5 goes to ground. You don't have to memorize this plug. Reminders will be given as the oscillator circuit is described. Within the oscillator sub assembly the oscillator tube is V101, 5763. Pins 3, suppressor grid, 4, one side of heater, and 7, cathode, are all grounded. The wire from pin 5, other side of heater, passes through a 1500 pf feed through capacitor then a wiring harness to pin 3 of P401, 6.0 volt heater supply. Pin 6, screen grid, is bypassed to ground by a 0.01 u f capacitor. Pin 6 also goes through a 470 ohm resistor then a 1500 pf feed through capacitor and harness to pin 6 of P401, wiper of XQ fine control. The plate of V101, pin 1, is connected through a trimmer capacitor, C129, 1.8,8.6 pf, to ground. Now we must face the band switch. Some abbreviation will save time. For example pin 3 on the stator will be indicated as SP3. Pin 3 on range 6 of the turret rotor will be indicated as RP3-6. S stands for stator and R for rotor. P stands for pin. On the rotor the order will be pin, band. In the schematic the turret is shown on band 5, 1.7 to 4.2 MC. The plate of the tube connects to SP4, that's pin 4 of the stator. On the rotor RP4-5 connects to one side of the plate coil. That's pin 4 of range 5 on the rotor. One section of the main frequency tuning capacitor connects to SP3. The other side of this capacitor is grounded. The other section connects to SP5 but the rotor has no connection to this pin on this band. The rotor connects to SP5 on bands 1, 2, 7, and 8. Pins R4-5 and R3-5 are tied together. RP4-5 connects through a 1 to 8 pf trimmer to ground. The grid of the tube, pins 8 and 9, goes through a 100 k ohm resistor to ground. The grid also goes through a 33 ohm resistor to SP6. RP6-5 goes through a 68 pf capacitor then to one end of the grid coil which has a 1500 ohm resistor in parallel with it. The other end of the coil and resistor go to RP7-5. SP7 is grounded. The two coils are magnetically coupled. The output coil which is also magnetically coupled connects between RP2-5 and RP1-5. SP2 is grounded. SP1 goes to the center of the coaxial cable that carries the oscillator signal to the hot wire resistor and thermocouple. SP2-5 goes through a 68 k ohm resistor to SP84. There is also a 10 k ohm resistor connected between RP8-5 and RP2-6. These resistors are used to apply different voltages to the top of the XQ coarse pot for different ranges. Pin 1 of P401 - J401 carries unregulated B plus through the harness and a 1500 pf feed through capacitor to one end of a 220 ohm resistor in the oscillator. The other end of this resistor is bypassed to ground through a 0.1 u f capacitor. The bypassed end of this resistor connects to stator pin 8 on the band in use. On the turret the resistor associated with pin 8 sends current to pin 2 of the band just above the one in use. The stator pin that is in contact with this pin sends current through a 100 ohm resistor then a 1500 pf feed through capacitor and the wiring harness to the top of the XQ coarse control. This is how the appropriate voltage is applied to the XQ control so the oscillator amplitude will not change drastically when the band switch is changed. The stator has contacts that ground pins 4 and 6 of the band just below the one in use, in this case band 4.

Turret wiring.

Band 1.

1, hot end of output coil.
2, cold end of output coil.
3, right end of plate coil, and 1 to 8 pf trimmer, and 47 pf cap.
4, center tap of plate coil.
5, tied to 3.
6, through 250 pf cap and 68 ohm resistor to right end of grid coil. Coil has 1000 ohm resistor in parallel.
7, left end of grid coil and other end of 47 pf cap.
8, left end of plate coil and through a 150 k ohm resistor to pin 2 of band 2.

band 2

1, hot end of output coil.
2, cold end of output coil.
3, right end of plate coil, 1 to 12 pf trimmer, and 33 pf cap.
4, center tap of plate coil.
5, tied to 3.
6, through 250 pf cap and 680 ohm resistor to right end of grid coil. Coil has 1000 ohm resistor in parallel.
7, left end of grid coil and other end of 33 pf cap.
8, left end of plate coil and through a 68 k ohm resistor to pin 2 of band 3.

Band 3

1, hot end of output coil.
2, cold end of output coil.
3, right end of plate coil, 1 to 12 pf trimmer, and 22 pf cap.
4, center tap of plate coil.
5, not used.
6, through 250 pf cap and 680 ohm resistor to right end of grid coil. Coil has 1000 ohm resistor in parallel.
7, left end of grid coil and other end of 22 pf cap.
8, left end of plate coil and through a 68 k ohm resistor to pin 2 of band 4.

Band 4

1, hot end of output coil.
2, cold end of output coil.
3, right end of plate coil, 1 to 12 pf trimmer, and 5 pf cap.
4, tied to 3.
5, not used.
6, through 100 pf cap to right end of grid coil. Coil has 1500 ohm resistor in parallel.
7, left end of grid coil and other end of 5 pf cap.
8, left end of plate coil and through a 68 k ohm resistor to pin 2 of band 5.

Band 5

1, hot end of output coil.
2, cold end of output coil.
3, right end of plate coil, 1 to 8 pf trimmer, and 10 pf cap.
4, tied to 3.
5, not used.
6, through 68 pf cap to right end of grid coil. Coil has 1500 ohm resistor in parallel.
7, left end of grid coil and other end of 10 pf cap.
8, left end of plate coil and through a 10 k ohm resistor to pin 2 of band 6.

Band 6

1, hot end of output coil.
2, cold end of output coil.
3, right end of plate coil, 1 to 12 pf trimmer, and 5 pf cap.
4, tied to 3.
5, not used.
6, through 47 pf cap to right end of grid coil. Coil has 1500 ohm resistor in parallel.
7, left end of grid coil and other end of 5 pf cap.
8, left end of plate coil and through a 3300 ohm resistor to pin 2 of band 7.

Band 7

1, hot end of output coil.
2, cold end of output coil.
3, not used.
4, through 50 u h choke to pin 8, and through 0.001 u f cap to pin 5.
5, to left end of plate coil, 1 to 8 pf trimmer, and other end of 0.001 u f cap.
6, through 150 k ohm resistor to 7, and through 47 pf cap to right end of grid coil .
7, left end of grid coil and right end of plate coil.
8, right end of 50 u h choke and through a 10 k ohm resistor to pin 2 of band 8.

Band 8

1, center tap of plate coil.
2, right end of plate coil, left end of grid coil, and pin 7.
3, not used.
4, through 12 u h choke to pin 8, and through 0.001 u f cap to pin 5.
5, to left end of plate coil, 1 to 12 pf trimmer in series with 0.01 uf cap to ground, , and other end of 0.001 u f cap.
6, to one end of 10 k ohm resistor also to one end of 33 pf cap. Other ends of R and C tied and go to right end of grid coil.
7, left end of grid coil, right end of plate coil, and pin 2.
8, right end of 12 u h choke and through a 3300 ohm resistor to pin 2 of band 1.
This completes the description of the Boonton Q meter schematic diagram
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