Tektronix TM 5006 manual Line Selector and 60 Hk Transformer, Rectifiers and Filters

Section 3-TM 5006

THEO RY OF O PERATIO N

Introduction

For ease in understanding, this description refersto the schemati cs located in the pullout pages at the rear of this manual. Also refer to the block diagram located in the pullout pages and the timing diagram in Fig. 3-1 . Each block in the block diagram is outlined on the schematics.

The TM 5006 uses α pulse width modulated switching supply for do voltages. Α 60 Hz transformer provides the ac voltages necessary for plug-in operation. Connections to the six plug-in compartments as well the seιί es pass transistors are shown on schematic 3s .

Line Selector and 60 Hk Transformer

Ac power is applied to the voltage select terminals through FL500 and α discrete line filter composed of Τ1050, L1030 and L1020. Line transients are filtered to ground through C1120 and C1140.

The two primary windings on Τ500 are connected in parallel for 115 V operation or in series for 230 V opera- tion . Winding taps are provided for various line voltages around the nominal values. The secondaries provide ac voltages to the various plug-in compartments .

Rectifiers and Filters 0

The ac line voltage is applied through negative temperature coefficient ιesistances R T1000 and RT1200 to the rectifier diodes. 4s these resistances are highest when cold, the surge currents, charging the high voltage capacitors when line voltage is applied, are limited, thus preventing component failure. These resistors then self heat to α low resistance.

In 220 V operation thefour diodes function as α bridge rectifier. See Fig . 3-2 When the voltage-select-circuit is set for 110 V operation, onlythetwoseries diodes operate. The circuit then becomes avoltage doubler with an output of approximately 350 V dc. The neon bulb in this circuit flashes to indicate when do voltage is present.

The rectified and filtered do is applied through L1320 and C1240, α low pass filter, and passes through 81430 and CR1540, to the collector of 01640.

20 kHz Output Stage

The output stage is α half bridge type with proportional base drive. The turns ratios and phasing of T1 740 are such that only α small amount of base drive power is needed to start conduction in either 01640 or 01650. Positive feedback from Τ1740 supplies base current for the remainder of the power cycle. When both base drive transistors, 01730 and 01731, (shown on schematic

are saturated, Τ1740 is essentially shorted, teι- mi λ(ating base current for either output transistor. Output transistors 01640 and 01650 alternately conduct at α 20 kHz rate. Their on and off times are adjusted by the regulation circuitry. Diodes CR1550 and CR1551 prevent base to collector current flow iη 01640 and 01650 at turn off. The base switching action of these transistors is improved by networks C1530 and 81530 for 01640, and C1560 and R1460 for 01650. Α series resonant filter between the transistors and the output transformer, Τ1710, is composed of C1430 and L1440. During 01640 and 01650 off time, the tank current generated by L1440 and C1430 passes through CR1541 and CR1551 .

The 20 kHz output voltage is stepped down to the correct levels by Τ1710.

Three sets of full wave diode rectifiers are provided for each of the three do voltage outputs. Schottky diodes are used in the +8 V supply for reduced forwardvoltage drop . All filters are L-C pi-sections . Bleeder resistors are provided for all filter capacitors.

Control Logic and Drivers

U1840E and U1840F are inverting amplifiers. Their outputs control the base drive transistors 01730 and 01731 . Collector voltage for these transistors is applied from the 10 V bus through α center tapped winding on the base drive transformer (Τ1740) . Reverse polarities across 01730 or 01731 are prevented by CR1730 and CR1731 . When either one or both of these transistors (01730, 01731) are on, either one or both of the output transistors (01640; 01650) are off. The bases of ο1 730 and 01731 are also controlled, through R1832 and 81950, by the collec- tor of 02240. During power up or power down, the collector of 02240 goes positive. This action t urns 01730 and 01731 on to turn the output transistors off. This is necessary as the control circuitry state is undefined during power up or power down.