HEWLETT PACKARD (HP) 686C
Service Manual

Model 686C 2) Check Magnet Regulator action by dropping line voltage to 103 volts. Current should not vary more than 0.025 ampere. 3) Repeat with line voltage at 127 volts. 4) Use an isolatedac voltmeter to check ripple voltage. See CAUTION below. Connect voltmeter to upper end of R31 (white wire) and right hand end of R27 (blue wire). The hum o r ripple voltage across the magnet must be less than 100 millivolts with the line voltage varied over the range of 103 to 127 volts. CAUTION

Section IV Paragraphs 4-8 to 4-10 5) Connect the @ Model 400D o r 403A a c voltmeter between the -150 volt bus and ground. The ripple voltage must be less than 3 mv over the range of 103 to 127 volts line. 6) Poor regulation and/or ripple are usually caused by weak tubes. Check by substituting for V107, V108, o r V109.
4-9. 300 V O L T REGULATED SUPPLY.

This supply uses the -150 volt bus for a reference voltage. Therefore the -150volt supply must be working perfectly and be accurately adjusted before the +300 volt supply is adjusted.

1) Connect the 412A between chassis and +300 volts

a voltageThe magnet power supply uses doubler circuit directly connectedto the power line. When the power cord is inserted in a polarized three-wire outlet that is properly wired, approximately 150 volts dc and 6.3 volts a c appear between electrical ground and the end of R31 to which the white wires are connected. I the line cord is reversed, apf proximately 122 volts ac as well as the 150 volts dc will be present on the white lead which may give false ripple voltage measurements.
Check cord polarity by measuring the voltage between the white-wire end of R31 and ground. U s e a multimeter type ac voltmeter with a dc blocking capacitor in one lead; you should measure 6.3 volts a c and not 120 volts ac. When checking the ripple voltage, the voltmeter case must be isolated from ground. Temporarily disconnect the voltmeter NEMA ground connector by using a two-prong adapter with the pigtail floating. Do not touch the voltmeter cabinet while making. this check. Poor regulation and excessive ripple are almost always due to weak tubes (V1 to V5) or noisy neon lamps (V6 through V11). First replace tubes, then t r y replacing the neon lamps. In the case where a neon lamp is noisy a good method for determining which lamp is at fault is to shunt a 470,000 ohm, 1/2 watt resistor across each lamp while watching for ripple indication on the 400D. If ripple decreases with the addition of the 470,000 shunt resistor, change that particular neon lamp. 4-8. -150 V O L T REGULATED SUPPLY.

at r e d wire, V106 Pin 1. (See figure 4-5).
2) Adjust the voltage to exactly +300 volts with R141. 3) Check the regulation by slowly varying the line voltage down to 103 volts and up to 127 volts. There should be no visible change on the 412A voltmeter. 4) Check the ripple voltage with the 400D o r 403A voltmeter. The ripple should read less than 10 mv over a line voltage range of 103 to 127 volts. 5) Recheck -150 volt supply and reset if necessary, then reset +300 volt supply.
4-10. HELIX SUPPLY.

1) Check the Helix Regulator by measuring the voltage from VlOl pin 1 to ground. U s e the 410B dc vtvm with 1OO:l multiplier probe. The voltage will vary from +400 volts when the FREQUENCY dial is at the low frequency end to +2,000 volts when the FREQUENCY dial is at the high end.

2) Adjust the FREQUENCY dial until the voltage is 1, 000 volts. Slowly vary the line voltage from 103 to 127 volts. There should be no visible change on the voltmeter. 3) Reduce linevoltage to 103 volts. Measurevoltage volts (FREQUENCYdial at high end).Set Helix to 2,000 on collector (pin 7 of V102). The voltage must be greater than 2,050 volts. I not, replace V102. f 4) Raise line voltage to 127 volts. Set Helix voltage to 400 volts (FREQUENCY dial at low end). Measure voltage on collector (pin 7 of V102). The voltage must be less than 2,600 volts.
5) TURN THE SWEEP OSCILLATOR POWER OFF.

1) Connect the 412A voltmeter from chassis to -150 volts at R129 (150K white wire). (See figure 4-5.) Adjust power line voltage to 115 volts.
2) Adjust R148 for exactly -150 volts dc. 3) Drop linevoltage to 103 volts and watch for change in -150 volt level. There should be no visible change. 4) Raise line voltage to 127 volts and watch for a change. None should be noticeable on the @ Model 412A. 00034-1

Disconnect the bwo tube high voltage connector. Connect four 100,000 ohm, 1 watt resistors in series and connect them from the bwo tube side of overload relay K103 to ground. Rotate the FREQUENCY dial to the low end. Rotate the CATHODE CURRENT control full counterclockwise. 6) Turn on the equipment and measure the voltage across the four lOOK resistors. With 400 volts, 1 ma will flow through them. 4-7