Field Change Notice FCN3-006
February 1, 1983
Effectivity: Chroma, Model 2103, Serial Numbers 0011 through 0354.
Reason for Change: Power Supply update modifications:
- To prevent PIV under high input line voltage conditions from damaging the analog rectifier diodes.
- To precisely adjust the pre-regulator voltage in the +5V digital supply.
- To improve feedback, increasing the current limit point and speed of the regulator in the +5V digital supply.
- (4) 1N4002, 1A, 100V, Diodes
- (1) 100 OHM, 1/4 W, 20%, Resistor
- (1) 6.8 OHM, 1/4 W, 5%, Resistor
- (1) 100 PF, 50V, 10%, Capacitor
- (1) 0.01UF, 50V, 20%, Capacitor
Parts Kit: Order Parts Kit KFC3-006.
REFER TO THE ACCOMPANYING DRAWINGS AS NECESSARY DURING THE FOLLOWING PROCEDURE!
ANALOG RECTIFIER CIRCUIT
Remove four 1N4001, 1A, 50V PIV, diodes CR6, CR7, CR8 and CR9. Replace them with 1N4002, 1A, 100 PIV diodes. CR10 does not need to be changed. Refer to Table One.
+5V DIGITAL LINEAR REGULATOR CIRCUIT
Remove C5, 0.00luf and replace it with a series network consisting of a 100 OHM, 1/4W, 20% resistor in series with a 0.01uf, 50V, 20% disc ceramic capacitor. Remove C4, 470pf and replace it with a 100pf, 50V, 10% disc ceramic capacitor. Remove R14, 9.1 OHM and replace it with a 6.8 OHM, 1/4W, 5% resistor. Refer to Table Two.
|C5||0.01uf||100 OHMs and 0.01uf (in series)|
|R14||9.1 OHMs||6.8 OHMs|
+5V PRE-REGULATOR CIRCUIT
The adjustments in the following procedure should be done on a cold supply that has been "on" for less than 20 minutes. A VARIAC is needed to control line voltage.
THE ADJUSTMENTS MUST BE MADE WITH A D.C SCOPE, NOT A METER!
- With the supply "off," remove transistor Q14, it will no longer be used.
- Remove the Overvoltage Protector Board containing Z4 from capacitor C3 and replace the wires on C3. Be sure to mark the correct polarity on the Z4 board as it will be replaced later.
- Connect a resistance substitution box across R3. Start with a setting of 100K OHMs.
THE FOLLOWING STEPS (DURING POWER ON) MUST BE DONE WITHIN 20 MINUTES.
- Using a VARIAC, set the input line voltage to 90VAC. With the Chroma on, press [Set Split], . All LEDs should illuminate and both display windows should indicate all eights and decimals. This loads the supply to maximum.
- Adjust the resistance across R3 until the minimum DIP of the pre-regulator voltage is at least 6.0 VDC. See waveform illustration.
- Press [Set Split],  to reset the Chroma, then select Program One. Thsi reduces the supply loading.
- Increase the input line voltage to 140VAC and make sure that the peak of the pre-regulator voltage is below 7.0 VDC. See waveform.
- With the power "off," remove all loads by disconnecting J1, J2, J3 and J4 at the supply.
- With the supply "on," check for HEXFET leakage. The input line voltage should be maintained at 140VAC. Leakage will cause the pre-regulator voltage to increase. Monitor the voltage creep for 20 seconds. Maximum pre-regulator voltage allowed is 7.5 VDC.
- Replace the resistance substitution box with a fixed resistor of the indted [sic?] value shunting R3. Replace the overvoltage protector, Z4 and the four supply connectors.
CAUTION: Connectors J1, J2 and J3 are mechanically but not electrically interchangeable. Be careful! If you are in doubt as to the proper connections, reer to the power supply connection diagram and go by the color of the wires.
The screws holding Z4 to C3 must be firly tightened to prevent excess ripple.
Rhodes Chroma National Service Manager