4-23
Problem
Possible Cause
Incorrect monopolar output
Bad calibration – Recalibrate
Ensure the load resistor is correct when checking output power.
Bad voltage or current feedback. Check:
•
A7 RF_MP_VSN, monopolar voltage sense
•
A7 RF_MP_ISN, monopolar current sense
•
These signals should match the Monitor voltage and current sense,
MRF_MP_VSN and MRF_MP_ISN, respectively.
Incorrect bipolar output
Bad calibration – Recalibrate
Ensure the load resistor is correct when checking output power.
Bad voltage or current feedback. Check:
•
A5 RF_BP_VSN, bipolar voltage sense
•
A5 RF_BP_ISN, bipolar current sense
•
These signals should match the Monitor voltage and current sense,
MRF_BP_VSN and MRF_BP_ISN, respectively.
4.12.1 HVPS Troubleshooting Hints
• Danger: Use an isolation transformer when
working on Mains side! A line on the printed
circuit board and on the schematic identifies the
Mains side of the HVPS.
WARNING: LOSS OF POWER SUPPLY
ISOLATION CAN CAUSE ELECTRICAL
SHOCK. WHEN SERVICING THE
HIGH VOLTAGE POWER SUPPLY,
ASSUME INTERNAL ISOLATION IS
COMPROMISED UNTIL VERIFIED
OTHERWISE.
• For best access for HVPS measurements, place
both the Low Voltage Power Supply and the
Transformer Boards in their service positions;
that is, mounted vertically in the appropri-
ate heatsink mounting slots. See mechanical
access section for details.
• Make sure the LED for the circuit you’re
working on is OFF when making connections
or touching circuitry.
•Observe both LEDs during POST. The PFC
LED should always be on when power is on.
It will become momentarily brighter dur-
ing POST. The Forward Converter LED will
normally be off unless it has been recently
activated. After activation it will slowly fade
to off. During POST, it should turn on dimly,
then brightly, then fade. The brightness of
both LEDs corresponds to the output voltage
of the corresponding circuit.
• Check Line Voltage input, HVSET, M-
HVEN, PFC_EN, and +15, +5, GND con-
nections for proper operation before replacing
parts.
• If LED2 does not light during POST, unplug
the RF Amplifier and cycle power – there
might be a short on the RF Amplifier.
• If fuses are blown, Q3 is probably shorted.
Also check surrounding components for fail-
ure if Q3 is found to be bad.
• If the Forward Converter output will not
exceed about 70Vdc, the PFC output voltage
is probably too low. (This limit will vary with
input line voltage.)
• The Forward Converter gate pulse width
should be close to 50% with no input voltage
while HVSET is low.
•If the PFC output voltage is changing with
heavy load, try increasing mains voltage with
a variable transformer. If this stabilizes the
PFC output voltage, PFC current sense cir-
cuitry is probably at fault. This is not gener-
ally a problem with 220V inputs.
• The PFC can be disabled for safer low volt
-
age Forward Converter troubleshooting – by
shorting TP11(PFC_EN) to TP4(-HV). Use
a variable transformer to set the desired input
voltage to the Forward Converter.
• Check the clock frequency and reference volt
-
age on both the PFC controller and Forward
Converter controller integrated circuits. The
PFC clock must have PFC enabled to func-
tion correctly.
• If the output voltage for either PFC or
Forward Converter is too high, check the
feedback resistors.
