Output Amplifier; Bias Board; Output Goes Unleveled (Err 900 Or Err 901) - HP 8560A Service Manual

Output Amplifier

The output amplifier low-pass filters the signal emerging from the Output Mixer and then
amplifies it into a usable range. The amplifier also contains a detector for leveling the output.

Bias Board

The bias board contains the ALC circuitry for the tracking generator and distributes dc power
from the Al4 Frequency Control Assembly to the rest of the tracking generator. The ALC
inputs come from the Al4 Frequency Control Assembly (for controlling the power level), the
EXT ALC INPUT on the rear panel, and the detector in the Output Amplifier. The ALC
loop drives the Modulator.
The following troubleshooting information is aimed at isolating tracking-generator-related
faults to either the A10 Tracking Generator Assembly, or one of the other supporting
assemblies, such as A14, A15, or A7. The A10 Tracking Generator is not field-repairable; a
rebuilt-exchange assembly is available.

Output Goes Unleveled (ERR 900 or ERR 901)

The ADC on the A3 Interface Assembly is used to monitor the control line ALC MON (ALC
Monitor) from AlO. If ALC MON is greater than +1.091 Vdc or less than -0.545 Vdc, ERR
900 TG UNLVL will be displayed, indicating that the output of the tracking generator (or
"TG") is unleveled. The TG can typically be set for +2.8 dBm output power and remain
leveled. In any case, the output should remain leveled for output power settings of less than
the maximum leveled output power. Refer to Table l-l in the Installation and Verification
Manual for more information on this specification.
It is normal for the TG to be unleveled at frequencies below 300 kHz. If the TG output
is unleveled and the start frequency of the TG is below 300 kHz, ERR 901 TGFrqLmt
(TG Frequency Limit) and ERR 900 TG UNLVL may be displayed (refer to General
Troubleshooting for information on checking for multiple error messages). If the start
frequency is changed to be greater than 300 kHz and the output is still unleveled, ERR 900
TG UNLVL will be displayed (see above).
The ALC MON line is monitored only at the end of a sweep. For this reason, it is possible
that the output could be unleveled during a portion of a sweep, and, if the output returns to a
leveled condition by the end of the sweep, ERR 900 TG UNLVL will not be displayed.
If ERR 900 TG UNLVL is displayed, proceed as follows:
1. Check at which frequencies the output is unleveled. Set the analyzer to zero span and
step the center frequency in 50 MHz increments. Note at which frequencies the output is
unleveled.
2. Check at which power levels the output is unleveled. Connect the RF OUT 5Ofl to the
INPUT 5052. With the analyzer in zero span, set the Center Frequency to 300 MHz or
one of the frequencies noted in step 1, with the analyzer in zero span. Press [AUXCTRL),
TRACKING GENRATOR, SRC PWR ON, MORE I OF 3, TRACKING PEAK. Wait for the
"PEAKING" message to disappear. Step the TRK GEN RF POWER setting in 1 dB
increments and note at which power levels the output is unleveled. It is acceptable for the
output to be unleveled only at power levels greater than the specified maximum leveled
output power level.
RF Section 11-19