HP 8970B, Service Manual, Page: 27 / 40

HP 8970B Option 020 Service Manual Supplement 5-3
Fault-Finding Tips
Input Assembly (08970-60097 or 08970-60125)
Input Assembly (08970-60097 or 08970-60125)
Figure 5-1: Input Assembly
The easiest way to determine any problems with this section is to use a signal generator
or sweeper which can cover the frequency range 10 MHz to 2047 MHz and can output -20
dBm.
The main source of problem with this assembly is that the pads are not switching or the
amplifier has been blown up. The common error when this is the case is error 27. The
philosophy of repair for this assembly is board level repair so these diagnostics are
intended to prove that the assembly is defective or not and replace it.
From the functionality viewpoint it is possible to check that pads and amplifier are
working individually by using Special 63.X.
To verify the pads and amplifiers of with the 8970B completely boxed up use the
procedure “RF Attenuator Checks” in the Service Sheet BD1 section of the standard
8970B manual. The voltages will be approximately correct.
It is also possible to measure the pads switching with the input section removed.
Using a 30 MHz signal at -20 dBm measure at J2 using a spectrum analyzer.
1. Select Special 63.0 and take a reference on the Spectrum Analyzer (straight
through).
2. Select Special 63.1. The level should go down by ~10 dB from 1 (PAD 1).
3. Select Special 63.2. The level should increase by ~20 dB from 1 (AMP).
4. Select Special 63.4. The level should decrease by ~10 dB from 1 (PAD 2).
5. Select Special 63.5. The level should decrease by ~10 dB from 1 (PAD 3).
If it is necessary to look for power holes in any of the components then sweep the input
and monitor the output over the desired frequency range.
The detector can be checked by going through the detector check as indicated in “Input
power detector Gain and Offset Adjustment”, on page 3-2.
10 dB
PAD 20 dB
AMP 10 dB
PAD 10 dB
PAD
Detector
3 GHz
LPF
2dB
PAD
PAD 1 20 dB
Amp PAD 2 PAD 3
Limiter
Limiter (on 08970-60125
Input Assembly only).