What a beast. I’ll have to find one as a project.
Peter
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On Mar 18, 2025, at 9:35?PM, David Feldman via groups.io <wb0gaz@...> wrote:
?I'm on (the other) HP 8673D repair project now. This one never worked in the time I have owned it, and I'd been storing it as a parts mule, but decided (now that I have some experience with disassembly and repair) that I'd like see if I can bring it back to life.
Primary symptom: Very low output (rarely approaching 0 dBm, at some frequency ranges much less), unleveled indicator (as the available RF would never reach the setpoint of the ALC circuit), yet never an "unlocked" indication.
Fault isolation so far: The service manual vol 2 08673-90022 (in section BD5) walks through several tests; it turns out RF level (so-called "baseband") 2.0-6.6 GHz out of YTO (then through a bias tee, modulator for pulse modulation, and a high pass filter) measures with adequate RF on all YTO frequencies (spec is >= 10 dBm; this is satisfied with about +18 dBm at bottom end of range and staying above +10 dB to the end of the range.) This signal is sent to the YTM assembly; output from YTM is very low and fluctuates widely across frequencies. The service manual details RF level tests at the high pass filter output (what goes to the power amplifier which is the start of the YTM system) and RF level tests at output of the YTM (this output is very low and uneven across frequencies, resembling output of the 8673D overall, so fault is in this area.
Further attempt at fault isolation: The YTM assembly includes a power amplifier (A1A11, part number in my (preliminary) manual is 08673-67004. The popwer amplifier in this 8673D part number is 08673-67011. The manual provides zero detail (that I can find) detailing expected output of the power amplifier that feeds the YTM itself. As that connection was SMA, I connected A1A11 power amplifier output (via attenuation) to the power meter and found ~0 dBm output (varying widely across frequencies), and after a few minutes in this configuration, A1A11 amplifier output went to zero and never returned.
Unsuccessful search for details in service manual: Attached are photos of the A1A11 amplifier and two associated PC boards, part numbers 08673-60140 (provides DC to the A1A11 power amplifier via pins E1 through E12) and 08673-60139. The DC test points on 08673-60140 (upper/larger board in photo) expect +5, -3.5, -6.5 and -9 VDC; I measured these (in system while running) at +5, -6.5, -6.5 and -9 (+/- some mV), so the -3.5V test point's voltage was incorrect.
I'm not able find schematics of either board in the (large/heavy) service manual, hence my posting here - does this ring a bell with anyone that may have encountered these modules in another context, has worked on an 8673D in this area, or has seen these mentioned in the service manual volume 2 (document number 08673-90022)?
In modern times, getting more gain and power at 2.0-6.6 GHz would not be at all difficult, however, the YTM is sensitive to the level of RF input, and it appears visually that the circuitry around the A1A11 amplifier is at least in part allowing the CPU to control output from A1A11 to keep drive within some (undocumented) acceptable level window.
Thanks for any clues!
Dave
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