HP 85047A contains HP 5086-7489 test port coupler which grossly fails simple directivity test (terminate with known-good APC-7 50 ohm load, and almost all source drive signal appears on the coupled port save for a notch/dip at mid-span.
As the history of the test set is not known, abuse is suspected.
Visual inspection inside the RF coupler subassembly shows no evidence of physical damage to any component or connection, nor absence of any component.
Question - is there any practical path to troubleshooting the coupler at the component level (with understanding that this would obviously negate any internal factory adjustments that might have been made at time of manufacture)?
Question - can the front APC connector assembly be removed from the coupler subassembly? If so, are there any torque limits I should be aware of (if there is a coaxial connection between the external APC connector and the coupler housing, I don't wish to damage that.)
Question - is the reference coupler (5086-7488) similar to the test port coupler (5086-7489) such that I could replace the coupler subassembly with a 5086-7488 (and ideally, transfer the external APC connector from the failed 5086-7489)?
Thanks very much for any advice!
So, I designed that coupler (actually a bridge) back in 1986. We'll see how good my memory is 33 years later.
you can test continuity from the bias tee to the output, it should be DC coupled. You can test continuity from the coupled port to ground, it should be 50 ohms.
If the bridge is over powered, then the R1 resistor or the R2 resistor can open up. There is a picture of the bridge on page 82 of my book and a representative schematic on page 81. If you open the bridge you can probe the R1 resistor (from the center conductor to the plate on the mono-block capacitor) and it should be 265 ohm. Occasionally the R3 resistor can fail for over power, and if that happens it is very difficult to repair. The R3 is under the coax outer conductor and sits in a pocket on a second small thin-film circuit under the main thin file. Those are saphire substrates so they are a bit fragile. Sometimes we see a crack in the upper substrate (it floats above ground). If you find that you can bridge it with some conductive epoxy.
The R channel is identical in body, but it might be hard to get the test port connector out. This is where my memory is a little fuzzy: the glass to metal seal at the test port I think is the same between the two bridges. So you can back out the threaded SMA connector and then take off the APC7 from the failed bridge and add them to the R channel body. It's a bit tricky to put in the pin-bead assembly (its easy to have if offset and miss the glass seal center conductor) but I've done it lots of times. But I don't think the R channel bridge has any DC feeds so the bias tee won't work.