Trying to repair an 8753E with a 6 GHz source module (08753-60146).? The source sometimes works fine, and other times has an output power that is 50 dB too low.? First, some symptoms:
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1)? No phase lock issues (regardless of whether this issue is occuring or not).
2)? When the output power is low, the S11 that it shows on the default display is -20 dB.? However, the response vs. frequency is still basically flat.? Usually when it is deciding to work if I quickly power cycle the machine it still works.? Likewise, if it is deciding not to work power cycling generally does not improve the issue.? I have tried to power up the machine in different orientations and this does not seem to change the issue.? When I change the output power of the machine in the menu from 0 dBm to +15, the output tracks (i.e. goes from -51 dBm to -36 dBm).?
3)? I measured the low output power first at the front of the machine (port 1), then at the source module itself, and then at the output of the mixer phase.? In all cases I got the similar results.
4)? Tested the source module in two different machines, and the symptoms are similar so I'm pretty sure that this is related to something in the source module itself and not anything else in the machine.
5)? Tested the output power of the YIG and it seems to be fine (around 3 dBm @ 3.8 Ghz when it was trying to create the 0 dBm output).? It's impossible for me to know if this output was when the source module was working properly or not, but my assumption for now at least is that the YIG is fine.
6)? I assumed that it might have to do with an electrical connection to an amplifier stage in the mixer portion, so I disassembled the mixer portion of the source and attempted to exercise the various connections in hopes of improving things.? No change in sympoms observed.
7)? It is possible to perform a cal and use the instrument when it's the -30 dB output power mode, but when I do so the response hops around a lot and the response below about 300 MHz is pretty much unusable due to noise and phase errors.
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I thought that I figured out a way to reproduce the issue.? There is a cable (W26) which connects the source assembly (A3) to the REFERENCE (A12J4).? When I powered the unit up and it had low output power, I disconnected this cable.? The output power stays identical (i.e. 50 dB down in power and shows the -20 dB return loss).? When I reconnected the cable, it turned the source "good".? I then powered down the unit and repeated (and it worked the same way a second time).? However, in subsequent tests I have not been able to get this "fix" to work.? This kind of makes me think that there is something wrong with respect to how the input from this cable is being processed - like a bad cap or something on the PCB below the mixer which is keeping this signal from providing the proper feedback???
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Attached are some pictures of the various measurements I described above.
1)? Output power of source (1 GHz tone) when it is not working properly.? Output power default (0 dBm target)
2)? Same as above, but output power +15 dBm target
3)? YIG output measured directly.? (default output power target, 1 GHz?)
4)? S11 (no load on port) when machine is working properly.
5)? S11 (no load on port) when machine is not working properly.? Note - the slight change in shape between #4 above and this measurement is likely cause by a different port coupler that I was testing (not related to this issue).? Basically the shape of the curves changes very little but everything drops 20 dB on the display.
6)? Source module showing the cable which I unplug to test.? When this cable is removed, nothing changes (if the machine is in the "bad" state).? When I plugged this back in (previously) it started working (generally).
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At this point, I'm confused so I'm hoping that one of the smart people here can point me to something that I can measure, test, or try to solve this elusive issue.
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