The RF unit
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I decided to first test the RF unit and leave the display for later. The RF unit was miraculously complete with all the boards. However, several screws and the plastic covers protecting the boards were missing. Additionally, the entire cooling fan assembly was not present. The tuning knob was missing from the front panel (fortunately i have some spares), and the covers were, of course, not there.
First, I installed a temporary fan to avoid overheating the unit, hoping to find the original one in the future. Then I turned on the unit and crossed my fingers...
The first power-up didn¡¯t go well, no image on the screen, and the right red LED was on. The CPU board showed the fourth LED lit during the self-test, indicating an I/O error. The manual explains the possible causes of the problem: it could be two faulty PALs or many other reasons. While I was inspecting the device, I noticed that the I/O board wasn¡¯t properly seated in its socket. I proceeded to remove, clean, and reinsert the board, and on the next power-up, I no longer had any errors, and the RF unit was alive!
Clearly, the backup battery was signaling depletion, so I replaced it. However, a big issue remained: there was no sweep, so the trace was stuck. I checked the amplifier board and noticed that someone had switched the input and output cables. Incredible... Once they were corrected, we had a perfect sweep.
At this point, I ran the first calibration, feeling quite confident, but unfortunately, it failed, even after adjusting the calibrator and frequency align to -10 dBm. I noticed that the calibration failed when it started exercising the attenuator. Indeed, there were some attenuator positions that didn¡¯t work.
I manually exercised the attenuator, and after 3 or 4 switchings, it started working perfectly again. There was clearly some oxidized contact. I relaunched the calibration, and this time, everything went well. The RF part works perfectly.
After testing some sample signals, I loaded the HP DLPs via GPIB and declared the repair finished...
Now it¡¯s time to move on to the 85662A¡
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