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| HP 8566B - What to look for? |
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| W4PJB:
Hi All, Hope everyone had a great 4th! Sorry I've been absent on keeping up with this thread. I've been on vacation for the past 8 days and, although this board is rather narrow in audience, I generally don't make announcements on the internet when my house will be empty for a week :) The one update I have is, the Marketplace gods have shined upon me, and this afternoon I picked up a "mostly" complete 8566 A/B (A converted to B) that happened to be for sale locally. The RF section is missing the entire A11 assembly, and it's also missing the 2nd Mixer from the A6. Otherwise the unit is complete, including the entire display section. The price was, let's just say, hard to turn down :D So far the only thing I've had time to test is swapping out the A6A7 current driver, which made no difference. In fact, I'd say the red light shone brighter than ever. At one point it went out briefly, then came right back on at full brightness. I didn't expect that to change anything but it's extremely easy to swap out, so I started there. Also, one clarification. I had previously mentioned I swapped out the YIG. I was referring to the "YIG Tuned Mixer, aka the YTX A6A8, not the A11A4 YIG Tuned Oscillator, in case that had caused any confusion. I haven't touched anything in the A11 section. Pic below of the parts unit. |
| W4PJB:
Massive news! In the spirit of "I have a bunch of extra parts now, let's see what happens if we swap a few out", I now have a signal that's now only 30dBm low! I tried swapping the A6A12 YTX driver card with the one from my newly acquired parts unit, and although nothing seemed to change at first, after a good warmup, with an input of 0dBm, I have a signal at -30 with a 10 dB attenuation set on the SA. I swapped in the old card (quickly, to preserve the heat in the unit), and it was dead again. A switch back to the new card and it instantly came back to life. I did this twice, with similar results. The problem exactly tracks the A6A12. LED on the A6A7 is now very dim. I think that pretty well confirms it's likely a simple current problem in the YTX driver, as others have suspected. Not sure when I'll get time to dive into this deeper than swapping parts, but the good news is I don't seem to have any significant hardware issues as I can consistently reproduce my results by swapping out the A6A12. I also have several other non-original parts inside, including the YTX and last converter that I need to return to original. But none of them made any immediate effect as the A6A12 did. Thanks again, and I'm certain more great news is to come! Edit: "SHFT - PRESELECT PEAK" brought it up by 10dBm. Entering manual DAC, I was able to get it up to 0 dBm by adjusting the DAC from 32 to 46! This was tested at 5.9 GHz. We're almost there! Edit Edit: Corrected board name to A6A12 |
| G0HZU:
That is good news! SHIFT - PRESEL PEAK worked wonders on my HP 8566B but I think you will probably also have to tweak some cal values to get it all to track correctly. I've never tried to do this with mine so I can't offer much advice in this respect. |
| W4PJB:
Thank you! Yes, I think I will have to tweak some values to get everything in alignment. I plan to replace the original YTX prior to making any adjustments. I also found a burned resistor on the old A6A12 board near the edge connector. Assuming it was not a shorted heater in the YTX that smoked the resistor in the first place, I may just swap the resistor from the parts board as the rest of the card should be calibrated to the unit from HP. I'll have to do some research to figure out what that resistor does prior to busting out the screw driver or soldering iron. More to come... Edit: Corrected board name |
| W4PJB:
Success!!! :-+ I decided not to reinstall the original YTX in the event a shorted coil damaged the A6A12 board to begin with. Rather, I used the YTX that was matched to the driver board from the donor unit. Seems that hunch was correct. The red LED on the A6A7 went out immediately upon powering up. On the original YTX, and to a lesser extent the "replacement" I had sourced online, the red LED stayed bright for 1-2 minutes after power up, and never completely went out. Even with the new A6A12 installed. To me, that's a good indicator the coil is failing, and potentially shorted, especially considering the burned resistor on the original card. So now the good news! After replacing the YTX, I was able to get a full amplitude signal at several frequencies, even before the OCXO was fully warmed up. Unfortunately, I currently have no means to test over 6 GHz, so we so at this point I'm calling it good and the covers are going back on. HUGE thank you to everyone who so generously shared their time and wisdom with me, even going so far as to check and confirm details on their own 8566. I absolutely could not have finished this project without your help. |
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