HP 8566B - What to look for?

[W4PJB]

Great info, thank you! That's a very generous offer to test your own 8566, it's much appreciated.

[G0HZU]

Thanks, and great point about the signal being attenuated twice. I did set the 8566 to 0dB. VNA was -10 dB, as G0HZU suggested.

Guess it's not looking too good for Uncle YIG?

It might not be being fed the correct control current for 2.7GHz (or any control current at all) so there still might be a simple fix for this issue...

Now is a good time to be cautious and not be too hasty pulling or probing connectors etc. I don't know how fragile some of these modules are or how tolerant they are of misprobing or shorts etc.

It's probably best to do as much external testing/diagnostics as possible.  Try the 'SHIFT' 'GHz' combination to bring up the DAC control and see if anything changes (even slightly) across the full DAC range.

[KE5FX]

Here's how mine looks (and you can see the parameters I used for both the 8753 and 8566 if you look closely):



Again, while it could be the YIG, you will want to check the coil current before making any assumptions.  It is more likely IMO that the preselector driver circuit has a bug.  At zero tuning current the YTX will probably not exhibit a meaningful dip anywhere.

[KE5FX]

Something else to note -- once you do observe an S11 response, you may get a deeper notch if you bump the 8753's point count up to 801.  Depending on span, the default 201-point sweep may not have enough resolution to show the full effect.  In fact, when making S11 or S21 measurements on a narrowband YTF using a wide VNA span, you can potentially conclude that a good filter is faulty because the notch lies entirely between two points.  BTDT

Also: use 0 Hz span on the 8566.  No reason to sweep the filter while you're trying to measure it!

[G0HZU]

You can also try pressing ‘SHIFT’   ‘PRESEL PEAK’ to restore the factory control of the YIG current.
My analyser had some serious holes in the YIG tracking, and pressing the above buttons fixed it. However, your symptoms are a little different.

[W4PJB]

Something else to note -- once you do observe an S11 response, you may get a deeper notch if you bump the 8753's point count up to 801.  Depending on span, the default 201-point sweep may not have enough resolution to show the full effect.  In fact, when making S11 or S21 measurements on a narrowband YTF using a wide VNA span, you can potentially conclude that a good filter is faulty because the notch lies entirely between two points.  BTDT

Also: use 0 Hz span on the 8566.  No reason to sweep the filter while you're trying to measure it!

I tried all of your settings, including the 801 point test, and still got basically no response. There was about a -4 dB drop at 2.7 GHz, compared to about -2 dB of other reflections. So the YIG is doing something, it's just not doing much. Or at least not much signal is getting into it.

[KE5FX]

Post a photo of the 8566's screen?

[W4PJB]

Post a photo of the 8566's screen?

[W4PJB]

Hi All, just a quick bump on this post. I managed to secure a supposedly "tested good" YIG oscillator and tried swapping it today. Absolutely ZERO change in response. Still able to see a signal blip at around -94 dB on 2-22 GHz, still everything is in perfect operation on the low band. So unless this YIG is failed exactly the same way as the previous one, I'm going to say the issue lives elsewhere. I've got two more evenings to poke at this, then going on holiday for a bit.

BUT!!!! I did notice something else this evening. Perhaps this has been the case the whole time, and I just didn't notice until this evening as it's rather dim, but the red LED on the A6A7 YTX Current Driver is lit. I only noticed when I shadowed it momentarily so it wasn't getting washed out by the fluorescent light over my bench.  This indicates the heater circuit is saturated and not reaching spec. Is this enough to cause my issue?  But it's a new breadcrumb.

As always, pictures attached.

[KE5FX]

Maybe I missed it, but did you ever check the YTX tuning coil current?

If not, again, do that. 

The heater circuit may or may not be OK but you don't usually lose 80+ dB when the heater goes south.

[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   

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.