HP 8566B - What to look for? - Page 1
Forum section: electronics / testgear

author: W4PJB
author_rank: Regular Contributor
timestamp: June 07, 2024, 09:31:21 pm
content: I'm going to look at an 8566B tomorrow. I can power it up and inspect it, but I don't think I'll be bringing my 8665B to test it as it's a little large to carryThe unit supposedly powers up and displays no errors, anything else i should look for? If i set it for "full span", is there anything obvious on the screen I should see if it's got a fault? I know the noise floor will shift a bit every few gHz (and that's a good sign). Anything else I can test?I do have an Anritsu MB2721B with TG I could use to create a signal up to 7gHz.

author: G0HZU
author_rank: Super Contributor
timestamp: June 07, 2024, 11:33:10 pm
content: Connect the 100MHz test signal (BNC) to the main RF input and see if it adjusts OK when doing the RECALL 8 and RECALL 9 tests in the manual.You can also try running the automated error correction routine by connecting the 100MHz test signal (BNC) to the main RF input and pressing the blue SHIFT button and then press the big FREQUENCY SPAN button. This runs an automated self correction routine that aims to improve performance by correcting errors between the RBW filters etc.This takes about a minute to run.Also put it on a 10MHz span and look at the 100MHz test signal. There will be some wobble seen on the signal when a 10MHz span is selected. A poor unit will show more wobble than a good unit.Make sure the 0-70dB attenuator works on all settings. Note that you have to press ATTEN 0 dB to get it to select 0dB attenuation. It won't let you select 0dB with the down arrow key.Probably the best advice I can offer you is to not buy the 8566B unless you really do need the extra frequency range it offers. It's unlikely to be very reliable in the coming years so you can expect to have to take it apart to replace tired capacitors from time to time.Also, the RBW filters may need to be aligned/adjusted as per the service manual. It is also very big and very heavy and the fan noise is very loud. They can also smell a bit musty if operated in a small room as the fans will spread the musty smell into the room.I've still got my 8566B but I rarely use it. I upgraded to something more modern about 5 years ago. Better performance, less noise and much easier to move about!

author: KE5FX
author_rank: Super Contributor
timestamp: June 08, 2024, 03:22:39 am
content: One thing you should look at is the CRT condition. All display elements -- text, trace, graticule lines -- should be razor-sharp at all reasonable brightness levels.The suggestion to hook up the calibrator and do a recall-8, recall-9, blue shift-W cycle, followed by a blue-shift-w command to inspect the results, is a good one. Take the time to read about that in the manual beforehand. It is unlikely that you will need to tinker with the RBW filters but if there is a problem, this procedure has a good chance of indicating it. Watch the shift-W cycle while it runs, keeping an eye out for gross errors in filter skirt shapes.Agreed that these units are likely to become more "maintenance-prone" going forward. But at least youcanwork on them. Mine will leave my bench shortly after I do, feet first.

author: G0HZU
author_rank: Super Contributor
timestamp: June 08, 2024, 08:27:50 am
content: If at all possible, try powering it on from a cold start to see how it behaves. Normally, you are meant to leave it powered in standby for maybe 15-20 minutes before powering it on. This allows the 10MHz ref oven to warm up and also other circuits need to be warmed up before it will be able to achieve reliable control of the swept LO system.However, if you deliberately power it on from cold it should report REF UNLOCK and OVEN COLD on the CRT display. If the analyser is in good health, this probably won't stop it from being basically functional on wide spans at least.However, when I first purchased my 8566B about 14 years ago, it would misbehave badly if given a cold start. It needed about 30 minutes to warm up and be functional. It showed all kinds of strange behaviour onscreen when started from cold. It would drift badly and in its attempts to try and sweep correctly it even wildly changed the centre frequency (to negative frequencies!) and basically went crazy.Over time this got worse and worse (almost certainly why the rental company sold it to me so cheaply) and it eventually refused to lock even when warmed up.This was traced to several failed capacitors on boards involved in the YTO (YIG) tuned LO system. Once these were replaced, it was basically useable from a cold start.It will also report the condition of the battery when you first power it on. If there is an issue with the battery it will show BATTERY on the CRT display.There are also a couple of red status LEDs along the lowest edge of the front panel. These should turn off once the analyser is booted and running correctly. If one or both of them stay lit then the analyser has problems somewhere.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 08, 2024, 02:18:20 pm
content: Thanks everyone for your replies, they are all very helpful! I certainly appreciate the warnings about it's age. That is, of course, a downside. But it's also part of the hobby. I'm the type that learns by doing; I need a problem to solve, I can't just sit and learn from a book. So if happens to kick down the road, I'm ok with that... I'll learn something! Whether I can fix it or not. Either way, the 8566 is just such a cool piece of gear I'd be kicking myself i missed the chance to own one for a few hundred bucks.I'll be sure to let everyone know how it goes!

author: nctnico
author_rank: Super Contributor
timestamp: June 08, 2024, 03:33:21 pm
content: If the 8566B costs a couple of hundred bucks, I'd save a bit longer and buy a much newer spectrum analyser. The ones from Advantest are typically not sure expensive. I'd avoid anything with a CRT screen.

author: KE5FX
author_rank: Super Contributor
timestamp: June 08, 2024, 04:49:18 pm
content: But the Advantest lacks service documentation, "tribal knowledge," and widespread parts availability. It has a substantially-inferior display, a less-expedient UI, all the soul of a Subaru, and worst of all, you miss out on the comforting roar that will remind you of your summer job at the airport as a teenager...One nice thing about the 8566B, especially with an LCD, is that it's up and running almost by the time your fingers come off the power switch. Seems like a goofy hill to defend but the fact that it's always ready when I need it is a big part of why it's still on my bench. Of all the spectrum analyzers around here, only the tinySA matches it in that respect. What typically happens is that the 8566B is used to lookforsignals, while a newer FFT-based analyzer (usually an FSP or E4406A) is then used when necessary to drill down and lookatthe signal.Admittedly, if I only had room for one, it wouldn't be the 8566B just because life is too short for swept analog filters. But everything with a modern signal-processing back end either has lower coverage limits like the OP's MS2721B, is in a higher price bracket like the FSP, or is intentionally compromised like the E4406A.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 08, 2024, 09:29:30 pm
content: Well, the good news is, the 8566B has a new home! I was able to run the self cal's at the sellers house and everything seemed to run, at least well enough to make an offer. The screen was bright and crisp. All controls responded well. YTO Unlock was on the screen briefly, but quickly disappeared after a minute or two of warmup. The other good news is, I only paid $400 for it.The bad news is, once I got it home and was able to connect it to my 8665B, I learned the 2.5-22 gHz range has no signal. Then, after about 10 minutes of operation, the screen went to a blurry dot, and the very familiar smell of capacitor filled the room along with a gentle puff of smoke. Not daunted on that, it's likely a display capacitor, as this poster experienced:https://www.eevblog.com/forum/repair/hp-8566b-display-problem/The fun part will be diving into the 2.5 gHz section once the display is fixed. Might as well get a few capacitors on order while I'm on DigiKey. Does anyone have a list of common caps to replace in these?

author: KE5FX
author_rank: Super Contributor
timestamp: June 08, 2024, 09:39:08 pm
content: Looks like a late-model unit that will clean up nicely.You'll want to get an ESR meter if you don't already have one, and then shotgun-check every axial tantalum cap on the PC boards under the plastic covers in the RF section. Any good-quality electrolytic (Ta or Al) with similar voltage and capacitance ratings can be used to replace most/all of them that are found to be defective. No need to replace caps that aren't actually defective.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 08, 2024, 09:56:59 pm
content: Quote from: KE5FX on June 08, 2024, 09:39:08 pmThanks! I was thinking it was a later unit, based on the encoder knob being the plastic / dimpled one, and not the aluminum / plastic one. Is there a way to date an HP unit by serial? Or only get a general idea based on sequence?My Fluke 123 Scopemeter can measure capacitance, and I also have one of those cheap transistor / diode / capacitor / resistor meters that sell for $20 on the big websites. Guessing the Fluke should be up to the task.Do you think it's likely my non-working 2.5-22 gHz section could be as simple as a bad cap? The only error that didn't go away in the first few minutes of warming up was "REF UNLOCK". It seemed to be blinking roughly in pace with the scan rate. But the cap in the CRT power supply went poof before I could let it warm up much.

author: G0HZU
author_rank: Super Contributor
timestamp: June 08, 2024, 11:59:28 pm
content: It does look to be in nice condition. I bought mine as an ex-rental about 14 years ago for just under £1000. At the time this was a really low price for an 8566B. It turned out to be an A version that had had the factory upgrade to a B. So it is older than I was expecting. I bought it from one of the main rental companies here in the UK (Microlease) so I had to trust them that it worked OK (it wasn't OK).Apart from the YTO instability, the CRT was also very fuzzy even when the focus control was optimised. I managed to fix the fuzzy CRT by servicing the Z axis board. I also went around all of the RF boards with an ESR meter and replaced several dead caps and this fixed the YTO issues.The other problem it had was a very poor frequency response across the 2-22GHz range. Playing with the YIG PRESEL button just made it worse in various parts of the 2-22GHz range.I'd recommend that you try pressing the blue shift key followed by the PRESEL PEAK button when on the 2-22GHz range. This should restore the factory control of the YIG preselector. In my case this fully restored the frequency response on the 2-22GHz range. There were then no more deep holes in the frequency response up to the 21GHz limit of my old 83752A sig gen.Hopefully, the high band mixer is OK on your unit. This would probably be expensive to replace assuming you can find a replacement.There is also a changeover relay that might have failed in some way. This relay switches across to the high band range.Is the high band range completely dead or is the displayed level just really low?

author: KE5FX
author_rank: Super Contributor
timestamp: June 09, 2024, 12:11:19 am
content: Quote from: W4PJB on June 08, 2024, 09:56:59 pmAdding 60 to the first two digits of the serial # will give you the date of the last design/manufacturing revision, which is a lower bound on the manufacturing date. E.g., if it starts with 27xx it will be 1987 production or later. Then there are the date codes that will be present on many of the caps and ICs.QuoteNo, you want an ESR meter. You can't reliably measure capacitance in-circuit, but you can read the ESR. It will be obvious which ones are open (and there will likely be a couple.) The axial tantalum electrolytics fail open much more often than they fail short.QuoteREF UNLOCK is normal during warmup, before the OCXO comes up to temperature. If it stays on, make sure the reference jumper and switch are set correctly on the back panel. YTO UNLOCK and other error messages are never normal, except perhaps BATTERY. There will be a Tadiran primary lithium battery on the CPU board that may need to be replaced if that message shows up.The 2.5-22 GHz section could fail for any number of reasons, most of them not too painful to find and fix. You'll probably need to go through the manual to track that issue down. Bad capacitors can cause almost any symptom including that one, but they are far from the only potential trouble source. Fortunately the high-band mixer is rarely bad, as the diode is well-protected by the YIG coupling loop.Artek Media has a good set of manuals (the free ones are pretty crappy.) I like to keep both paper and .PDF copies on hand, but the paper manuals take up almost as much space as the 8566 itself...

author: W4PJB
author_rank: Regular Contributor
timestamp: June 10, 2024, 07:45:30 pm
content: Thanks again for your reply!The first two digits are 31, so that should make my unit a '91 or later model. Great!I will look into a better LCR/ESR meter for this. Never hurts to have one on the bench anyway. The DE-5000 seems to have a good following here for the money.Also great info on the REF UNLOCK error. Unfortunately I was not able to keep it in warm-up long enough to see if it went away. YTO ULOCK did come up for a minute or so when I powered it up at the sellers house, but it quickly went away. It did not come on again when I powered it up at home, just REF UNLOCK. They got the SA from an estate sale, and are not electronics savvy, so I unfortunately have zero history on the unit.Once I get an ESR meter I'll check all the caps in the RF section as well, hopefully that does the trick for the high band. I plan to replace the large caps in the power supply too, while I'm in there. I'll be sure to keep this thread updated as I progress, but now we're entering the waiting phase to get test kit, parts, and most importantly TIME to mess with it.Thanks for the tip about Artek Media. The whole set of 5 manuals is only $15. I'll be placing that order right now!

author: KE5FX
author_rank: Super Contributor
timestamp: June 10, 2024, 09:36:16 pm
content: Quote from: W4PJB on June 10, 2024, 07:45:30 pmDefinitely a later model. If you had $61.5K to burn in 1991, you could have bought this 8566B or a new Porsche 911.I would absolutely not recommend replacing the large power supply capacitors (or any others) unless they are actually defective. They should be checked, of course, but the truth is that the newer capacitors you replace them with will not necessarily be any better or last any longer. (Plus, have you checked the price of new 'computer-grade' electrolytics lately?)

author: W4PJB
author_rank: Regular Contributor
timestamp: June 11, 2024, 02:08:52 am
content: I'm continually blown away, and simultaneously humbled, by the brain trust on this forum. Thank you!I took the liberty of popping a few panels off tonight, just to see if I could find an obvious "smoking gun", since, well a pretty significant jet of electrolytic smoke emanated from the side of it.From a quick visual nothing obvious, and on a positive note, the unit was SUPER clean inside. Not a speck of dust. This must have been in a clean environment, rarely used, or both. Judging by the quality of the display (when it worked), and the softness of all the wire jacketing, I'm guessing this may have pretty low hours on it.One thing I DID find, when pulling one of the small side panels off of the RF unit, was a few tell-tale sunflower seed shells. So perhaps I need to be on the alert for critter damage as I go through the unit.Also, how on earth did humans design this thing? In the 1970's, no less? If I can't get this fixed, I'm pulling the covers off and hanging it on the wall. What a masterpiece of technology.Apologies for the weird color cast in right side of the photo, it's an artifact from the rolling shutter in my iPhone and the crappy fluorescent light above my bench. Everything inside is as clean and silvery as it should be

author: W4PJB
author_rank: Regular Contributor
timestamp: June 11, 2024, 02:18:21 am
content: Quote from: G0HZU on June 08, 2024, 11:59:28 pmI only had a few minutes until the display quit, but from what I could tell, dead. I had a source going in at 0dbm, through a 30db pad, and 10db of attenuation in the 8566, and I got absolutely nothing on the graph. The low range peak was exactly where expected. As soon as the relay clicked into the 2-22 gHz range, I couldn't get a reading anywhere. I have a 6 gHz source and tried at 2.5, 3.0, 4.0, 5.0, 5.5, and 6 gHz, and nothing registered on the screen.The high band noise floor looked like the one in this photo (not mine, it's from the web).Thanks!

author: KE5FX
author_rank: Super Contributor
timestamp: June 11, 2024, 03:58:04 am
content: Yeah, that's a low-hours unit. The CRT HV lead is the real telltale, and it looks like new. Should be well worth fixing up!Trivia: they started work on the 8566A in the early 1970s, andit didn't ship until 1979. Its code name was "Doomsday." The earliest drawing I've seen is from 1971 (and never mind where it came from):

author: W4PJB
author_rank: Regular Contributor
timestamp: June 11, 2024, 01:50:08 pm
content: That's a very fascinating site, thank you! Not surprised it was code named "Doomsday". It doesn't take a vivid imagination to figure out what kind of projects would have motivated development of a cost is no object, 22 GHz spectrum analyzer in the 1970's.

author: nctnico
author_rank: Super Contributor
timestamp: June 11, 2024, 02:02:26 pm
content: Quote from: W4PJB on June 11, 2024, 02:08:52 amThis looks like new indeed. I'll admit that looking at the photo of the internals does put a smile on my face

author: G0HZU
author_rank: Super Contributor
timestamp: June 11, 2024, 05:12:33 pm
content: Yes, it does look to be as new inside. I recently acquired a late model HP 8405A vector voltmeter that looks inside like it was made yesterday. It's absolutely sparkly clean inside.To give some contrast, I know the history of my 1500MHz HP 8568B analyser back to about 1987 and it was made in 1986. It was donated (as part of a contract) to the company I work for back in about 1987.I joined in 1990 and this analyser was used for >8 hours every working day for about 15 years since 1987. It was never used in an ATE rack so was switched off every evening. The company gave it to me (I made a small donation to a charity to secure it) in about 2005 after it had fallen out of use at work for a few years.When I took it apart, I was surprised at how much dust there was around the CPU board. It was thickly coated in dust to the point it wasn't possible to make out the ribbon cables. The display unit showed quite a lot of black soot around the HT areas.I'm not sure why it was so dusty and sooty after maybe 30,000 - 50,000 hours operation time. It may be that the filters in the fans weren't working well for many years or maybe there was more dust in the air than I realised in the RF labs at work!My HP 8566B is a few years older but the display unit was very clean inside. Not nearly as sparkly clean as yours but I assumed that it had been internally cleaned and serviced at some point because it showed none of the soot and dust that I found inside the 8568B. I suspect that my 8566B has seen a lot of use as it was an ex-rental analyser from Microlease.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 11, 2024, 06:38:18 pm
content: Well, mine has a .gov tag on it, so it was probably used twice, put in a storage facility for 30 years, then went to auction for pennies on the dollar

author: G0HZU
author_rank: Super Contributor
timestamp: June 11, 2024, 07:17:00 pm
content: My HP 8568B was also a govt owned analyser. The govt donated it to the company when it was about 1 year old.They also donated a nearly new Advantest TR4172 analyser a few years later and this would have cost them about $60k due to the high cost of the Yen in the late 1980s.I also now own that Advantest analyser as the company gave it to me about 20 years ago. I had to make a charitable donation for it though. It was a worthy competitor to the HP 8568B. It wasn't as polished as the 8568B in terms of all round performance, but it outclassed it for spurious free dynamic range. That's why it was donated to the company. The 8568B was good, but in terms of SFDR it wasn't good enough for the stuff we were doing for govt back then. The TR4172 also had really low phase noise at 100kHz offset. IIRC it was typically about -128dBc/Hz. It also had a tracking generator and could be used as a crude VNA with an external RL bridge.If you can get that 8566B fully working, you could probably sell it to a collector for a lot of money as it looks to be in fantastic condition inside.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 12, 2024, 02:26:18 am
content: Since it looks like I'll be working on the CRT power supply section, any advice on how to ~not~ kill myself?Just kidding... mostly. I'm very aware of the voltage levels in CRT circuits, even potentially when unplugged. Do the HP CRT boards typically have bleeder resistors to discharge the capacitors when unplugged? I see nothing in the service manual instructing to manually discharge anything, or wait "x" time before servicing; it simply states to not work on this area of the unit with the power cable connected. I could always cobble together a drain circuit to discharge it in a controlled manner, but it isn't exactly easy to get to HV lead on the CRT on these units.Thanks!

author: KE5FX
author_rank: Super Contributor
timestamp: June 12, 2024, 03:12:38 am
content: Quote from: W4PJB on June 12, 2024, 02:26:18 amIt is indeed described in the manual:It's worth following this procedure, as the CRT really will hold a charge for quite some time. Contact with this voltage probably will not kill you but it will hurt.Be sure the CRT shield ground wire is in place. How I know this is a long and boring story, but trust me.All of that being said, whatever went 'poof' and smoked is probably not in the HV section.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 12, 2024, 11:45:12 am
content: Edit: Now that I knew what to look for, I found the procedure in a different manual from EMCTEST. Thank you for sharing this!Also, the manual from EMCTEST is a 1990 printing, so likely more representative of my unit, rather than the 1984 printing from Artek, so added bonus.-----------------------------------------------------------------------------Thank you for your reply. Two things I find interesting here:1: My manual is completely different, and does not list this procedure at all.2: HP engineers advise using the wire on a screwdriver trick on a sensitive $60,000 piece of RF equipmentI would have expected something a bit more.... controlled.I'll post below the only two pages in my manual that reference a service procedure for the A1A3 or the CRT. The only place I see a reference to a discharge procedure for a high voltage in the A1 section is in the A1A8 section, where it advises, "The time constant of the filter capacitor circuit is about 25 seconds. Before servicing, allow adequate time for discharge after line power cable is removed.". But this is referencing mains voltage levels, not the HV circuit. Even the CRT removal procedure does not have any flags about HV.Was your manual sourced online? I'm using a manual from Artek, and the page numbers for the A1A3 stop at "A1A3 9/10", which is the schematic diagram for this board. I don't see it in the manual on Xdevs, either.

author: KE5FX
author_rank: Super Contributor
timestamp: June 12, 2024, 03:26:46 pm
content: I'm not sure where my copies came from, I think they were originally from Agilent's own site. At least some of the files, including the copy of 85662-90085 I clipboarded the above from, are available on theInternet Archive.The problem is, whoever OCR'ed them didn't GAF what they were doing, and used some seriously-inappropriate compression settings As a result various bits and pieces of text including many numeric values were replaced with the OCR'ed text, rendered in the OCR app's best guess as to the correct font.This actually changes the information in some cases:So you don't want any part of these files, believe me. That's one of the reasons I keep paper copies on hand. It's unfortunate if Artek's files weren't captured from a later edition, but at least they don't screw up stuff like this.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 12, 2024, 03:39:11 pm
content: I had never even considered OCR, as I never use it when scanning PDF's. But that's an EXCELLENT point!Thanks again!

author: W4PJB
author_rank: Regular Contributor
timestamp: June 16, 2024, 04:53:02 pm
content: It's always the last place you look, right?In this case, it was literally the last board in the unit there was to check.I found capacitor C22 was missing its magic smoke. This coincides with the approximate location of the volcanic eruption I saw from the right side of the display unit when it failed.An equivalent seems to be available from Vishay (DigiKey 150D225X9020A2BE3) and I will probably go ahead and just replace all 3. C21 measures OK at 2.19uf but it's seen some heat damage from C22, and C20 measures around 2.8uf and has about 20% less phase rotation according to my LCR meter, so I'll guess that one on its way out next.Once I get the display up and running then on to the 2-22 GHz problem. Thanks for everyone's help!

author: KE5FX
author_rank: Super Contributor
timestamp: June 16, 2024, 05:07:26 pm
content: Yep, the neighboring tantalum caps are a good exception to the "Don't replace unless defective" advice.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 20, 2024, 09:51:19 pm
content: Well I got the new caps in today and viola! We have a screen!Now onto the real work; figuring out why I have no signal on the 2-22Ghz band. Thanks for everyone's help!I'll start with checking all of the voltages very closely, I understand the YIG can be very sensitive to even small errors here.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 20, 2024, 10:40:16 pm
content: Voltages all looked good, except the "18V Typical" on the A18 was low at 16.2v. The -10v side of that test port was bang on, relative to the "REF GND" on the A19.C4 and C19 on the A21 board are looking a little sus, though in-circuit they seem to measure OK. They're also over $40 EACH!

author: KE5FX
author_rank: Super Contributor
timestamp: June 20, 2024, 11:02:38 pm
content: Just use whatever's handy to replace them, same or higher voltage, same or (slightly) higher capacitance. Might have failed shorted if they appear OK on the ESR meter while looking like that.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 21, 2024, 01:23:31 am
content: Have you checked the RF switch? Had an issue with another model where the switch clicked but the contacts didn't actually make contact well.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 21, 2024, 01:33:52 am
content: Quote from: EggertEnjoyer123 on June 21, 2024, 01:23:31 amThanks for your reply. Not yet.... but it's high on the list! I've been doing through the service manual and three things are pointing me in that direction. 1: looking at common points of failure, that's one of the first in the chain for this issue. 2: I do not have any errors, such as "YTO Unlock", on the display. 3: Switching from 2-22Ghz to the low band the "click" from the relay is crisp and fast. Switching to 2-22Ghz is kind of "lazy".But there's a bit of teardown to get to that component, and with a "real feel" of 107*F in Maine today, this evening I've switched from inspecting vintage HP instrumentation, to inspecting a nicely chilled bottle of Chianti. Meaning it's probably best I keep my hands out of electrical devices for the evening. Cheers!

author: W4PJB
author_rank: Regular Contributor
timestamp: June 21, 2024, 04:28:38 pm
content: Well, it wasn't the switch. Pic of a 1MHz - 7GHz sweep attached. Ignore all of the ripple, it's most likely reflections in the cheap SMA cables.On to the next thing! I have no errors on screen, so I doubt there are any issues with the YIG itself, but if it saw some DC it's likely the mixer diodes are fried. It's my understanding this will not cause any error messages. So perhaps I'll pick one up and try it, they're cheap enough these days.

author: KE5FX
author_rank: Super Contributor
timestamp: June 21, 2024, 09:39:06 pm
content: Messing with the YTX is probably not going to be productive in this case. It could be damaged by DC, sure, but the damage would take the form of burning out the input coupling loop rather than the diodes. Application of enough power to damage the YTX is likely to damage the attenuator as well, which it sounds like you've ruled out.Refer to figure 1 in the August 1979 HPJ issue and you will see what I mean. (As with many HP instruments, HPJ actually provides a better technical introduction than anything in the official manuals.)If you were to replace the YTX you would have to recalibrate all of the the trimpots that define its tuning-current curve. You really don't want to go there if you can help it. Instead, try applying a CW signal at a few GHz and tracing the resulting 321.4 MHz IF through the Figure 2 block diagram in the HPJ issue, using (e.g.) your Anritsu. If you can't find the IF signal at all, check the current through the YTX tuning coil to make sure it's not open, and that its driver circuit is functioning at all.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 21, 2024, 09:43:30 pm
content: Thank you. I've been working on this during my spare time in the evening, so I haven't had much time to really dive into the block diagrams yet. I have a lot more time to do research over the weekend.Also, I always think of the archival value of these posts. Hopefully at some time in the future, someone may find the information here helpful, just as I have learned from past posts on this forum and on others!

author: G0HZU
author_rank: Super Contributor
timestamp: June 21, 2024, 10:05:38 pm
content: Good to see you got the display working again!If you get a bit of free time, could you do a quick screenshot of the phase noise at 100MHz on a 100 kHz span as in the image below?The image below is taken from the manual and it shows the front panel 100MHz (-10dBm) calibrator test signal being measured on a 100kHz span with the RBW set to 1kHz.Could you quickly show me what your analyser looks like with this test setup please? I'm interested to see how much cleaner the later models are in terms of phase noise. I checked the serial number of mine and it was made in 1982 so it would have been a fairly late A version converted to a B with the factory upgrade. My analyser looks very similar to the plot below. I'm interested to know if the loop bandwidth was changed at this span setting on the later models. It seems to be very narrow in the image below? On narrower spans the loop bandwidth appears to be wider on my analyser.I think my analyser has 1989 firmware as it flashes up 1989 when booting but the actual hardware appears to have been made in 1982. I'm not sure what the latest version of firmware is for these analysers.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 21, 2024, 11:40:00 pm
content: Ah ha! Progress is being made! I set the 8566B to 3 GHz, 10 kHz span, and low and behold, a little 4 dB blip showed up right at 3 GHz. (Sweep rate is slow so I have time to hit "Peak Search" and grab a photo before the next sweep.)Also, I connected my Anritsu to the 1ST LO out, and as to be expected, 3.321 400 GHz.So the great news is, everything is working! At least somewhat, enough to know the main components of the RF section aren't dead. It just has a very, very weak signal that disappears in the noise floor unless it's set to a very narrow span. I checked the attenuator and as expected, it's working fine.So, once again, on to the next thing!

author: W4PJB
author_rank: Regular Contributor
timestamp: June 21, 2024, 11:48:34 pm
content: Quote from: G0HZU on June 21, 2024, 10:05:38 pmMy pleasure! Is this what you are looking for? Or did I miss a step?

author: KE5FX
author_rank: Super Contributor
timestamp: June 22, 2024, 03:18:43 am
content: You're set to 1 dB/division, so you can't see anything but the peak. Try these settings:(from 27xx serial #)

author: G0HZU
author_rank: Super Contributor
timestamp: June 22, 2024, 10:42:14 am
content: Thanks! The plot I posted up is taken from the B manual but it is a reprint from the A manual. It looks like the loop response is different on the later models. Close in phase noise is better but the phase noise at 50kHz offset is higher.Quite surprisingly, my analyser still uses the same loop response on a 50kHz span and this really doesn't look good. Maybe there is a way to upgrade it to emulate the later versions. However, that is a topic for another day...QuoteCan you set it to 10dB/div as shown by KE5FX. I think you will then see a similar plot as the one from KE5FX's analyser.

author: G0HZU
author_rank: Super Contributor
timestamp: June 22, 2024, 10:49:24 am
content: If you have access to a VNA (even a nanovna) that can cover up to 3GHz or higher, then you should be able to connect the VNA to the input port of the 8566B and then set the 8566B to (say) 2.7GHz on a 1MHz span and then set the input attenuator to 0dB. Then set the VNA to an s11 measurement. Set the VNA source power to -10dBm or less.You should then be able to see a dip in the return loss at 2.7GHz if the YIG preselector is working and tracking correctly. It should show a return loss of >10dB across about a 20MHz? bandwidth (at a guess).It is then also possible to press 'SHIFT' 'GHZ' and this should bring up an oncreen YIG DAC control feature. You can rotate the wheel on the front panel of the 8566B and then the YIG filter frequency can be tuned over a limited frequency range either side of 2.7GHz.This should give an indication of the health of the YIG spheres and also the YIG tuning and control sections without having to take anything apart inside the analyser.If you don't have a VNA then maybe make or borrow a directional coupler and use it with your Anritsu analyser + tracking gen to do the same thing.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 22, 2024, 01:02:33 pm
content: Quote from: G0HZU on June 22, 2024, 10:49:24 amA 3 GHz VNA... funny you should mention thatThank you for your advice!

author: W4PJB
author_rank: Regular Contributor
timestamp: June 22, 2024, 08:36:15 pm
content: Quote from: G0HZU on June 22, 2024, 10:42:14 amHere you go! Sorry about the first go-around; my unit is butterflied out on my bench right now, so everything is either upside down, sideways, or both, depending on how I crane my neck to see anything.

author: G0HZU
author_rank: Super Contributor
timestamp: June 22, 2024, 09:22:24 pm
content: Thanks! That looks very similar to the plot from KE5FX.Mine is probably about 1-2dB noisier than the (noisy) image I posted up from the user manual. It used to be exactly the same as the image in the manual, but I think my analyser needs a service. The phase noise has become a bit fluttery at times and I think I will have some more capacitors to replace. It has been quite reliable for the 14 years I've owned it, and it was in regular use for the first 8 or 9 years.When you get the VNA set up, I think it's worth trying the input s11 test at 2.7GHz. You have to set the analyser to a narrow span at 2.7GHz to keep the YIG preselector in one spot. I would suggest using a 1MHz span or maybe less. Also, make sure you select 0dB attenuation and turn the VNA source power at or below 10dBm.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 22, 2024, 09:45:36 pm
content: Glad that was helpful! I would imagine the 8566 was on for about 10 minutes or so when I took that, so perhaps it would settle in a bit more after 30 minutes or so.I tried the VNA (that has it's own thread, now) and was not able to get much resolution. Just as you suggested, I set the 8566 to 2.7 GHz, 1 MHz span (I even tried 100 kHz), 0 dBm, and set the VNA to -10 dBm, 2.7 GHz, and several different sweep widths. I calibrated the VNA on each change of span. Even at 100 MHz, there was no discernable notching in the reflection anywhere across the span.

author: KE5FX
author_rank: Super Contributor
timestamp: June 22, 2024, 10:18:21 pm
content: Try entering ATTEN 0 dB on the 8566. You're probably looking at the YTX through the 10 dB attenuator (which is 20 dB once the VNA stimulus signal goes through it and comes back.)They make you enter 0 dB explicitly to help protect the mixer from static damage. Be very careful with ESD and signal levels while 0 dB ATTEN is in effect.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 22, 2024, 10:35:36 pm
content: 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?

author: G0HZU
author_rank: Super Contributor
timestamp: June 22, 2024, 10:39:39 pm
content: Give me some time and I'll power up my 8566B and an old 8714B VNA. Both are in storage (under the staircase!) but I can dig them out and power them up together. It's very late here in the UK so I won't be able to do this until tomorrow but I'll try and produce some images of the return loss you can expect to see if the YIG is tuning OK.As KE5FX warns, be careful of ESD, especially if you start taking things apart and checking stuff with a meter or other test gear.I think you should see a dip in s11 of at least 10dB at 2.7GHz and it should be about 20MHz wide. It might be a bit wider than this, but I think it will be no wider than 50MHz. You do have to select 0dB attenuation with the 8566B. With 0dB attenuation the return loss will probably only be 1-2dB outside the passband of the YIG and maybe 10dB to 15dB inside the ~20 MHz bandwidth of the YIG.This assumes that all is well with the YIG/mixer of course.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 22, 2024, 10:41:09 pm
content: Great info, thank you! That's a very generous offer to test your own 8566, it's much appreciated.

author: G0HZU
author_rank: Super Contributor
timestamp: June 22, 2024, 10:44:14 pm
content: Quote from: W4PJB on June 22, 2024, 10:35:36 pmIt 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.

author: KE5FX
author_rank: Super Contributor
timestamp: June 22, 2024, 10:45:17 pm
content: 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 itcouldbe 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.

author: KE5FX
author_rank: Super Contributor
timestamp: June 22, 2024, 10:50:15 pm
content: 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. BTDTAlso: use 0 Hz span on the 8566. No reason to sweep the filter while you're trying to measure it!

author: G0HZU
author_rank: Super Contributor
timestamp: June 22, 2024, 11:11:50 pm
content: 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.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 22, 2024, 11:14:14 pm
content: Quote from: KE5FX on June 22, 2024, 10:50:15 pmI 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.

author: KE5FX
author_rank: Super Contributor
timestamp: June 22, 2024, 11:15:54 pm
content: Post a photo of the 8566's screen?

author: W4PJB
author_rank: Regular Contributor
timestamp: June 22, 2024, 11:23:08 pm
content: Quote from: KE5FX on June 22, 2024, 11:15:54 pm

author: W4PJB
author_rank: Regular Contributor
timestamp: June 26, 2024, 01:29:12 am
content: 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.

author: KE5FX
author_rank: Super Contributor
timestamp: June 26, 2024, 01:46:51 am
content: Maybe I missed it, but did you ever check the YTXtuningcoil 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.

author: W4PJB
author_rank: Regular Contributor
timestamp: July 07, 2024, 12:51:05 am
content: 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 weekThe 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 downSo 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,notthe 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.

author: W4PJB
author_rank: Regular Contributor
timestamp: July 08, 2024, 10:20:19 pm
content: 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

author: G0HZU
author_rank: Super Contributor
timestamp: July 08, 2024, 11:23:28 pm
content: 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.

author: W4PJB
author_rank: Regular Contributor
timestamp: July 08, 2024, 11:39:57 pm
content: 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

author: W4PJB
author_rank: Regular Contributor
timestamp: July 09, 2024, 05:45:07 pm
content: 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.