Products > Test Equipment
HP 8566B - What to look for?
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G0HZU:
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?



KE5FX:

--- Quote from: W4PJB on June 08, 2024, 09:56:59 pm ---Thanks! 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?
--- End quote ---

Adding 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. 


--- Quote ---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.
--- End quote ---

No,  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.


--- Quote ---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.
--- End quote ---

REF 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...
W4PJB:
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!
KE5FX:

--- Quote from: W4PJB on June 10, 2024, 07:45:30 pm ---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!

--- End quote ---


 :-+ Definitely 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?)
W4PJB:
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 :)


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