Teardown: HP 8642B Signal Generator - Page 1
Forum section: electronics / testgear

author: idpromnut
author_rank: Unknown
timestamp: March 19, 2014, 11:36:04 pm
content: I have to confess something however... I started out by testing this piece of hardware (where the testing was VERY loosely done as I don't have the equipment to test this beauty very thoroughly), and then decided to see what was under the cover. When I finally popped the top, and saw all the sealed metal cans, I realized that I am not skilled enough to do this properly (i.e. without screwing it up). So enjoy the pictures, and I may the get courage to do a more complete teardown in the future

author: G0HZU
author_rank: Super Contributor
timestamp: March 20, 2014, 12:29:50 am
content: That's a very high spec generator you have thereI've often considered buying one of these fabulous generators but they are very big and extremely heavy so unfortunately I can't really accommodate one in my work areasI think they weigh something like 33kg?If it isn't faulty then I'd strongly recommend you don't take it apart any further.The key performance areas for that generator are high accuracy and low noise/spurious and I would not risk compromising the RF sealing of those screened enclosures for the sake of a 'teardown'However, it's up to you and I would certainly watch a teardown video if you did one...

author: alex.forencich
author_rank: Frequent Contributor
timestamp: March 20, 2014, 12:57:07 am
content: Quote from: idpromnut on March 19, 2014, 11:36:04 pmI have one of these myself, and it is a very nice unit. Mine does have an intermittent issue with the ALC components. I did do some poking around to try to figure out what might be going wrong, but I did not turn up anything obvious. The seals on the boxes are not terribly delicate as far as I can tell, but you have to be careful when removing some of them as at least one of them has an RF transistor bolted to the box.I have been mulling over doing a teardown and signal path walkthrough video, but I have not had the time. I do know a few things about how these RF circuits are designed and I figured it is about time someone shed some light on the 'black magic' of RF design. Sadly, I have been far too busy lately.Also, they actually include a wrench for the SMA and SMC connectors. Take a look near the fan. I also shelled out for the complete service manuals on CD for the unit. Gobs and gobs of PDF'd schematics and block diagrams. Unfortunately, most of the repair advice is 'replace the faulty module'. Sigh.

author: idpromnut
author_rank: Unknown
timestamp: March 20, 2014, 02:46:37 am
content: alex.forencich, I would love I video (or few!) on even the broad strokes of some of the RF magic involved in these kinds of generators.And G0HZU, I hear you! I weighed this unit at 76.5lbs !!! I bought it off a local ebayer who wanted 150$CDN to ship it to me... I ended up going to pick it up myself. I thought the dude was a wimp when he waddled out with the unit in both hands. Imagine my shock at lifting it upAlso, I got this unit super uber duper cheap at 200$CDN. The guy was a dealer primarily in computer parts and equipment and so had no idea how to test or what this unit was reasonably worth I think. Happy times.And yes, I think I will leave the beast intact for the moment. I imagine that there will come a time when I will need to fix it, at which point I will post pics and more complete teardown.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: March 20, 2014, 02:57:38 am
content: Quote from: idpromnut on March 20, 2014, 02:46:37 amThat's a darn good price for the unit. I think I paid about $400 US for mine. The dude was actually selling it for parts or not working because it when he turned it on it displayed "err: ref oven cold" - which is actually a good thing! It has the high stability OCXO reference, but it also has the rear connector option. Which is not a big deal; I just have an SMA cable that I leave connected to the output on the back.Also, my generator is at the bottom of a stack with an RF switch unit and five power supplies on top of it. Not so easy to get to!

author: idpromnut
author_rank: Unknown
timestamp: March 20, 2014, 03:01:33 am
content: Yup, sounds about right; I have option 1 on mine as well, but no option 3. But no biggie as I Macgyver'd a chassis to mount it in.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: March 20, 2014, 03:09:02 am
content: Quote from: idpromnut on March 20, 2014, 03:01:33 amThat actually looks pretty darn good for a Macguyver. I wonder if it would be a good idea to add a cross piece on the bottom to hold the angle, taking some of the force off of the rack ears and top mounting.

author: idpromnut
author_rank: Unknown
timestamp: March 20, 2014, 03:13:05 am
content: I was actually wondering how this would work in a rack... I didn't see any mounting ears or bars in the back of the unit but with the weight I would think it would need to be a 4 post mounting in a rack.There is actually very little horizontal stress on the ears... without the bolts it actually balances in about that angle. Totally intended that

author: alex.forencich
author_rank: Frequent Contributor
timestamp: March 20, 2014, 03:16:43 am
content: Quote from: idpromnut on March 20, 2014, 03:13:05 amYeah, I was thinking about if I racked mine up, it would most certainly have to go on a 4 point shelf. No way all that weight would be supported just on the front rack ears. Mine actually didn't even come with rack ears, not that it would have made all that much difference.It's intersting that the unit balances at that angle, I suppose the center of gravity is quite a bit closer to the bottom than the top. Not what I would have guessed. Go figure.

author: G0HZU
author_rank: Super Contributor
timestamp: March 20, 2014, 12:34:24 pm
content: I would expect that there will be some kind of sliding rack bar available from HP if you want to rack mount it. I bought an old HP analyser a few years ago that had a long heavy duty sliding rack bar kit fitted along each side. It allowed the analyser to slide in and out of a rack like a drawer.I got rid of the heavy duty rack bars because they are unsightly and get in the way but from memory, the analyser had the side handle straps removed and the rack bars screwed to the vacant side handle strap fixings and it was also designed to fit snugly into the handle strap recess. There may have been other fixing points because I remember removing lots of screws. I found a set of surplus carry handle straps to fit to my analyser (to get it back to standard) and I now have it on a sturdy trolley rather than a rack system.

author: TerraHertz
author_rank: Super Contributor
timestamp: March 21, 2014, 04:41:35 am
content: Quote from: idpromnut on March 20, 2014, 03:13:05 amThe usual way is sitting the unit on simple L slides in the rack. The front brackets then only act to prevent it falling out.Lightweight equipment can be mounted by the brackets only, but when you're lifting heavy units in and out of a rack, you definitely want it sitting on slides.It also means each unit can be removed independently.Edit: some equipment comes with mounting holes for extending slide rails, that let the unit sit in the air in front of the rack for servicing. IF you have extending anti-tilt foot bars on the rack! But annoyingly when you get second hand equipment that was once mounted on extending rails, almost invariably you only get half the rail set. They unclip, and sellers just unclip them, leaving half in the rack. Infuriating, but that's the way it is.That 8642B is really nice. And they are sometimes affordable I see. Added to my want list.Looking at the service manual pdf you bought, does it include ALL the schematics, for every module? (Personally I prefer to buy original paper manuals, since they're much more usable on the bench. But that's just me.)

author: alex.forencich
author_rank: Frequent Contributor
timestamp: March 21, 2014, 06:11:24 am
content: Quote from: TerraHertz on March 21, 2014, 04:41:35 amIt is a nice unit. Definitely try to get one with the plugs in the front, though, if you can. It's a little inconvenient having everything on the back, like on mine.The service manual has complete schematics, including every individual resistor, capacitor, inductor, transistor, IC, front panel button, etc. It's four volumes, each one is between 300 and 700 pages. Along with a LOT of troubleshooting information. This glorious set of PDFs is something more than worthy of the term 'service manual', unlike some of the other 'service guides' I have seen that have no real useful information at all. I got it from Artek Media for $25, IIRC. The four PDF files are 140 MB.

author: KJDS
author_rank: Super Contributor
timestamp: March 21, 2014, 08:22:09 am
content: Thanks for the teardown. I'm picking up a 8640B next week, I'll remember to take a weightlifting belt if it's that heavy.

author: idpromnut
author_rank: Unknown
timestamp: March 21, 2014, 11:35:09 am
content: I forgot to take pictures (twice!) but I have to replace one of the BNC's from the front panel. I ordered the BNC shields from Agilent (they had 14 left in stock after my purchase).The interesting thing about the front panel is that it is mounted on a hinge on the left side. So with a couple of screws on the top and bottom of the front panel removed, the whole front assembly just swings out. Very nifty design and saved me from pulling the entire instrument apartIf I pull the unit again I will take and post pics of this (but don't hold your breath, that thing is HEAVY).

author: AlphZeta
author_rank: Regular Contributor
timestamp: September 09, 2014, 01:53:14 am
content: OK, I just did a full teardown of mine.https://www.eevblog.com/forum/testgear/a-full-teardown-of-hp-8642b/

author: Mr Simpleton
author_rank: Unknown
timestamp: September 09, 2014, 08:29:33 pm
content: If I recall correctly the 8642 have superior phase noise close in at carrier, but 8662/3 do have the edge a bit further out.And yes lifting the 8662/3 is not for wimps eitherWould not mind have any of these at my lab....

author: idpromnut
author_rank: Unknown
timestamp: January 07, 2015, 01:26:57 am
content: So, apparently I spoke too soon about this signal generator; it looks like this will turn into a bit of a repair project. I seem unable to set an amplitude of less than 0dBm.