HP 8640B Repairability - Page 1
Forum section: electronics / repair

author: ignilux
author_rank: Unknown
timestamp: March 11, 2019, 04:30:44 pm
content: Hi, all-Having just acquired a 8568B for a helluva bargain, I'm quickly realizing that I need a signal generator that goes above the 12MHz top end of my 3312A. Clearly being a vintage HP fan boy, I went searching for an older HP RF sig gen. There is a 8640B locally to me, but it is listed as having "no RF output" and a bent knob. I could, of course, ask the seller what they've tested so far, but I'm hesitant in case they happen to find an easy fault and jack up the price. Here's the meat of my post:Is there anyone out there who can describe the relative repairability of these units? I did a google and forum search, but didn't find anything as far as this specific model goes. AFAIK the main obstacles would be the extensive RF isolation and shielding inside, and the fact they use a custom ceramic hybrid for the output stage. It's a huge, heavy unit with daughterboards protruding vertically from a horizontal motherboard on the bottom of the chassis, so that aspect is a definite plus. Anyway, any input here would be very appreciated.Thanks in advance!

author: bd139
author_rank: Super Contributor
timestamp: March 11, 2019, 04:51:02 pm
content: There are a ton of problems with the gears and leaf siwtches falling to bits in them and a lot of the parts aren't replaceable now. If it has been dinked I wouldn't touch it.However, there are suppliers of replacement gears for them!https://www.ebay.com/itm/153024403695If it's relatively cheap I'd grab it as a parts mule in case a good one comes along.33312A is a very nice bit of kit though. Keep that around

author: ignilux
author_rank: Unknown
timestamp: March 11, 2019, 05:01:32 pm
content: That was my gut feeling as well. I narrowly missed out on a low-priced 8657A, maybe I'll wait for another to come around. If I'm not mistaken, though, the 8640 was one of the last all-analog sig gens that HP made. 8657 is synthesized-- certainly not the end of the world, but there's a certain elegance to having an analog signal path.Also, the 3312A has served me well. That one came at a great price, too, as it was listed as not working. Knowing how many mechanical switches and knobs there were, I took a chance and basically immersed the unit in Deoxit. Ta-da! Good as new

author: bd139
author_rank: Super Contributor
timestamp: March 11, 2019, 05:08:03 pm
content: Keep your eyes peeled - lots of good RF gens creep past. 8640B is sort of synthesized and is mostly digital actually. There's a cavity oscillator which is phase locked and then a whole bunch of digital dividers off that followed by filters and attenuators. Edit: and an ALC feedback loop.There's a simplification here which works the same as an analysis point:https://www.cwtd.org/AnalyzeThis/8640jr.pdfhad a 3312A as well with the same problems. Quick squirt and it was good as new. Regret selling it if I'm honest. Thing of beauty inside.

author: 0culus
author_rank: Super Contributor
timestamp: March 11, 2019, 07:39:25 pm
content: I sent you a PM about an 8657A that I have.As bd139 pointed out, the major weakness of the 8640b is the mechanical front panel. I have an HP spectrum analyzer with a similar type of front panel (8569B) that I was lucky to get intact with only minor issues. You just want to be ginger with the controls. Don't throw them around. Other than that, it is a solid performer for it's bandwidth.