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HP 8662A Signal Generator - Broken RF connection to front panel N connector
Posted by aa7ae on 15 Feb, 2022 04:51 -
I was working on this HP 8662A signal generator fixing the bad display board when this happened!
(See photo)
The RF connection leading from the reverse power protection circuit to the N-Connector on the front panel has busted. I don't know why they have to use this hardline stuff, it's not designed for moving back and forth like this over time.
How can I fix this and make it more future proof? The connection on the reverse power protection is not a standard type. It's basically hardline shoved in there and tightened down with a nut and washer. urrrgh!
Thanks for anyone's ideas!
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Standard connector
YES it is, it is an SMA, just fit a new one.
G Edmonds -
I think the idea is to remove the nut holding the 'N' connector to the front panel before pulling the panel straight out and folding it down. Can't remember exactly how it works, but there is a way to pull the panel safely while leaving the hardline connected.
Look around on eBay for MiniBend SMA jumpers, it's good stuff. E.g., this or this. It doesn't have to be 0.141, 0.086 is fine.
Of course, that'll only work if the reverse power module really does have an SMA-F jack, and if you don't mind losing the common-mode choke on the outside of the cable. It's unlikely you'll notice the latter, as these things were never the most leakproof signal generators to begin with. You can always use a clip-on ferrite or two. But obviously the former is a showstopper.
Be sure to swap out the NiCd batteries while you're in there. -
The N connector nut is on the wrong side to be removed.
The SMA is a crimp on, from the photograph, these are not a good solution.
The Outpur from the RPP module does NOT look like an SMA connector.
You should NOT remove the two screws retaining the N mounting plate, if you do the hard line will be dammaged every time.
G Edmonds -
The N connector nut is on the wrong side to be removed.
The instructions in the manual make no sense, as they actually tell you to pull on the RF connector:
So it must work like the 8663A, in which you indeed remove the N connector nut and pull the panel straight out, leaving the connector in place.
QuoteYou should NOT remove the two screws retaining the N mounting plate, if you do the hard line will be dammaged every time.
Correct, if it's like the 8663A those two screws need to remain installed. In his photo, it looked like the N connector bracket was welded to the panel frame, but I can see now that's not the case.
Not going to be fun to fix that broken hardline.
I think I'd try soldering the connector back on, actually. If the center pin of the .141 hardline can be straightened out, it might work OK. The risk is that the SMA female center pin will be damaged if it's crooked.
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The old SMA can be recovered. Remove the the old tin soldered, push back the center pin, the live pin, remove the bad section of rigid cable, and solder the connector again. Make and solder rigid cable with connectors is easy when you catch the trick.
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Hi
Both of my 8662A's have knurled nuts on the outside of the N connector like the 8663A.
It is hard to see from the photograph, but it looks like the hardline may be aluminium jacketed and cannot be soft soldered to, hence the crimp SMA.
G Edmonds -
Thanks everyone for your replies, it's eye opening. I wish I had known about the situation of the N-Connector before I opened the front panel.
I was able to press the SMA connector back on and re-solder it by adding some flux and it seemed to take solder well, but I can't say the integrity of this cable is very good now.
I did find one on E*BAY, but they want $50 for it including $10 shipping which seems a lot for this tiny bit: https://www.ebay.com/itm/351215935948?hash=item51c619e1cc:g:50sAAOSwGWNUWSvZ
The original problem I had is the last digit of the display for the frequency was acting up. Dots would randomly disappear or go completely out on that digit. I was able to find another A1A1 display board for $18 on E*BAY.
Just swapping the single digit didn't work, but problem persisted, so there is something wrong with this board, either with bad solder joints or a crack in the board itself. Neither of which I could see from inspection.
I fitted the E*BAY A1A1 board and now the digit seems to be OK, although it doesn't display a 0 only 1-9 which must be by design (milihertz). It does display 0 if I press status button as expected.
These single dot-matrix digits themselves are rare and expensive. I saw some on E*BAY for at least $20. Broadcom makes a modern replica of them, but they're over $50 each on digikey. Really crazy.
If I had purchased one of these digits, the problem wouldn't have been solved.
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I've had a few of these generators with flaky LED displays, and have never found the smoking gun. Taking everything apart and putting it back together seems to fix it for a few months or even years, but the problem always comes back.
Each group of digits has its own rear-mounted heat sink on the ones I've seen. The heat sinks are usually loose, rattling around with nothing but a few specks of dried silicone between them and the back of the LED elements, and it's never been clear to me how they were supposed to be held in place. I doubt the display glitches are a heat-related problem, since they always seem to get better when the various connectors are removed and replaced, but those heat sinks have always bugged me.
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How about shortening the OEM semi-rigid and attach a new SMA F or M, then use a short pig tail with flexible coax to N connector? I doubt you'll even notice the difference.