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HP3577A 5Hz-200Mhz VNA Teardown

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Cubdriver:

--- Quote from: Berni on July 29, 2018, 11:56:43 am ---I am just in the process of repairing one of those exact CRT modules in my HP 4145B since the NewScope5 is a bit pricy

It uses the same switches on the front panel and mine also have an inconsistent feeling. Some are soft and some are clicky. I am guessing that all of them ware supposed to be clicky but the springs got bent enough with use that they don't provide the buckling action anymore. But i prefer the soft feel of the worn out ones. I was guessing if jabbing the spring with a Flathead screwdriver would bend it enough to "break" the clicky action of the switch.

--- End quote ---

Berni, actually the 'soft' ones are those that are working properly.  They're the bog-standard leaf spring switches that HP used in skazillions of instruments back in the day.  The leaf spring is supposed to have a slight curve in it that lets it bend smoothly, but in older instruments they get sticky and don't want to bend properly, resulting in the feeling you describe.

Below is a link to a YouTube video posted years ago by a guy showing how to remove and reform the springs.  I've recently read that the issue can be caused by old lubricant gumming up and preventing the springs from moving properly, and that it can be fixed by flushing the switches with alcohol to clean out the grease rather than taking them apart as shown in the video.

I've flushed a few of them with denatured alcohol and been successful in getting them working again, though some were stubborn and ultimately needed to come apart.

I'd try an alcohol flush first (you may need to hit them two or three times) before going to the trouble of pulling the springs.  Easy stuff first, right?

Here is the video link:
http://www.rbarrios.com/projects/HPSWITCH/

Good luck with it.

-Pat

Berni:
Oh thanks! I will give that a try.

Not only does my HP 4145 have this problem but also my HP 8566B, all use the same switches. I have expected them to be harder do disassemble but yeah will definetly try cleaning them first since doing this on every single switch could take a while.

sixtimesseven:

--- Quote from: Cubdriver on July 29, 2018, 05:59:38 pm ---
--- Quote from: Berni on July 29, 2018, 11:56:43 am ---I am just in the process of repairing one of those exact CRT modules in my HP 4145B since the NewScope5 is a bit pricy

It uses the same switches on the front panel and mine also have an inconsistent feeling. Some are soft and some are clicky. I am guessing that all of them ware supposed to be clicky but the springs got bent enough with use that they don't provide the buckling action anymore. But i prefer the soft feel of the worn out ones. I was guessing if jabbing the spring with a Flathead screwdriver would bend it enough to "break" the clicky action of the switch.

--- End quote ---


Berni, actually the 'soft' ones are those that are working properly.  They're the bog-standard leaf spring switches that HP used in skazillions of instruments back in the day.  The leaf spring is supposed to have a slight curve in it that lets it bend smoothly, but in older instruments they get sticky and don't want to bend properly, resulting in the feeling you describe.

Below is a link to a YouTube video posted years ago by a guy showing how to remove and reform the springs.  I've recently read that the issue can be caused by old lubricant gumming up and preventing the springs from moving properly, and that it can be fixed by flushing the switches with alcohol to clean out the grease rather than taking them apart as shown in the video.

I've flushed a few of them with denatured alcohol and been successful in getting them working again, though some were stubborn and ultimately needed to come apart.

I'd try an alcohol flush first (you may need to hit them two or three times) before going to the trouble of pulling the springs.  Easy stuff first, right?

Here is the video link:
http://www.rbarrios.com/projects/HPSWITCH/

Good luck with it.

-Pat

--- End quote ---

Nice link. How did he get the key of the switch (I mean the actuall boutton on top of the switch)? I tried to remove the keys since they have yellowed and I tought I could try the Peroxid/UV trick on them... But I didnt manage to remove them.

I had really sticky switches and all I did was spray them with WD-40 contact cleaner spray and they work perfectly now.

precaud:

--- Quote from: rx8pilot on July 29, 2018, 05:50:59 pm ---Did you build some AC coupled adaptors? Like you, I would prefer not to test my luck! Addressing this now could save any 3577A owner a lot of trouble.

--- End quote ---

Yes, I had a couple old Tek input capacitance normalizers which I repurposed. I used 100nF caps, which give -3dB @ 3.18Hz. There's probably room for 220nF's in there if I find that cutoff is too high. But I doubt that will be the case. If I need to examine xfer functions below 10Hz, an FFT-based DSA is a much better tool, anyway. Sweepers take forever at such low freqs.


--- Quote from: precaud on July 29, 2018, 02:51:03 pm ---: No built-in power splitter. A silly omission.
--- End quote ---

--- Quote ---Is it worth buying a commercial splitter for under 200Mhz or is it easy enough to DIY? Searching around for some info - they only seem difficult when the BW goes much beyond 500Mhz.
--- End quote ---

Probably not worth buying. I made mine, using closely-matched 1/2W axial MF resistors I had on hand. It's good beyond the 30MHz upper limit of the Anritsu VNA I mostly use. I've never used it with the 3577A. And given Berni's excellent results with 0402's, making one is a no-brainer.

For power supply/PDN stuff you probably won't be working above 10MHz, anyway. When you do, come back and tell us about it!  :)

Cubdriver:
I used one of those crook-necked lab squeeze bottles to flush the springs with alcohol - it lets you direct the flow and get some force behind it so it gets into the area under the spring.  Actuating the switch as you squirt it seemed to help, too.

Ideally you'd gently (so as not to dislodge the springs) blow them out afterwards with compressed air, but my compressor was down when I did the ones on my 8662A so I just let them air dry.

-Pat

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