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Old 26th Dec 2020, 8:16 pm   #1
Alan_G3XAQ
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Default HP8640B reed relays in RF filter

My "good" HP8640B signal generator, with brass frequency range select gears, has joined ranks with my other two and is poorly. I have no output on the lower ranges and on the lower half of the upper ranges. I can hear the relays "tick" as I tune from the high (working) to the low (not working) part of the upper ranges. The relay coil voltages seem to switch OK so I suspect one or more of the 4 relays in the A10 RF Filter unit is faulty. Yes?

The part number is Elec-trol 0490-1073. It's a 5 volt coil with SPCO contacts. I have failed to find a replacement or even a datasheet. Can anyone help? Is it anything special?

Thanks, Alan

Last edited by Alan_G3XAQ; 26th Dec 2020 at 8:26 pm.
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Old 27th Dec 2020, 6:26 am   #2
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Default Re: HP8640B reed relays in RF filter

Picture of relay?

Andy.
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Old 27th Dec 2020, 10:35 am   #3
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Default Re: HP8640B reed relays in RF filter

Might be unlikely that several relays have failed together. Try isolating the contacts and checking operation before looking further.
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Old 27th Dec 2020, 10:53 am   #4
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Default Re: HP8640B reed relays in RF filter

I was asking more from the angle of wondering if anyone knew if HP had chosen something unusual. Perhaps esoteric contact material or unusual coil resistance or maybe an inbuilt flyback protection diode.

I have another HP8640B so after sleeping on it I think I'm going to swap the entire filter board across.

Thanks, Alan G3XAQ
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Old 27th Dec 2020, 5:57 pm   #5
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Default Re: HP8640B reed relays in RF filter

A French chap on the HP groups.io group found some genuine NOS relays on eBay so I've invested there.

73, Alan
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Old 27th Dec 2020, 6:19 pm   #6
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Default Re: HP8640B reed relays in RF filter

Given the lengths of the tracks shurly any DIL SPCO reed relay will do?
 
Old 27th Dec 2020, 6:38 pm   #7
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Default Re: HP8640B reed relays in RF filter

Depends on what frequency they're switching. These are switching filters in an RF sig gen. I don't know these, but some people put type C reeds inside grounded aluminium tubes to create a coax structure. The coil went round the outside.

David
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Old 29th Dec 2020, 5:40 pm   #8
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Default Re: HP8640B reed relays in RF filter

I've swapped the filter board for the one in my scrapper while I await a replacement relay in the post. Sadly the 30-way Cinch edge connector on that one is cracked at the end. I tried epoxy in the crack (clamped overnight while it set) and the sig gen did work for a while until the crack opened again so now I'm trying a splint of PCB material epoxied across the whole area.

While warming the adhesive with a hair drier the Rifa PME271Y522 EMC capacitor across the mains let go with a cloud of blue smoke. I've just clipped it out for now. I believe this make has a bad reputation. Which type/brand do folks favour as a replacement?

And the transistor array chip to repair my HP8568B that I ordered on the 17th *still* hasn't arrived. Sigh.

Alan
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Old 30th Dec 2020, 1:18 am   #9
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Default Re: HP8640B reed relays in RF filter

Quote:
Originally Posted by Alan_G3XAQ View Post
While warming the adhesive with a hair drier the Rifa PME271Y522 EMC capacitor across the mains let go with a cloud of blue smoke. I've just clipped it out for now. I believe this make has a bad reputation. Which type/brand do folks favour as a replacement?

Alan
Anything that is a paper based class X/Y type made by either RIFA (now Kemet) or WIMA should be avoided, look for either metallised PP or PET or ceramic, finding them with long enough leads seems to be harder these days.

Examples of the sections to search through;
https://uk.farnell.com/c/passive-com...ety-capacitors
https://uk.farnell.com/c/passive-com...ion-capacitors

David

Last edited by factory; 30th Dec 2020 at 1:27 am.
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Old 30th Dec 2020, 9:14 am   #10
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Default Re: HP8640B reed relays in RF filter

Might have a spare 30 way Cinch connector,will look. That PCB is a work of art, I'd rather stick it on my wall than a Van Goth.

Andy.
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Old 30th Dec 2020, 1:03 pm   #11
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Default Re: HP8640B reed relays in RF filter

The epoxy/PCB splint seems to be holding for now. I agree it does rather destroy the Rembrandt appearance but at least it is buried out of sight. Perhaps I can revisit this if/when the repair fails.

OK on the correct replacement Y capacitor. Would it be very wicked to leave it out and put the covers back on?

On the plus side, the donor unit has the OPT 003 reverse power protection option so I'll swap that across. And the tiny angled 1/4" AF spanner was still in the clip on the side of the attenuator. The OPT 002 1024MHz doubler option is there too but AIUI it's not something for mortals to swap across and I'm not a VHF aficionado anyway.

Thanks all,

Alan G3XAQ
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Old 30th Dec 2020, 3:24 pm   #12
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Default Re: HP8640B reed relays in RF filter

I have no options fitted to my 8640B, but have found instructions for fitting the internal doubler (option 002).
HP 11698A 8640B Internal Doubler Kit.pdf

From what I can see, there is a replacement A26A1 board & the output amplifier hybrid gets swapped over to the new board, a wire link is added too.
A relay & diode are removed from the A26A2 board & a link fitted and two resistors on the A26A4 board are changed. A low pass filter is also fitted between the output amplifier board & the attenuator assembly and another wire link added to the A9 connector.
The only other changes are cosmetic, i.e. replacement range knob & option sticker.

There are a couple of adjustments required and performance tests (five of which are labelled as being critical).

The external doubler seems a lot simpler.

I did see an option 001 board (variable audio oscillator) offered for sale a while back, but decided it was useless without the correct control knobs to go with it.

David

Last edited by factory; 30th Dec 2020 at 3:33 pm.
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Old 31st Dec 2020, 3:43 pm   #13
Alan_G3XAQ
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Default Re: HP8640B reed relays in RF filter

Yes, David, it looks fiddly to install but particularly difficult. The "critical" testing for harmonics is beyond the ability of my test gear. I'll leave it in the scrapper for now.

My HP6840B #2 does have OPT 002 factory installed (and 001 too, but not the more useful 003 reverse power protection). I switched it on to check it works OK before putting it back on the shelf as I reassemble my shack/workshop after redecoration. Guess what? It had a fault. This one seems to be the divide by 64 prescaler in the display module. It reads all zeros on Internal and when fed with an External signal it works on the 0-10MHz range but reads zeros on the 0-550MHz range. Never having been inside the counter before I pulled it out of the chassis and opened the box. Hmm, two PCBs face-to-face with a short riser and Cinch connectors. Not having an extender board I can't look for the fault so I've just swapped over the complete counter module from my #3 scrapper.

These faults on 40+ year old gear is making me increasingly uneasy but as David has said before there is no commercial market for low noise signal generators these days. So what to do? My Marconi 2019A is a fair bit younger but isn't as quiet.

I keep looking at the Analogue AD9910 datasheet and thinking the performance curve in fig 15 looks pretty reasonable at low HF. But the proper eval board is over £500. There again, there is a Ukrainian PCB on eBay for a tenth that price. Hmm, I wonder why? And there would still be the challenge of homebrewing a switched attenuator with a 150dB range. I suppose if it was easy Rigol and Siglent would be selling this kind of stuff for a grand.

73, Alan G3XAQ
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