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Checking sampling diodes


 

I acquired an S-1 sampling head for cheap, "Untested, for parts only" (Yeah, right, since it came from a surplus test equipment company!). We all know what that means.

Anyhow, as I expected, it doesn't work properly. From the bizarre trace I think that one of the two sampling diodes has failed, probably from input overload.

How would you recommend initial-checking them at DC? After looking at the specs for a replacement kit from Tek (https://groups.io/g/TekScopes/topic/7636255), it appears that 9 volts reverse, and at least 1 ma forward, should be ok. My Fluke 73 has a diode check function but I need to measure the max current available. I think it's 1 ma. Or I can just apply a little DC to the input connector and measure current to ground from the bridge outputs (pins H & I from the sampling board), which have 200 ohms in series with each diode.

If they're bad, it looks like there are plenty of Russian GaAs diodes available. The specs probably aren't quite as critical since this is a 350 ps head, not a 25 ps...

Thoughts?


snapdiode
 

Sampling diodes aren't tunnel diodes. The S-1 is pretty low bandwidth, any reasonable Schottky will work.


 

Yes, thank you, I already mentioned that :)
So, once again, how electrically fragile are they? Will I damage a good one with 1 ma of DC forward current?


snapdiode
 

You said " Russian GaAs diodes", I assume you refer to the mountains of Russian tunnel diodes, since Schottky diodes aren't GaAs.

Well, for the diode test, I'd stick a resistor in series just to be sure.


Tom Lee
 

As others have said or implied, a 1GHz sampler is not demanding of the diodes. HP/Avago/Broadcom HSMS-281x (obsolete but still not hard to find) Schottkys are a close electrical match (a slight upgrade, actually). Your principal challenge will be mechanical if you do need to replace them.

The stock diodes are hardy enough to survive testing with a regular DMM's diode test function. But if you are a "belt and suspenders" guy, go ahead and add a 1k series resistor.

Cheers,
Tom

Sent from an iThing; please excuse the terseness and typos

On Feb 13, 2021, at 7:53, "Charles" <charlesmorris800@centurytel.net> wrote:

I acquired an S-1 sampling head for cheap, "Untested, for parts only" (Yeah, right, since it came from a surplus test equipment company!). We all know what that means.

Anyhow, as I expected, it doesn't work properly. From the bizarre trace I think that one of the two sampling diodes has failed, probably from input overload.

How would you recommend initial-checking them at DC? After looking at the specs for a replacement kit from Tek (https://groups.io/g/TekScopes/topic/7636255), it appears that 9 volts reverse, and at least 1 ma forward, should be ok. My Fluke 73 has a diode check function but I need to measure the max current available. I think it's 1 ma. Or I can just apply a little DC to the input connector and measure current to ground from the bridge outputs (pins H & I from the sampling board), which have 200 ohms in series with each diode.

If they're bad, it looks like there are plenty of Russian GaAs diodes available. The specs probably aren't quite as critical since this is a 350 ps head, not a 25 ps...

Thoughts?





 

Sorry, I skipped a couple steps in my train of thought. There are also lots of GaAs diodes (not tunnel diodes) from Russia, for mixing and microwave applications. I figured on using a pair of those to make sure I'd meet the 350 ps spec.

Anyway I'll read some Schottky diode spec sheets and pick one, unless you have a p/n to suggest?

Meanwhile... I did check with a power supply set to 1 volt, and a 1K series resistor. Sure enough, one of the diodes was open. So I carefully removed the holder per the manual without incident... but while manipulating it with a hemostat and test probes, I not only managed to break the aged and brittle plastic holder in two,. but also pull the lead off one of the incredibly tiny diodes. I think it's the good one, too. Oh well, they are supposed to travel in pairs anyway.

It looks like I can either fabricate a mount to span the gap in the PC board where the holder slid in, or perhaps carefully solder the diodes (with very short leads) directly across the gap. As we've noted, 350 ps is not quite as critical as the faster heads are.


 

On Sat, Feb 13, 2021 at 11:44 AM, snapdiode wrote:


You said " Russian GaAs diodes", I assume you refer to the mountains of
Russian tunnel diodes, since Schottky diodes aren't GaAs.
Some are.
https://www.mouser.com/datasheet/2/249/MA4E20xx_Series-938308.pdf


Tom Lee
 

The HSMS-2812 is a dual Schottky that can replace that pair well.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/13/2021 14:45, Charles wrote:
Sorry, I skipped a couple steps in my train of thought. There are also lots of GaAs diodes (not tunnel diodes) from Russia, for mixing and microwave applications. I figured on using a pair of those to make sure I'd meet the 350 ps spec.

Anyway I'll read some Schottky diode spec sheets and pick one, unless you have a p/n to suggest?

Meanwhile... I did check with a power supply set to 1 volt, and a 1K series resistor. Sure enough, one of the diodes was open. So I carefully removed the holder per the manual without incident... but while manipulating it with a hemostat and test probes, I not only managed to break the aged and brittle plastic holder in two,. but also pull the lead off one of the incredibly tiny diodes. I think it's the good one, too. Oh well, they are supposed to travel in pairs anyway.

It looks like I can either fabricate a mount to span the gap in the PC board where the holder slid in, or perhaps carefully solder the diodes (with very short leads) directly across the gap. As we've noted, 350 ps is not quite as critical as the faster heads are.




 

Thanks! I just looked at Mouser and they are non-stocked, obsolete. I'm sure I can find them elsewhere, but is there something you can recommend that is more recent?

Would these work? They're only 0.8 pf (vs. 1.3 pf for the HSMS-2812):
https://www.mouser.com/datasheet/2/348/rb886cst2ra-e-1017124.pdf


Tom Lee
 

Yes, as I mentioned, they're obsolete but still widely available (but not from the usual distributors). An example is http://www.danssmallpartsandkits.net/

Dan's has HSMS-2825 dual diodes, 10 for a buck, plus shipping. These are two isolated diodes in one package. They'll be well matched and thermally coupled. They're lower in capacitance than the stock diodes, so they'll exhibit a bit less kickout, which is nice. I've used the 282x series for lots of stuff at 1GHz. They're my go-to diodes for that frequency range. They're not super-delicate (still, observe appropriate ESD protocols), so you don't have to worry about a cross-eyed look zapping 'em from across the room. The only ding on Dan is that his packages come with a bonus supply of tobacco smoke. Open them up outdoors. Seriously.

I see that our very own Brad Thompson (https://radioexchange.tripod.com/id12.html) might have some HSMS-2815, as well. These diodes are very close to the stock diodes in specifications. And not only is Brad a damn fine gentleman, the air in his packages seems to consist almost entirely of oxygen and nitrogen.

The Mouser diodes you linked to look like they'd work fine as well, so you have lots of options.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/13/2021 15:30, Charles wrote:
Thanks! I just looked at Mouser and they are non-stocked, obsolete. I'm sure I can find them elsewhere, but is there something you can recommend that is more recent?

Would these work? They're only 0.8 pf (vs. 1.3 pf for the HSMS-2812):
https://www.mouser.com/datasheet/2/348/rb886cst2ra-e-1017124.pdf




 

Thanks for the recommendations. Dan has raised his postage rate to $8.00... and Brad's email address bounced (at least the one on the website you linked, which was updated last in 2008.

Anyhow, I ordered from Digi-Key:

https://www.digikey.com/en/products/detail/MA4E1339B1-1146T/1465-MA4E1339B1-1146TCT-ND/12820007
(which is their recommended substitute for the HSMS-2812), five at $1.53 each (won't break the bank);

and ten of the Rohm RB886CS single diodes I asked about earlier, at only 34.8 cents apiece.

Since I ordered multiple spares, that guarantees I'll never blow another one (kind of like carrying an umbrella prevents rain) ;)

Any advice on mounting issues? If I have to extend leads I assume a short wide piece of brass or copper foil is the lowest inductance. I think the SOT package will be just a hair short of spanning the gap in the board.


Tom Lee
 

Hi Charles,

You'll certainly have enough diodes to prevent rain for quite a long while! If you ever need to induce a longer drought, you can find Brad's posts here and use that address to contact him.

As far as extending the leads, copper foil tape cut to about the same width as in the original would be just fine. At 1GHz, you've got some freedom to be a little sloppy and still enjoy success.

--Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/13/2021 17:56, Charles wrote:
Thanks for the recommendations. Dan has raised his postage rate to $8.00... and Brad's email address bounced (at least the one on the website you linked, which was updated last in 2008.

Anyhow, I ordered from Digi-Key:

https://www.digikey.com/en/products/detail/MA4E1339B1-1146T/1465-MA4E1339B1-1146TCT-ND/12820007
(which is their recommended substitute for the HSMS-2812), five at $1.53 each (won't break the bank);

and ten of the Rohm RB886CS single diodes I asked about earlier, at only 34.8 cents apiece.

Since I ordered multiple spares, that guarantees I'll never blow another one (kind of like carrying an umbrella prevents rain) ;)

Any advice on mounting issues? If I have to extend leads I assume a short wide piece of brass or copper foil is the lowest inductance. I think the SOT package will be just a hair short of spanning the gap in the board.




Brad Thompson
 

Charles wrote on 2/13/2021 8:56 PM:

Thanks for the recommendations. Dan has raised his postage rate to $8.00... and Brad's email address bounced (at least the one on the website you linked, which was updated last in 2008.
<snip>
...Earlier, Tom wrote...

I see that our very own Brad Thompson (https://radioexchange.tripod.com/id12.html)
<snip>

Hello, all--

Well, *that's* embarrassing. Several years ago, a gentleman asked if I'd like to add my parts-FS list to
a ham gear and components-FS marketplace (radioexchange) that he was building on the tripod site. Sure, I said,
what could possibly go wrong?

Somehow, the gentleman lost control of his marketplace web site and thereafter could neither edit nor delete
his site-- thereby stranding my parts list forever.

To compound the problem, a few years later my original e-mail provider (valley.net) went out of business
invalidating my e-mail account.

I apologize for the confusion.

Fortunately, I still have small quantities of most components listed on the frozen site, so don't hesitate  to
inquire about price and availability. One of these days (*), I'll edit the list and make it available
on demand.

You can reach me at...

brad.thompsonaa1ip@gmail.com

Thanks, and 73--

Brad  AA1IP

(*) A 577 curve tracer awaits my attention


Dave Wise
 

?If the original diodes went in clips like the 1S1, you could use my technique. I took a bit of bare FR-4, ground it into a "T" shape with the crossbar fitting the clips, glued the SOT-23 diode in the middle, wrapped each end with a few turns of fine wire?, and soldered to the diode. Why a "T"? To give you something to grip while working on it. It was pretty easy to make four of these.


HTH,

Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Charles via groups.io <charlesmorris800=centurytel.net@groups.io>
Sent: Saturday, February 13, 2021 5:56 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Checking sampling diodes

Thanks for the recommendations. Dan has raised his postage rate to $8.00... and Brad's email address bounced (at least the one on the website you linked, which was updated last in 2008.

Anyhow, I ordered from Digi-Key:

https://www.digikey.com/en/products/detail/MA4E1339B1-1146T/1465-MA4E1339B1-1146TCT-ND/12820007<https://www.digikey.com/en/products/detail/MA4E1339B1-1146T/1465-MA4E1339B1-1146TCT-ND/12820007>
(which is their recommended substitute for the HSMS-2812), five at $1.53 each (won't break the bank);

and ten of the Rohm RB886CS single diodes I asked about earlier, at only 34.8 cents apiece.

Since I ordered multiple spares, that guarantees I'll never blow another one (kind of like carrying an umbrella prevents rain) ;)

Any advice on mounting issues? If I have to extend leads I assume a short wide piece of brass or copper foil is the lowest inductance. I think the SOT package will be just a hair short of spanning the gap in the board.


 

"?If the original diodes went in clips like the 1S1, you could use my technique. I took a bit of bare FR-4, ground it into a "T" shape with the crossbar fitting the clips, glued the SOT-23 diode in the middle, "

Unfortunately they do not. There is a (brittle and now broken) white plastic holder for both the sampling diodes, which slides into a slot in the board. D6 in the manual picture, (D5 is in the same holder on the other side).
https://groups.io/g/TekScopes/photo/260744/0
I want to add as little capacitance and inductance as I can to the SOT package leads, too.


Siggi
 

On Sun, Feb 14, 2021 at 12:15 PM Charles <charlesmorris800@centurytel.net>
wrote:

"?If the original diodes went in clips like the 1S1, you could use my
technique. I took a bit of bare FR-4, ground it into a "T" shape with the
crossbar fitting the clips, glued the SOT-23 diode in the middle, "

Unfortunately they do not. There is a (brittle and now broken) white
plastic holder for both the sampling diodes, which slides into a slot in
the board. D6 in the manual picture, (D5 is in the same holder on the other
side).
https://groups.io/g/TekScopes/photo/260744/0
I want to add as little capacitance and inductance as I can to the SOT
package leads, too.
There are good photos of the holder with the diodes in place on the TekWiki
page: https://w140.com/tekwiki/wiki/S-1. The clip would lend itself to SLS
printing such as with ShapeWays - I see people printing these clips (
https://www.shapeways.com/product/CGNES54EY/tektronix-351-0449-00-pcb-bracket)
occasionally, and I've never had a complaint that they've failed. It would
also be possible to machine a replacement, though it would have to be done
in two halves due to the "facial features" on the inside of the clip.

How you'd mount your replacement diodes, though, I'm not sure.


 

It is going to be interesting, certainly.

Now waiting on Digikey to mail them. The SOT-323 package (two series diodes) will be hard enough to work with. I just looked at the dimensions for the individual Rohm diodes - the package is just 0.9 x 0.6mm (.035" x .024") wide! I don't think my hands are steady enough to handle and solder them, although I do have a tiny Oryx 6 watt pencil iron and forceps. Another reason I bought spares...Time to break out the dissecting microscope!

I think the best approach will be to mount the dual diode package by its common pin (to the board's input trace). Then it will be in the right location and I can solder bits of copper foil to the board and tack them to the remaining two pins. We shall see.


snapdiode
 

On Sun, Feb 14, 2021 at 09:15 AM, Charles wrote:

Unfortunately they do not. There is a (brittle and now broken) white plastic holder for both the sampling diodes, which slides into a slot in the board. D6 in the manual picture, (D5 is in the same holder on the other side). https://groups.io/g/TekScopes/photo/260744/0

Fantastic... That looks like it's made of Tektronix's go-to material in the 1960s, Delrin, ungluable by mere mortals.

This also looks like the clippy in the 1S2...


Tom Lee
 

Delrin is a bit of a challenge, isn’t it? A company called Reltek makes BONDiT, which does a good job of gluing Delrin. I’ve also been told that Loctite makes a plastic-bonding epoxy that works well on Delrin, but I’ve not used it. But if (and that is perhaps a big if) you want to put that back together, it appears you have some options.

—Tom

Sent from my iThing, so please forgive typos and brevity.

On Feb 16, 2021, at 10:02 AM, snapdiode via groups.io <snapdiode=yahoo.com@groups.io> wrote:

On Sun, Feb 14, 2021 at 09:15 AM, Charles wrote:

Unfortunately they do not. There is a (brittle and now broken) white plastic
holder for both the sampling diodes, which slides into a slot in the board. D6
in the manual picture, (D5 is in the same holder on the other side).
https://groups.io/g/TekScopes/photo/260744/0
Fantastic... That looks like it's made of Tektronix's go-to material in the 1960s, Delrin, ungluable by mere mortals.

This also looks like the clippy in the 1S2...





 

I'm not sure I would want to glue it even if I could - because then I would need a pair of diodes that fit the holder. For some reason I suspect they are unobtainable too ;)

Anyhow, as previously discussed I will use an SOT-323 two-diode package soldered in place, if my eyes and hands will permit it. I am reasonably confident I will not zap the replacement, being aware of the sensitivity of such units - and if I do, a solder repair won't be significantly harder than removing the holder without breaking anything :)

The diodes have left Thief River Falls MN and were last tracked at Grand Forks ND. Still a long way from southern MO.