Schottky diode 5082-2811 replacements ?

[paulfifties]

inside my Marconi TF-2015 (10MHz-520MHz AM/FM signal generator), on both the A16 and A17 board, the ALC circuit uses 2 Schottky diodes in series, of type :
HP 5082-2811 (according to the schematic diagram).

The diodes actually mounted show "HP2 835 405" (in 3 lines of 3 digits). So I suppose they are HP2835... (Is that the same as 5082-2835 ?)

As I suspect the diodes having gone way out of specs (or even defective), I would like to replace all 4 of them.
I already tried BAT83s as a replacement, but that is not working for some reason.

So I wonder which ones I should use.
The original ones (and are those 5082-2811 or 5082-2835?) are very expensive, and apparently only available from the internet...
I have the feeling though that the diodes are very critical in the diff amp in which they are used.

Any advise is appreciated.
Paul

[Karen O]

I think I might have some of them somewhere.

I'll have a rummage.

[MrBungle]

These aren't particularly special diodes. They are selected from 1n5711 silicon. You can possibly get away with matching a set if it's performing a mixing function. RS sell 50 for £3.85+VAT (free delivery): http://uk.rs-online.com/web/p/rectif...iodes/6870763/

Both have 410mV drop. The 1n5711's have slightly higher min capacitance and higher reverse leakage, although I've got to be honest, a lot of the modern silicon performs way better than the datasheet says it does.

If you can find a 1n5712, that's even closer.

Datasheet for the entire series for ref: http://pdf.datasheetcatalog.com/datasheet/hp/1N5712.pdf

[paulfifties]

Apparently, while researching one comes always down to the same Agilent's Technical Data sheet for the Schottky diode family

I noticed the 1N5711 and 1N1712 equivalents too, but from the Electrical Specifications table , I found the 1N5711 and 1N5712 show a much larger Max Reverse Leakage Current.
However: while closely examining, these values apply at (much) higher VR values (at which the original 5082-2811 would have broken down already...).

Examining the "Typical Parameters" graphs, I would conclude that the 5082-2810 and 5082-2811 are very close (the Reverse Current for the former may be slightly less at low values of the Reverse Voltage), while from the Dynamic Resistance graph, it shows that the 1N5711 has a higher value for an identical Forward Current than the 1N5712, which is coinciding with the 5082-2811 graph.
So, if I am to choose between 1N5711 and 1N5712 as a replacement for 5082-2811, I think indeed the 1N5712 looks the better match.

On paper, that is...

[Radio Wrangler]

The old HP Schottky diodes live on. HPA was spun out and floated off as Avago, and have recently bought out Broadcom. They must have liked Broadcom's name more than their own because they've adopted it.

You'll find a lot of them on Farnell's site in SMT packages beginning HSMS-

The Broadcom website is a bit tedious to navigate to find datasheets.

David

[MrBungle]

I'm surprised. If you're in the IT trade, the word Broadcom makes people cry!

Good to hear about them still being issued. Also it looks like they come in two per package in SOT-23 which is even better as they're likely to be on the same die and therefore better matched.

[Craig Sawyers]

I've used this family of devices to replace a fritzed sampling bridge in a Tektronix S3A sampling probe.

The ones I used were two SM dual diodes back to back.

[paulfifties]

Originally Posted by Radio Wrangler

The old HP Schottky diodes live on. HPA was spun out and floated off as Avago, and have recently bought out Broadcom. They must have liked Broadcom's name more than their own because they've adopted it.

You'll find a lot of them on Farnell's site in SMT packages beginning HSMS-

I noticed prices (even way) over € 10 a piece for 1N5712, while I find in Belgium a 1N5711 for € 0,20 a piece, but they don't offer any 1N5712 unfortunately.
Is there any logic in that ?

Otherwise, I could maybe try to make a "hanging construction" replacement using the HSMS-281x series, but I'm a little afraid of soldering to this tiny tiny package, soldering a wire to it, which in turn is to be soldered to the original place of the Schottky diode on the PCB...

[MrBungle]

They're actually easier than they look to solder on to things. Just buy a few extras as if you ping one onto the carpet which does happen then you'll never find it again. TBH they're quite easy if you have a flux pen.

[Radio Wrangler]

Small soldering iron, tweezers, some magnification and a steady hand. It's doable.

Once you've got a bit of appropriate gear and a bit of practice you can suddenly do things others are scared of and a lot of things become possible.

David

[MrBungle]

Indeed. I built a discrete 1W audio amplifier in SMD a while ago. Used FR4 with cut board (Stanley knife) and a scalpel, flux pen and Antex XS25. Whole thing was .75" square.

Then I learned the hard way about lower power dissipation of SOT-23 packages and turned the thing to ash