Need diode ID help

[jeffjmr]

In the process of bringing an HP 428B back to life.

Among other things, I suspect some diodes may be part of the problem.

Here is how they are described by the parts list:

si 30 wiv 50 mA 2 pF 2ns
A part number, PG512 is listed for this one, but a web search comes up empty. And a capacitance spec on a diode is something I haven’t seen before, and 2ns Trr seems unobtainium. The fastest I can find on Mouser and Newark is 4ns.

germanium 60 wiv 1 microsecond Trr
This one has no part number, and is that Trr spec for real?

Pics attached. They don’t look like anything special, nor do I see any signs of them having been replaced. There are some numbers on them but I’ll have to pull them to read them.

Any ideas on what I could use for replacements for these would be greatly appreciated.

Thanks,
Jeff

[bdunham7]

The first one is a 1N3956, the second a 1N54AS.  Both are available from American Microsemiconductor--for $95 and $492 each.   

The 1N3956 probably needs to be replaced with the same thing.  The NTE 177 is purportedly a substitute, but I wouldn't trust that.  There's a local mil-spec surplus supplier near me and I may be able to obtain 1N3956 from them, but I'd have to go there when I'm in their area for something else.  They show them as available, but until they are in hand you just don't know.

The 1N54AS might just be what they used for a germanium diode and perhaps a 1N34A will work.  How is it used in the circuit?  I can't find datasheets at all for the 1N54AS, but I can for the 1N54A.  However, they are so brief that comparisons are difficult.

Edit:  1N54A has much lower reverse leakage current, I'd assume the 1N54AS is a 'select' version with even lower I_R.  So you may have to do some scrounging and/or selecting

http://pdf.datasheetcatalog.com/datasheets/newjerseysemiconductor/1N54A.pdf

[jeffjmr]

$2158 worth of diodes, eh? Well, if those are indeed faulty, then my 428B will be an attractive paperweight.

Even one of the $95 units exceeds the cost of the meter.

Thanks for the quick response.

[bdunham7]

That's just the hoarder/reseller silliness.  They hope someone will approve a purchase out of desperation.  I'm sure you can round up something for a few bucks.  You might just get 10 or so 1N54A or NTE109 from eBay or Digikey marketplace and select the ones with the lowest leakage.  Or, I wonder if a 1N916 would work.  Hmmm.

Let me know if you want to to scrounge the 1N3956.