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A quick look at ePay reveals 3 IBM 5110s, and two 5120s all asking in
excess of $1200 (all but one are OBO, so you might be able to get a
discount). Shipping on these things is going to be bear, however. There are
lots of watchers on each one.

The 5120 is especially interesting looking, with a larger display, two
vertically mounted 8 inch floppies, and a generally "rounder" look to the
case: the thing just screams 1975. If I had any interest in these machines
I might be tempted to pick one up.

搁别苍é别,

what I used to replace the same is MUR 160 ... the 4148 was not fast enough

But the datasheet for the MUR160 gives Trr of 60 ns, while the 1N4148 gives Trr of 4 ns. How could the 1N4148 be too slow?

Also the capacitance of the MUR160 is 20 pF compared to the 1N4148's 4 pF, so the MUR160 would throw the capacitance of the whole sub-circuit off.

Finally, when I look at alleged pictures of MUR160s they seem to have really fat leads, which makes sense given their higher current rating, but that will only make it more difficult to place them in the existing holes, especially since I'm having to do the rework without access to the backside of the PCB.

Dave,

Regarding your question "How are places like Mouser and Digikey with small time onsie-twosie buyers..."

I have found that they are amazingly cooperative with small orders. I have been refurbishing many of my personal test equipment units each of which need a variety of replacement electrolytic caps. I end up ordering "kits" of parts after spending a significant amount of time using the search engines on the supplier's web site to find high quality replacements of suitable size and value. I choose only "in stock" parts because the stated manufacturers lead time for non stocked parts is just an estimate and usually requires a substantial minimum order quantity. I also want my "kit" to arrive complete so I can get on with the project. During my last order I found that Mouser didn't quite have all the parts I needed but Newark did and the Newark pricing was better. Digikey is a good supplier too. I just haven't needed to use them recently.

Tom

Hi Jeff,

Please call me Tom. :)

Some critical diode parameters are rarely discussed in EE curricula, so you have every right to be flummoxed by datasheets. It is also not infrequently the case that manufacturers want to overwhelm you with a lot of data to hide the fact that they didn't tell you the one thing you really wanted to know. One of these is "reverse recovery time". It happens that PN junction diodes don't turn off right away. Until they do, the diode maintains a 0.7V anode-to-cathode voltage, even if that means that the current reverses direction. As you might imagine that can be a Very Bad Thing(tm) in certain circumstances. The time taken for a diode to return to the non-conducting state is the reverse recovery time. This value depends on both the forward and reverse current, but under typical conditions, the 1N4148 recovers in the several-nanosecond range. The venerable 1N400x is much slower, with 5-20us being typical. The UF400x is a fast recovery version of the 1N400x series, with typical recovery times in the range of 50ns-100ns.

On top of all that, you might care about capacitance, as in any circuit that has to support a high bandwidth. Frustratingly, manufacturers often leave out that data, making it difficult to figure out whether a given diode might be an acceptable replacement. In that case, you have to guess. A good guide to guessing is to assume that the junction capacitance is 10-20pF per ampere of rated forward current. This figure applies to junction diodes, and not to Schottky or other types. (But the same rule of thumb applies to the output capacitance of a bipolar transistor, as it happens.) The logic is that both capacitance and current-handling capacity are functions of device area, so the two should be correlated. Other factors affect the relationship as well, but current handling capacity is the simplest proxy for capacitance that is quantitatively useful.

In your circuit, you're looking for a diode with a breakdown voltage of 175V and a forward current rating of 100mA. The 1A rating of a 1N4004? or UF4004 means that it can handle those two fine, but will have 10 or so times the capacitance, and so will likely be unacceptable. A Schottky unfortunately has much higher capacitance per ampere, so it is also probably not a good choice here.

After all that, I often replace these types of diodes with series combinations of 1N4148, if there's room, because I am always impatient to fix the thing now, rather than wait for the right part to get shipped to me (if I can even find a vendor of these often-obsolete components). In this circuit, two in series should suffice to replace an FDH2161. Some fussy folks would advise using a resistor in parallel with each diode to guarantee equal voltage division, but they're unnecessary here, especially if the diodes come from the same batch.

Good luck with your continued debug. You are going to get this to work!

--Cheers,
Tom