Zener Diode failure modes and Diode identification

[Gtx21]

I am working on repairing a 1995 era audio system that is comprised of all through hole components on a 2 sided pcb.   It has somewhere around 80-100 diodes of at least 5-6 different types, almost all of which are in glass packages with no labels save for a single color stripe.
   
The forward breakdown voltages are pretty consistent (650-700mV) but the reverse breakdown voltages are very inconsistent even between diodes that I’m pretty sure are the same type. 
 
When Zener diodes fail, do their reverse breakdown voltages ever go higher?   Or vise versa, do reverse breakdown voltages of regular diodes ever fail in way that makes them appear to be Zener-like?
 
For instance, I have 2 identical diodes with the same forward voltage (and capacitance) where one has a reverse breakdown of right around 2.6V and the other somewhere >> 15V (max of my current supply).   Either one of them has failed or I have 2 diodes that look identical in every way physically but are different.

[SeanB]

Normally zener diodes fail by going short circuit, open circuit or getting really leaky.  Generally is they have a blue band  or a green one, they probably are ISS1 diodes, which are pretty much a 1N4148 or 1N914 in most cases.  Breakdown at 2V6 in circuit or out, it makes a big difference, but to see if it is a regular diode or a zener you will need to reverse engineer or find a circuit diagram of the amplifier.

Out of circuit about the only test aside from using the manufacturers test conditions is to use a curve tracer to show up leakage or noisy diodes.

[Gtx21]

Normally zener diodes fail by going short circuit, open circuit or getting really leaky.  Generally is they have a blue band  or a green one, they probably are ISS1 diodes, which are pretty much a 1N4148 or 1N914 in most cases.  Breakdown at 2V6 in circuit or out, it makes a big difference, but to see if it is a regular diode or a zener you will need to reverse engineer or find a circuit diagram of the amplifier.

Out of circuit about the only test aside from using the manufacturers test conditions is to use a curve tracer to show up leakage or noisy diodes.

80% of them have either a single blue or single green band, but no text printing.
 
Is ISS1 a brand, model, or line of diodes?   Google seems vague on it.

[SeanB]

1990's audio was generally made in the far East, so Japan, Hong Kong, or around there, and often used Japanese made components. Thus the common diode was the ISS1 diode, and yes these are part of a whole range of ISS numbered diodes, including zener and other diodes, all in the same glass package with a band. Thus you will have to reverse engineer the circuit to see what the actual use of the component was, as they can vary greatly while looking identical, and only the reel had the part number on the auto insertion machine that made the equipment in question.

What make and model, you can perhaps find a service manual for it online, and this tells you what they used where, often with PCB diagrams as well showing you the location of the component on a grid, that was part of the parts list.