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I am currently reverse engineering the electronics of a Pentax Electro Spotmatic camera from the early 1970s. I have the circuit mostly figured out but now I want to find out more about the particular components used.

The component in question is a diode that measures with a forward voltage of about 1.3 V. There are three of them in the circuit with forward voltages varying between 1.2 and 1.3 V. I have not measured the reverse breakdown voltage, yet, as I would need to desolder the diode for that. My meter in diode mode does show the diode as "OL" in reverse, indicating that it is not a Zener diode, if I'm correct.

UPDATE: I unsoldered the diode and checked it in reverse bias, as suggested: Zero reverse current observed with up to 15V reverse bias (checked with 10k series resistor). I did not want to go higher with the reverse voltage. Definitely not a Zener diode.

UPDATE2 USE OF DIODES: One of these diodes is used as a voltage reference in a voltage regulator circuit. The two other diodes are used in the so-called "log expansion" network that discharges the timing capacitor. In this latter context, the two diodes are in series and the result is similar to a diode with ~2.5 V forward voltage with a rather soft knee. For log expansion, the circuit makes use of the exponential characteristic of the diode to time the exposure.

The diode body is small, black, glossy, and roughly football-shaped. One end of the diode (measuring as the cathode) is marked with a blue dot. Please see the attached picture.

Which type of diode could this be?

image of unknown diode

UPDATE: Rough V-I curve traced with an octopus (1N4148 Gimp-ed in as a reference):

V-I curve

UPDATE3, schematic: Per request, here is the schematic of the Pentax Electro Spotmatic PCB I've analyzed. Note that the off-PCB wiring of the camera is not shown and the circuit cannot be fully understood without that. I have not yet drawn that part. When the reverse engineering is done, full results will be published, probably on pentaxforums.com.

The mystery diodes can be found in sections "Vcharge voltage regulator" and "timing capacitors".

schematic

As per the accepted answer, this seems to describe the package correctly: It is one of the 30* package types listed here for the KB-269 and similar components, though a bit smaller than the "30G" package listed for the KB-269 (maybe it is 30D or 30E): https://www.datasheetarchive.com/pdf/download.php?id=ec42375149ccc73d9ce7707084f7ae1d482959&type=M&term=KB269

package types "30"

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    \$\begingroup\$ I second Marcus - unsolder one leg and apply an adjustable DC voltage through a 10k resistor to measure the reverse breakdown characteristics. Most "diode check" meters struggle with Zener breakdown as their compliance voltage is too low (like 2V.) \$\endgroup\$
    – rdtsc
    May 18, 2022 at 11:52
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    \$\begingroup\$ @Edwin Steiner. Yes, but before silicon diodes became the norm, Copper Oxide and Selenium diodes were available. Not suggesting your diode is one of them without knowing its markings, but coincidentally these earlier diodes had a 1.3V - 1.7V forward drop due to the low efficiency, so it may well be the case ... \$\endgroup\$
    – citizen
    May 18, 2022 at 12:03
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    \$\begingroup\$ @MarcusMüller and rdtsc: breakdown voltage is > 15V. I added this and a rough V-I curve to my question. Thanks for the comments. \$\endgroup\$
    – Edwin Steiner
    May 18, 2022 at 13:18
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    \$\begingroup\$ Maybe a portion of the schematic would help understand what they are. Could it be that two series-connected Si diodes were produced in a single 2-lead package for use as a voltage reference? \$\endgroup\$
    – Theodore
    May 18, 2022 at 13:40
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    \$\begingroup\$ It could be two standard silicon diodes in series. \$\endgroup\$
    – Hearth
    May 18, 2022 at 15:30

3 Answers 3

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I'm going to take a wild-a** guess and suggest this is a stabistor that is intended to allow the use of a non-Hg battery after trade in those cells became restricted (typical mercury battery voltage was 1.35V and was very stable so they were popular in measuring instruments).

A similar Jedec type would be 1N4156.

Nowadays, we would use something like an LMV431 and a couple resistors as a shunt regulator, or use a series regulator.

Edit: A plausible Japanese part from the day would be the ユニゾン KB-269 which comes in a similar case with a blue dot. Vf 1.32 to 1.44V @ 3mA. Tempco -3.4mV/°C, which implies it may be a series dual junction with some doping adjustment.

Drawing (dot indicates model by color and cathode by position):

enter image description here

Original scan from: enter image description here

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  • \$\begingroup\$ Thanks for pointing me to the 1N4156. It seems to have very similar characteristics as this old diode (e.g. ~1.5V forward voltage at 10mA). This seconds @AnalogKid's answer about this being two PN junctions in one package. \$\endgroup\$
    – Edwin Steiner
    May 18, 2022 at 17:03
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    \$\begingroup\$ You can see in this datasheet the number of junctions from 1 to 4 is disclosed. Stabistors seem to have attracted a bunch of wrong descriptions online such as "varistor" and "varactor". The term is not that common these days, for obvious reasons. \$\endgroup\$
    – Spehro Pefhany
    May 18, 2022 at 17:09
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    \$\begingroup\$ Just don't build a robot out of these! \$\endgroup\$
    – pipe
    May 18, 2022 at 20:34
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    \$\begingroup\$ I agree this likely has some regulation effect, allowing more modern batteries (1.55V alkaline) to be used in a 1.35V camera (mercury cells). Consider scrapping the whole circuit and just implement a modern VOLTAGE REGULATOR . No diode-magic will do as well as an actual linear regulator. In 1975, these didn't exist. Today you can get an SMT regulator with 1.35V output for about $0.25. I did exactly this to my Dads Pentax camera a couple years ago... The modern battery caused the electric eye to function incorrectly \$\endgroup\$
    – Kyle B
    May 18, 2022 at 22:24
  • \$\begingroup\$ @KyleB, it is not such a straight-forward use of the stabistor in this context. Please see my updated answer for the PCB schematics I reverse-engineered. BTW, the PCB works fine, including the three(!) voltage regulator circuits. I wanted to identify the diode to improve my understanding of the circuit and to be able to specify a modern equivalent for people who might have the same board with a broken component. \$\endgroup\$
    – Edwin Steiner
    May 20, 2022 at 7:11
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It probably is two standard rectifier diode chips in series in one package. I encountered this more than once back in the day. The blob style package was common in Poly Paks assortments.

https://1980computer.blogspot.com/2016/01/poly-paks-mail-order-electronic-parts.html

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  • \$\begingroup\$ That's quite plausible given that the V-I curve looks very much like two standard silicon diodes in series. Unfortunately, I see no way to confirm it without destroying the package. Thanks for confirming that such double diodes were a thing. I did not find anything on today's internet about that. \$\endgroup\$
    – Edwin Steiner
    May 18, 2022 at 16:15
  • \$\begingroup\$ Seeing the old company name "Poly Paks". Long long ago, 1975, used to purchase from them. \$\endgroup\$
    – Marla
    May 18, 2022 at 16:54
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    \$\begingroup\$ "I did not find anything on today's internet about that." - Being a geezer comes in handy. \$\endgroup\$
    – AnalogKid
    May 18, 2022 at 17:45
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This answer is intended to complement Sphero's and others excellent and anciem regime related answers:

This 268 page Mullard 1985 "Quick Reference Guide" provides 4 references to Stabistors.

Page 56 - BA314 as "like" a stabistor

enter image description here

Page 61 - BA314 AS a stabistor

enter image description here

Pages 64-65 - BZV46 stabistors

enter image description here

This may be the closest match.
The Panasonic MA27xxx family is in some places described as a stabistor and in other places as a varistor.
In this datasheet MA27 Series Silicon epitaxial planer type variable resistor the terms "diode" and "stabistor" do not pass their lips - but they are both.
Tables and graphs of characteristics are provided.

Digitron (who?) family of stabistors

  • TIGHT TOLERANCE STABISTORS
    Feature 1N4156, 1N4157, 1N4453, 1N4829, 1N4830, 1N5179, MPD100-MPD400A
    High-reliabilit y discrete products and engineering services since 1977
    Available as “HR� �� (high reliability) screened per MIL- PRF-19500, JANTX level.

1N4156 et al datashete here

enter image description here

Digikey lists 4 obsolete no stock higher voltage stabistors here

Finally, this interesting Google ngram plot shows the rise and fall of the term with time - peaking in 1965, and now virtually unknown. enter image description here

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