There are diodes called 1T243, 1T243M and 1T243S used in SONY products. These diodes are often used as Zener diodes, but the measured Zener voltage values in the SONY product service manuals often vary depending on the equipment and location used. It may also be used in the forward direction rather than in the reverse direction as in Zener diodes. Furthermore, the outer shape of this diode is also TO-92 with 3 terminals and 2 terminals. For this reason, audiophiles and radio repair enthusiasts are confused about what type of diode to replace when repairing SONY products.
The cause of this happening is as follows.
(1) Actually, 1T243, 1T243M and 1T243S are NPN type bipolar transistors themselves, and SONY uses the emitter-base terminal as a diode in SONY products.
(2) 1T243, 1T243M and 1T243S have different types with different characteristics, and each has a model number starting with 1T243 called 1T243 *** (*** is a subdivision number and is a subscript such as alphanumeric characters).
(3) The application as a diode is not one type, but differs depending on the type indicated by ***, and there are Zener diodes, small signal diodes, and varicaps.
(4) There are multiple types of 1T243 used as Zener diodes, each with a different Zener voltage.
(5) Since the service manuals for SONY products rarely describe the subdivision numbers indicated by ***, the valid 1T243 cannot be determined. Therefore, if the characters in the *** part are replaced with 1T243 ***, the circuit will not work properly.
(6) The standard voltage described in the service manual of SONY products is an example of measurement using a tester with an internal resistance of 20 kΩ / V, which was common at the time of product sales. Therefore, when the voltage is measured with a modern digital multimeter, it is often higher than the voltage described in the service manual.
(7) There are individual differences in the Zener voltage of the Zener diodes, and it varies by about ± 5% from the nominal value.
(8) Zener voltage is not constant and changes slightly depending on the current. If the current is lower than the rating, the Zener voltage may be about 0.5V lower.
(9) SONY outsourced the production of type 1T243M to Mitsubishi Electric.
This time, from the SONY's document, I found some of the ratings of these diodes, especially the Zener voltage when used as a Zener diode, so I will show it in the table below.
Then there is information that the 1T243 has two Zener diodes in the internal circuit, which is incorrect. Since 1T243 is originally an NPN transistor, it has two sets of PN junctions and is recognized from the outside as two diodes. Since the breakdown voltage between emitter-base terminal is low and its characteristics are similar to those of Zener diodes, it is used as a Zener diode, but between base-collector terminal, the breakdown voltage is 50V or more, which makes it a characteristic as a diode for small signals.

２　1T243/1T243M/1T243Sの定格

S	M	D

1T243M-40,1T243M47W,1T243M-49W