Old zener identification - Page 1
Forum section: electronics / repair

author: torquil
author_rank: Regular Contributor
timestamp: October 04, 2019, 09:55:29 am
content: Hi!Sorry if this is the wrong section. I need some info on a old zener diode, and I figured there were plenty of people here who know about old electronics since this is the repair forum.In order to lower the B+ voltage after having changed from tube to solid-state rectification on a homebrew audio amplifier based on an EL33 power tube (converted from an old tube radio), I am going to put a Zener diode in series with the centre tap of the secondary HT winding on the power transformer.I found an old zener in my parts bin, but it would be nice to find the data sheet to see if it is up for the task of passing around 50mA. My cheap "multi-function tester TC1" tells my that it is a 26V zener diode. Does anyone recognize the manufacturer marking on this component?:Best regards,Torquil Sørensen, Norway

author: Twoflower
author_rank: Frequent Contributor
timestamp: October 04, 2019, 10:09:30 am
content: That's more likely a transistor:http://www.web-bcs.com/transistor/tc/b0/B407.html

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 04, 2019, 10:14:47 am
content: That's not a Zener diode, it's a PNP transistor - B407 is short for 2SB407, and any part that begins with 2SB is a PNP transistor. The logo is Sanyo and a quick search of the usual datasheet websites shows this is a Ge transistor:https://alltransistors.com/transistor.php?transistor=10272

author: torquil
author_rank: Regular Contributor
timestamp: October 04, 2019, 10:44:04 am
content: Hmm, I didn't see that one comingI didn't consider that it might be a three-pin component with the case as one pin... Unfortunately, my multi-function tester is not able to correctly identify it when connecting all three pins. It detects only a two-pin component and says that it is a resistor in that case. When I attach it to the zener connectors on the tester it says that it is a zener diode....However, I'm a bit suspicious of it now, because it seems to be shorted between collector and emitter. Around 0 Ohms both ways and a 0V voltage drop both ways in diode test mode on the DMM. Collector-Base and Emitter-Base seems fine.

author: Gyro
author_rank: Super Contributor
timestamp: October 04, 2019, 11:08:04 am
content: Quote from: torquil on October 04, 2019, 10:44:04 amSadly, it looks like an ex-2SB407 then.P.S. Your zener tester may have been detecting Base-Emitter or Collector-Base reverse breakdown. Use Zener testers with care (i.e. not at all) on transistors. Base Emitter reverse breakdown is destructive in a transistor - well it damages its gain characteristics anyway.P.P.S. It's fairly common for Germanium transistors, particularly power ones, not to register correctly on simple transistor testers due to their high leakage current (not that it would have mattered in this case).

author: torquil
author_rank: Regular Contributor
timestamp: October 04, 2019, 06:51:04 pm
content: Thanks everyone, that was very useful information. I knew this place wouldn't disappoint!- Torquil

author: Tomorokoshi
author_rank: Super Contributor
timestamp: October 04, 2019, 08:02:45 pm
content: For a 26V drop at 50 mA, you could put two 13V Zeners in series:https://www.digikey.com/products/en?keywords=1N5350BGOS-NDThese are rated to 5W, derate somewhat starting at 25 C. Plenty of margin. Use other values to tune the exact voltage you need.