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I found this circuit board from an 1980s old humidity controlled chamber. This definitely looks like a temperature regulator of some sort. It has a M logo which means made by Motorola. It has the labels E03G-3-9 and 8739. Does anyone have any info on what this part is?

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    \$\begingroup\$ Without looking up part numbers, that looks very much like a power diode, not an SCR. THere's 4 of them, so this is probably a "bridge" (i.e. turn AC into rectified DC) en.wikipedia.org/wiki/Diode_bridge. If your meter has a diode check function, this is easy enough to verify \$\endgroup\$ – Kyle B Aug 12 at 19:21
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It matches rectifier Case 175-03 or 253-03 used on SCR's with an obsolete P/N made in 1987 week 39 (8739). Appears to be rated for about 25A with 4 switches to serve as 2 half-bridges for 2 loads with variable phase or ON/OFF control. It takes 2 SCR's to make a Triac.

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Also obsolete from 1987 is the latest cell phone. My friend had one of these. enter image description here

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  • \$\begingroup\$ I also saw those "latest" cell phone in the movies back then. We nick named it "big brothers" because only those high rank gang guys with high enough status could own one. At that time everybody used "pagers". No body heard of cell phone. I forgot it was 2G days. Time flies. This morning the “3” shop emails me that my 4G phone is "out" and would lose face if not immediately upgrade to Samsung S20 5G, only for a big discount price of HK$5,200! \$\endgroup\$ – tlfong01 Aug 13 at 0:57
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    \$\begingroup\$ My "3" shop had a laughable name called "3" because they stupidly though that 3G would last forever, so they bid the domain name three.com, perhaps for millions of dollars, and also the world class marketing and design experts the "3" company logo for perhaps another million dollar. Anyway, this is the big discount promotion 5G email I got this morning from "3": web.three.com.hk/edm/200807/s20addon_offer/index.html \$\endgroup\$ – tlfong01 Aug 13 at 1:07
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    \$\begingroup\$ @Tony Stewart Sunnyskyguy EE75 You are correct. It is an SCR. I have the third type of SCR like in the picture you have posted in your answer(stud version). So, after some digging found the shorter lead is Gate, long is cathode and body is Anode. After feeding the Gate-cathode with a 1.32V from a NiMH cell via a 10 ohm resistor I was able to get the Anode to cathode to conduct. Without the battery connection it doesn't conduct so confirmed. Thank you so much for the information. Though I wish I could find the voltage and current specifications for the Thyristor. \$\endgroup\$ – The_Vintage_Collector Aug 13 at 20:13
  • \$\begingroup\$ It’s gonna be about 25A for that size and voltage unknown but you can test it with 300V and 1M resistor for the breakdown. You might be able to get 15A from a LiIon across it and measure current via 50mV shunt. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 13 at 20:21

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