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Old 13th March 2012, 04:41 PM   #1
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Default Help finding obscure transistor

I'm looking to replace a power supply transistor, and I can't find any data about it. The schematic shows it as an npn transistor, but with no part number. The part is marked:
(Motorola M) 752

I believe the second line is a 1975 date code - it comes from a power supply that was built around 1976.

The package is sort of like a TO-220, except that the plastic body extends onto the mounting tab, with a round clearing around the hole so that a bolt or standoff will contact the metal tab on top as well as on the bottom.

The 3 pins and the metal tab (top and bottom) are gold plated.

I've tried searching for BD742, MJE752, 2N752, etc. and can't find anything that matches.

Any help identifying this part and finding a data sheet would be appreciated!
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Old 13th March 2012, 05:23 PM   #2
kevinkr is offline kevinkr  United States
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First thing you need to do is tell us exactly what this transistor is used in - bipolar power transistors in most cases aren't too difficult to substitute if a little is known about the circuit where it is used. There are exceptions of course, but hopefully not the case here.

Post a hand drawn schematic or link to a legally hosted copy of the schematic as having the context where it is used is helpful.
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Old 13th March 2012, 05:39 PM   #3
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The numbers you list are house part numbers for your particular power supply.

The package sounds like the old Motorola "case77", which became the TO-225. It was used in the early 70's and I remember building several Li'l Tiger audio amps in the late 60's that used that package. My ADD powered brain can't remember important stuff, but I can remember almost every transistor and tube number I have ever blown up. The Li'l Tiger used 2N4921 and 2N4918 transistors.

The case 77 package was popular back then and there was several types of transistors and SCR's made in that package. It is slightly smaller than the TO-220. Motorola also made a bigger package that had no plastic around the screw hole. It wasn't as popular.

Motorola spun off their discrete semiconductor business in the late 90's. ON Semiconductor is the new name for that business unit and they still make several parts in that package including the 2N4918 and 2N4921. One is PNP the other is NPN. There is a high voltage NPN part, the MJE340-D and a PNP, the MJE350-D. Perhaps one of these will work in your circuit.
Attached Files
File Type: pdf 2N4921-D.pdf (90.8 KB, 13 views)
File Type: pdf MJE340-D.pdf (67.6 KB, 3 views)
File Type: pdf MJE350-D.pdf (133.7 KB, 3 views)
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Old 13th March 2012, 07:32 PM   #4
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I thought the case was a Case 77 at first too. But it is not - on the Case 77, the plastic goes all the way to the hole. The case on my transistor looks just like this one:

Surplus Sales of Pennsylvania

(This part makes me suspect that for my transistor, 752 is the date code and 1075 is part of the part number.)

The package difference is important - several of these transistors get clamped between posts pressed into the PC board and an aluminum plate heat sink - so the package thickness is everything.

Additionally (and annoyingly), the circuit depends on an electrical connection from the top side of the tab, which is connected to the collector.

Answering the other question: The transistor is used as a pass transistor for a power supply controlled by an LM723 regulator IC. I need to find the real deal or a close match, because this power supply has three of them in parallel to get the necessary current output. (I know - I could replace all three with 3 new, matched transistors, but I'd rather not.)
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Old 13th March 2012, 07:38 PM   #5
Mooly is offline Mooly  United Kingdom
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Your link just takes me to a login screen. Post a picture of the real part

To help determine a replacement what are the voltages and current we are working with.
The transistor has to be rated for the full supply voltage, not just the differential across it in use. What is the output output current of the PSU ?

Maybe also post pictures of the mounting arrangements.
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Old 13th March 2012, 07:50 PM   #6
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I don't have any specs for the power supply - it's just good luck that I have a schematic, though non of the components on the schematic are labeled.

The load is a couple of relays and stepper motors - so it's a pretty inductive load. The load draws 2.8A "average power", according to the spec. (It's a pair of vintage Pertec 510 8" disk drives.)
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Old 13th March 2012, 07:57 PM   #7
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The case is called "Case 199-04". It is the same case used on the MJE3055K

(MJE3055K datasheet and application note, data sheet, circuit, pdf, cross reference, pinout | Datasheet Archive)
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Old 13th March 2012, 08:14 PM   #8
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Default PHoto of transistor and schematic

:-) You make me figure out how to post a photo. Attached is the transistor, lying on the relevant portion of the schematic, if I did it right.

The unregulated power supply voltage (the input) is around 37V - probably designed for 40V. The output is 24V. The output continuous rating is probably 3A, with some additional rating for inductive surge. There is a protection diode on the output, together with an electrolytic cap, just to the right of what you can see in the photo.

Attached Images
File Type: jpg MotTransistor.JPG (100.8 KB, 140 views)
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Old 14th March 2012, 08:09 AM   #9
Mooly is offline Mooly  United Kingdom
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You seem to have got the hang of attaching pics but more here, post #11. The attach box is slightly different now to when I wrote this but it's near enough.
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In all the years of being involved in electronics I have never come across that package.

You say the circuit depends on the connectivity from the tab. Without seeing the actual unit and how that works and how it all connects together mechanically it's hard to advise how to get around the mounting problem for other device. Spec wise there are dozens of suitable transistors. The collector is always the metal tab on any power device.
There must be ways and means of attaching other devices to the heatsink even if it means using an insulating kit and hard wiring the collector connection to where ever it goes.
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Old 14th March 2012, 10:12 AM   #10
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Sir Mooly,

in that case, you probably never saw leads such as these.

(Motorola manufactured a lot of MJE types with the Case 199-04 package, or Case 90, a few mm wider than TO-220)
Attached Images
File Type: jpg MJE2011.jpg (105.4 KB, 127 views)
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