ID Needed for "H" Transistor

Hi. This is from a early 90’s vintage instrument if anyone has any idea what it is. Thanks for looking


Not a very distinctive mark, that… If you can find documentation for the appliance in question, there’s some possibility that further information may be available. Otherwise, you may be stuck reverse-engineering things to discover the device’s function. Being stuck on a heat sink next to a big electrolytic in an 90’s era device, there’s a fair chance of it being a linear regulator of the 78xx flavor, or something similar.

Thanks. I appreciate the input. This one is working. I have a fried pair on another unit that needs replacing.

The “Q2” marking on the PCB implies it is a transistor (a regulator would usually be U2).

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It’s looking like a Fairchild subset model “H” 60V MOSFET transistor. If I can get a probe into the small space I’ll test for PNP or NPN next.

Hi Pixietube,

It may be that the device is mounted to the heatsink backwards, so all the important markings are on the other side of the device. Wonder if you could remove the fried devices from the board, remove the heatsink, clean the device from the grease and take a photo of the backside?

heke, AsamaLab

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Yes, you are exactly correct. I ordered another unit on Ebay with the heat sinks mounted properly and the markings visible. I’ll post later just in case anyone else has this problem.

Please do post a pic of the other unit to satisfy my curiosity.

All the TO-220 packaged devices I’ve ever seen, admittedly not many compared to many techs/engineers, have one flat side with metal running down into the resin housing to attach to the heat sink. The other side has a distinct step between the housing and the tab. This makes it only work correctly, or even fit reasonably, when mounted in one orientation.

The photo you posted appears to show a distinct step within the boundary of the heatsink indicating that it is mounted in the correct orientation (flat side toward heat sink). I’m curious if what I’m seeing is just an optical illusion.

Hi Paul, Pixietube,

Indeed would be nice to see the photo of the backside.
The package seems to be a TO-202 which can be mounted both ways
(may require bending the tab).

Heke, AsamaLab

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I’ve never used or read about TO-202’s before thanks for the tip @heke.

Appears to be an Aavid TO-202 heatsink that includes a cut out to clear the resin housing and make good contact with the tab. That must be quite a bit less effective than the TO-220 style which has metal to metal contact in the area of the silicon in the body. I suppose that’s why under single transistors the 220 package has ~2000 parts listed while the 202 has only 22, and in regulators there are ~2800 TO-220’s with no TO-202’s.

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Here is a pic without the heat sinks on another unit. Despite being marked Q they were all linear regulators. Maybe that was an 80’s -90’s thing.

LM341T-15/NOPB 500-mA, 35-V, linear
LM7915CT/NOPB3-Terminal Neg Vltg
UA7815CKCT 3Pin 1.5A Fixed 15V

Thank you all very much for your input! That was very educational.

Hello Asama the parts are package of TO-220-3
heat sink looks to be HS107-ND or similar to it

LM341T-15/NOPB 500-mA, 35-V, linear
LM7915CT/NOPB3-Terminal Neg Vltg
UA7815CKCT 3Pin 1.5A Fixed 15V

we have in stock

Definitely not an 80’s 90’s thing, I’ve been doing this since the 60’s, professionally since the 70s, and “Q” has been the standard transistor designator and “U” the standard IC designator the whole time. IIRC it came into use right at the start of the commercialization of the transistor in the 1950’s.

My guess is that they either just screwed up the designator, or they wanted to mislead anyone who didn’t have access to the the schematic.

A third explanation for Q might perhaps be that in the original design they probably used pass transistors and zeners at the base, but due to poor line and load regulation, or other reason, upgraded to monolithic regulators without redesign the PCB.
There is a DIL packaged IC (AD7552) - I wonder what is the designators for it on the PCB (U or Q)?