Mosfet identification

I’m trying to identify the Mosfet in the attached pictures.
The markings are :

Motorola logo

They are used on an old Nutone (LBC-55) doorbell

Thank you,

Welcome to the technical forum. I am not able to find any data on this part. It appears it is a regulator. I just have no idea what the values are . Though others will be checking this also. So maybe they have more of an idea . Sorry. I could not be more help.

Based on the markings of the PCB/parts I see they are Motorola three terminal regulators manufactured in 1980 & 81 at their facility designated QQ.

Based on the part numbers of 36698 & 36699 I’d guess they are similar in function but with different voltage outputs.

Nothing comes up in a quick search and being Motorola it’s likely they are private labeled versions of some of the standard Motorola 3 terminal regulators of the 1980s.

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Agree with @Verna_1353 and @PaulHutch.

Based on a Google search of the NuTone LBC-55, I found an image here which shows the whole board, and one can see that the right of the two TO-220 parts is marked with LM340T-x (I can’t make out the “x” portion), which confirms it’s a regulator. The only question is to what voltage is it regulating, as that’s what the “x” defines in the part number.

That website I linked to above appears to be quite a good resource to help with issues related to the NuTone LBC-55. I would recommend you go there and see if they can be of further assistance.

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After looking at the photos on the site @David_1528 linked I think I know the circuit topology.

The heat discoloration of the power supply area on the PCB reminded me that the standard USA doorbell transformer puts out a high rectified voltage, up to 26VDC (at low current and high line voltage).

A common trick (because switching supplies were like gold back then) was to use multiple stages of linear regulation to spread out the power dissipation. I did this back in the early 80s for a product I designed for work. I put a crude and cheap zener as a pre-regulator before a 78M05 5V regulator.

I’m guessing the two regulators are 5V for the Intel microcontroller and 12 or 15V for the pre-regulator. So possibly they are a 7805 and 7812 or 7815 for the pre-regulator. Since LM340T is a higher current version of the 78xx series I’d bet the photo is showing the replacement with a higher current part to hopefully extend the life.


I think you’re right on. I believe they run a 24Vac system (edit: the document listed below states it’s a 16Vac system) and rectify that. On that same website, I just found where they keep manuals and schematics for it, here.

Page 5 of the NuTone Musical Electronic Chime Models LA55 LAC55 LB55 LBC55 Service Manual shows a 5V and 15V regulator, and they use a 10 Ohm 10W resistor to burn off some of the power before feeding the 5V regulator.


Excellent find, yeah, I was thinking a 16VAC xformer at less than 1/2 full load current and input of 125VAC can easily end up @ 26VDC.

So the replacements in the photos should be LM340T-5 and LM340T-15.

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Based on the board markings and the schematic, the part with “Z5” next to it is the 15V LM340T-15/NOPB and the part with the “Z6” designation on the board is the 5V LM340AT-5.0/NOPB.