fet location on 5 volt board for headlight control please show photo

Alan,

If the headlight is always "on", that means that he FET has brown and is allowing track power to go directly to the headlight bulb. This will, in all probability, causing the bulb to burn out in the near term.

If you're going to operate the engine in the mean time, I suggest that you disconnect the headlight bulb from the board.

that's why I want to know where the fet is located on the board so I can replace it thanks Barry I know it will burn out that's why i'm asking the fet location. thanks 

The headlight FET is Q215.

where is it located John would you have a picture please? the board I have is not labeled Q215 and not any markings at all

Alan

Didn't you ask this question in another thread?  I posted a photo.

yes Stan I didn't think that was correct when I looked at your board picture do you know the part number and where I can purchase it maybe digikey I did not think that was a fet looks like a transistor to me and the pin 5 did not look like it went anywhere on the board I tried tracing where pin five went to, did not see a solder trace on either side of board. thank Stan any help would be appreciated. 

Allan,

I did not think that was a fet looks like a transistor

It is. A FET is a Field Effect Transistor.  

Alan Mancus posted:

I did not think that was a fet looks like a transistor to me

Transistors and FET's come in that same package.  You can't tell by the package if it's a transistor or FET.

Well, to make sure we're on the same page, why not pop-out the crimped wire from position 5 on the 12-pin housing.  Simply bend up the retention clip (like a diving board) and the wire should easily slide out.  In this case it's the orange wire.  The headlight should go OFF.  Obviously, if the headlight stays ON then you have a separate wiring issue between the board and the bulb.

Then, take your meter and measure the continuity between pin 5 on the board and the top pin of the 3-pin transistor in the photo.  They should be connected.  Note that the circuit board has inner layers so you may not see the traces on the visible surfaces.

If there is continuity from pin 5 of the connector to the transistor, then you've found the right transistor.  You can then remove the transistor and confirm the headlight is now OFF (after re-inserting pin 5 wire back into the housing).

As for a replacement part, I'd hoped one of the MTH Service guys would have popped up with the "official" replacement part.  I've read threads where they get a "cheat sheet" with a locations of all the output transistors for the lights, smoke, couplers, etc. to help them repair boards.

In absence of their input, I suggest the FDN5630 transistor as used on the PS2-3V board - the lighting electronics is essentially the same between the 5V and 3V boards so I'm 99% confident it will work.  This transistor is available from DigiKey for about 50 cents so you'll pay more for shipping!

thanks Stan I know it is a board problem because I installed another board in engine and the headlight worked correctly. Barry I know the difference between a transistor and field effect transistor and a transistor , transistor has a n or p junction and has emitter base and collector base turns the emitter and collector on when bias  correctly, fet work with a gate that turns on a source and drain inside the device . check out picture below you'll see why I thought the device was  a transistor look at the 5 area marked in yellow, did not realize fet was under capacitor! Alan

your right John I repaired a ps 2 3 volt slave board the trace was cooked completely and I had to run a jumper wire to repair the board the fet was so cooked it crumbled when I unsoldered it .after the repair the board is still working been over a year ago!

thanks for the input john!

Alan 

gunrunnerjohn posted:

...

Don't discount the possibility of a blown trace, I've had a couple of them to these small FET's, apparently the little suckers can pump enough current to kill the little traces.

Though in this particular situation, if the trace blew (like a fuse) it would break the connection to the transistor and hence headlight would be OFF. 

In any case, remove the transistor and confirm the headlight is now OFF before installing a replacement.  This only takes a minute and would give you 100% confidence the transistor was indeed shorted between its S(ource) and D(rain) pins.

Stan I did check with a digital ohm meter between the source and the drain and shows infinity, but ill still try what you suggested thanks so much for all your help just ordered the fet's  from digikey thanks again for that info too!

Alan

ps those fet's   are really tiny

But you did confirm continuity from pin 5 to the transistor D(rain) right?

Not that it's conclusive but does the transistor "look" damaged?  Perhaps charred, perhaps pock marked, perhaps a bulge, etc.?   The wildcard (not conclusive) is if you're indeed reading infinity between S and D, the transistor may be OK and the control wire going to the transistor (the Gate pin on the lower left) is stuck "ON" when the electronics turns on.  This would be an exceedingly rare failure mode for this type of circuit design but it's one of those never say never!  In any case, since DigiKey is so efficient, your transistors are already in a box on their loading dock!  

So as before, I think the way forward is to remove the transistor, confirm the headlight goes out, wait a couple days for the DigiKey box, and then just install the replacement and hopefully case-closed!

FWIW, I have seen a failure of the driving circuit, which in this case is directly from the uP.  In that case, you're toast, there's no fix for that.

Stan . But you did confirm continuity from pin 5 to the transistor D(rain) right?

yes Stan I confirmed pin 5 to drain was good. parts are ordered and on there way thanks. yes John hope it is not the up you mentioned, will wait and see after replace fet from digikey!

thanks everybody for your assistance!

Alan

ps does anybody have sheet with pictures for locations of fet on board and function of each headlight ,smoke fan motor, heater control fet etc for ps 2 5 and 3 volt boards? 

Yes, but it is tech information.  Go to MTH School and they will provide it too you. G

GGG posted:

Yes, but it is tech information.  Go to MTH School and they will provide it too you. G

While you're there, shell out around $700 or so for all the test sets.

I am going to tech school but have to wait until next class last class was full. I'm hoping next school will be in October.

Alan