[Dr_C2]

   

I plan to focus on part replacement for two versions of the C8515 - namely the widely used rev G version andvthe more recent higher spec rev H version. I’ll leave the very old rev F version for another time.

With the plastic DPR hatch removed, the rev G is on the left and the rev H on the right.

Next let’s zoom in and look at part numbers.

Remember our interest is the n-channel MOSFET (motor control/PWM), the p-channel MOSFET (brake control/PWM), and the respective upstream npn bipolar transistors.

c

[Dr_C2]

The C8515 rev G
===========

   

Please see below the MOSFETs and npn transistor types as referenced by markings as per attached zoomed-in photo.

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Marking ‘NO2’ = ZXM61N02F (Motor)
Marking ‘PO3’ = ZXM61P03F (Brake)
Marking ‘A8E’ = LMUN2215LT1

Note: LMUN2215LT1 is a npn transistor with R1=10k prebias.

[Dr_C2]

C8515 rev H
========

Please see below the MOSFETs and npn transistor types as referenced by markings as per attached zoomed-in photo.

c

   

Marking ‘34N’ = DMN3404L (Motor)
Marking ‘R1’ = CJ3401 (Brake)
Marking ‘H11’ = UMH11N

Note: UMH11N is a dual npn with pre-bias.

[Drifter2]

It's nice to see there is enough room to work on these even though they are quiet compact.

[Dr_C2]

Hi Drifter2 - yes the space around the parts does help us DIY repair enthusiasts :)

And, the space adds another dimension. These MOSFETs can handle a lot of power yet they have no visible heat sink to help keep them cool. Instead they use the electrical ‘drain’ pin as a heat sink onto the copper surface below. The drain pin is the one which sits alone opposite the other two pins. So larger spacing  can provide a larger area of copper underneath for more effective cooling of the part.

As an aside, the above photos now clearly show the MOSFET and npn parts used by Hornby in rev G and rev H decoders at that time.

The original part for motor control for the rev H variant appears to be a Diodes manufactured DMN3404L. With reference to the Diodes datasheet we can see this part was manufactured in November 2017, with final assembly and test in Chengdu. 

Global supply chains are all part of this great hobby!

c

[Dr_C2]

To keep this thread tightly on track - next I will share some soldering techniqueswhich can be used remove and replace the faulty MOSFETs without damaging the underlying copper pads and tracks. This is not difficult but it does require the correct tools and some care.

All the above MOSFET use a package type known as SOT-23.

As proposed by Woodcote I’ll start with a photo of the workbench set-up and required tools. These include a 18-25W soldering iron , a tip cleaning pad and a pair of electronics grade tweezers. Also we will need some electronics grade lead-tin solder. Lead free is better for the environment but still a bit tricky for this type of rework.

Photos in the next few days :)

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[Dr_C2]

So before I show a ‘typical’ repair workstation I would like to focus on health and safety for a moment. To see the fine soldering work we need to put our eyes quite close to the work. Also we need to keep in mind that solders contain a corrosive flux which is designed to remove copper oxides and prepare the surface for clean solder bonding. So unless protected, these flux fumes head straight towards our eyes and face.

So… I advise two precautions:

1/ eye protection in the form of either safety glasses or a large workspace magnifying lens to provide the desired separation.

2/ addition of a simple extraction fan with filter which draws the flux fumes away from the operator’s face.

End of safety lecture :)

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[Drifter2]

So holding your breath and doing the "safety squint" wont cut it?   :)

[Dr_C2]

LOL, I think not.

[Dr_C2]

Next let’s take a look at the tools and workbench setup required for decoder repairs. I am sure others will have different options and methods. So what follows is my approach. 

TOOLS AND WORKBENCH

   

Please click on the photo for high res image which even has the correct orientation :)

Items by clockwise rotation starting at the top.

1/ Filtered fume extractor fan
2/ Electronics grade solder
3/ Soldering iron (mine is an Antex XS 25W) plus heat-proof holder.
4/ Safety glasses
5/ A fine tipped soldering bit (the ‘pointy’ fine-tipped version works well for this type of repair)
6/ the decoder plus a mounting jig. I use the plastic DPR hatch. Remember always rotate the work-piece for the most convenient access. Often tweezers in one hand and soldering iron in the other.
7/ Tweezers - note the tips - one is narrow and sharp and the other is specially shaped to hold parts without risk of scratching. I use both types.
8/ A solder tip cleaner (copper coiled wire). Use this to clean the tip every time you remove the soldering iron from its holder - unusable old solder (dull appearance) gradually builds up on the tip and so the tip requires regular cleaning.

One thing I don’t show is a heat-proof surface to protect the worktop. There are some nice silicone options.

THE CHALLENGE

   

Finally, if we are to attempt decoder repairs we need to be confident we can handle a very hot soldering iron with careful precision and without burning fingers etc! Photo shows the scale of the challenge.