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Technical bulletin: Repairing C8515 decoders


Before getting into the specifics of decoder repair, first a technique for removal of surface mount components without damaging the underlying thin copper pads and tracks.

We will use the technique for removing three pin devices know as SOT-23 package. The same technique can be applied to larger packages such as TSSOP-14 as used for the microcontrollers in standard SSD decoders. It can even be used for successful removal of the 64 pin microcontrollers inside the APB. 

So the technique involves three steps.

Step 1 - add fresh lead/tin solder to each of the three pins to build up a small ‘ball’ of additional solder. This serves two purposes, first it dilutes the earlier applied solder which will be lead-free so melts at higher temperature. The dilution effect lowers the melting temperature making the task easier. Second, the increased volume of solder helps retain heat during the next step.


Step 2 - This step is about raising the temperature of all three pins so that the part can be removed. Here we apply the soldering iron to each pin in turn - holding the soldering iron tip in place for approx 1 second before moving to the next pin. After approx 10 rotations, gently ‘tap’ the part sideways with the tip of the iron. If the solder at all three pins is molten the part will move sideways without causing any damage. If the part doesnt move, repeat heating rotation for another 10-30 seconds.

Once the part has moved, it can be retrieved with tweezers.

Step 3 - The final step is to clean the solder pads (three in this case). Here the technique is to apply some fresh solder using a nicely cleaned soldering iron tip. Fresh solder applied direct to the pad will be drawn towards the tip of the iron as the iron is withdrawn - leaving a nicely conditioned pad surface for future attachment of a new part.

With all of the above steps, practice makes perfect. As mentioned the technique can be applied, even, to the removal of 64 pin microcontrollers - but definitely good to learn to walk before learning to run Cool

I hope this is of interest to at least some forum members.

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We saw in post #3 an outline of a repair procedure for C8515 decoders. Further posts have covered identification of the Hornby parts as factory fitted. Then, more recent posts have covered a ‘typical’ tool set on the repair bench, and finally a ‘technique’ guide to removal of surface mount components with focus on the SOT-23 package.

Next up will be a set of magnified images of the repair process step-by-step. As I don’t possess any faulty decoders I used a perfectly healthy decoder for the photo-shoot. No decoders were harmed in creating the upcoming photo sequence. Yes, the decoder functioned perfectly both before and after.  Checkeredflag

The decoder is now happily re-fitted into a Scalextric Rover SD1 with blue roof lights.

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Repair Step 1
Test diodes with a Digital meter (in diode test mode) and replace any that are faulty.

I will discuss diode testing in a future post - basically we set our test meter to ‘diode test mode’ and probe across each of the four diodes in turn. For each diode we check with the leads in each polarity. In the forward direction the meter should read in the range 0.2-0.7 Volts. In the reverse direction the meter should read infinite Ohms or there abouts. If any of the diodes give an anomalous reading remove from the board, test again and if necessary replace with a new one.

Repair Step 2
Remove both output MOSFETs and replace only the n-channel MOTOR drive MOSFET with a new one. This is the MOSFET labelled N02 on the rev G decoder board.

Please click on photos for a full frame and full resolution image.

Photo: The two MOSFETs which are to be removed.
Photo: After removal and pad clean-up.
Photo: N02 refitted (This should be a new replacement part).

Repair Step 3
If the decoder then functions, i.e. motor is controllable, then replace the p-channel MOSFET. This is the one labelled P03 on the rev G decoder board.

Photo: P03 refitted. Again, this should be a new replacement part.

If the decoder then functions i.e. motor and brake are controllable the job is done. If not a more advanced diagnostic is required.
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A further photograph showing how the MOSFETs are fitted. The MOSFET is held in place with tweezers (see left and right of the MOSFET). Then, heat is briefly applied to one of the pins to ‘tack’ it into place using residual solder on the pad. This locks it in the correct position. Next, fresh solder is applied to the other two pins, and finally the first pin is revisited with fresh solder.

Result - a correctly positioned MOSFET with three reliable solder connections.

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That's a truly awesome resource Dr_C  Thumbup

I shall get a new pointy tip for my 18W Antex CS18 iron - and wait to blow up a chip!
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When it comes to fitting replacement MOSFETs it makes sense to replace faulty MOSFETs with the same parts as originally fitted. And, all the original parts are currently available from major electronics conponents retailers (as of 1/12/23).


C8515 rev F and rev G

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

C8515 rev H

Marking ‘34N’ = DMN3404L (Motor)

Marking ‘R1’ = CJ3401 (Brake)

In the new year I will share an updated list of alternative MOSFETs which represent an upgrade and/or improve overall decoder resilience.


Not quite the New Year, but I have been testing out my 2024 upgrade recommendations for the C8515 rev G and the C7005 in-car decoders. This involves replacing the four rectifier diodes plus, of course, replacing the two motor control MOSFETs (labelled N02 and P03).

My test cars are NSR Mercedes AMGs with Evo King 21.4k motors. I consider these relatively high torque motors and so a good test for upgraded decoders.

It’s all looking promising, the decoders are staying cool and the wheels can be locked at full throttle without any unwelcome smoke. More testing and then I expect to report back on time in early January.

Meanwhile I hope you enjoy the festive season.

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Chart showing the original fitted MOSFET part numbers for the following SSD decoders:

The left hand column is for the C8515 (revs F & G) and C7005.

The right hand column is for the C8515 rev H.

Clearly these MOSFETs can be used to replace failed MOSFETs and in doing so retain the OEM specification. All of these MOSFETs remain currently available.

The centre column provides an upgrade path for the C8515 (revs F & G) and C7005 which increases maximum motor current from 1.7Amps to 4.0Amps (at 25degC). If carrying out this upgrade the four rectifier diodes, also, should be upgraded as will be described in a separate post.

While not a current upgrade for the C8515 rev H, the same centre column also provides a useful repair route for the C8515 rev H. In this case the replacement MOSFETs add robustness against high voltage spikes thereby improving protection against electrostatic damage (ESD). This is achieved by integrating a pair of back-to-back Zeners.

So these are my suggestions as of 2024 for MOSFET repair and/or upgrade to SSD decoders.

If anyone wishes to try the centre column i.e. my suggestions (recommendations) - please let me know how you get on. 

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I have just let the smoke out on a decoder. The brake Mosfet and transistor failed when I accidentally shorted the decoder. 
This guide came in very handy as a reference to find parts on a donor decoder. It's all working now.
Thanks Dr_C.  Thumbup
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