Repairing blown function output?

[Pillar]

Due to incorrect wiring while testing out a breadboard project I managed to fry the yellow function output on a Hornby 4 function decoder . Other than the faulted function being suck 'on', the rest of the decoder still seems to work correctly.

 

Looking at the circuit board, it seems fairly obvious which component emitted the magic smoke (see attached). It appears to be marked 'H46', but I can't find a sensible match for this so far in any lists of SMD devices. I'd expected perhaps a PNP transistor, but it uses a 6 pin package.

 

Has anyone had success with identifying and replacing parts like these in the past?

 

Thanks,

 

Liam

 

 

Edited January 28, 2020 by Pillar

[Crosland]

Looks like a SOT23-6 or a SOT323-6 package. It's probably a dual transistor driving two outputs. Having two such on a 4-function decoder would add weight to that. A single transistor would be 3 legged.

 

[edit]It is most likely an NPN transistor.

 

Tracing the PCB tracks would give a clue to the pinout. I would expect the output to be the collector, the emitter connected to common and the base to the processor. You may need a part with integrated base resistors. Unfortunately I can't find a reference to the H4* marking code that fits the bill.

 

Once you know the pinout it shouldn't be too difficult to find a suitable candidate.

 

How are you at soldering? My technique in this case would be to slice down through the legs with a fine scalpel blade. Remove the bits that are left with a fine soldering tip. Tin each pad, a little flux, hold the replacement in place and reflow each pad. Use solder paste if you have it, but everything needs to be suitable for electronic work, not model making. No Power Flow plumbing flux, or the like!

 

Edited January 28, 2020 by Crosland

[Pillar]

Hi Andrew,

 

Thanks for your reply. There are actually four similar ICs on the decoder (another photo attached). It does seem as if there might be one per function; but as you say, single transistors would be 3 legged.

 

My soldering is ok, but I haven't done any SMD work before. Your method of slicing through the pins before desoldering sounds like the way to go since the chip is dead anyway.

 

[Edit] - I don't generally use separate flux for electrical work, but I have some ancient brown BICC flux which has worked well for stubborn joints in the past.

 

Do you have a particular reason for thinking the transistor would be NPN? My understanding was that DCC is common positive and most DIY lighting circuits seem to use PNP transistors.

 

Cheers,

 

Liam

Edited January 28, 2020 by Pillar

[Crosland]

2 hours ago, Pillar said:

Do you have a particular reason for thinking the transistor would be NPN? My understanding was that DCC is common positive and most DIY lighting circuits seem to use PNP transistors.

 

Yes, functions are common positive (the blue wire). The load goes between the blue wire and the function output. The output is pulled low by an NPN transistor driven from the microcontroller.

 

See "Using an NPN transistor" here: https://elinux.org/RPi_GPIO_Interface_Circuits

[RAF96]

Board codes Q11 and Q12 are both UMH4 dual devices, one driving F0 and F1 and the other driving F2 and F3.

 

 

https://www.datasheetarchive.com/pdf/download.php?id=a21482717862d88452a1f05b2e8c2c5fccd630&type=M&term=UMH4

 

 

[NIK]

27 minutes ago, RAF96 said:

Board codes Q11 and Q12 are both UMH4 dual devices, one driving F0 and F1 and the other driving F2 and F3.

 

 

https://www.datasheetarchive.com/pdf/download.php?id=a21482717862d88452a1f05b2e8c2c5fccd630&type=M&term=UMH4

 

 

Hi,

 

I hope you don't mind me saying well done on finding the part.

 

I may have interpreted the datasheet wrong but I think the UMH4 has integral 10k base resistors:

 

Regards

 

Nick

[RAF96]

Integrated base resistors was what Andrew said was likely.

[Crosland]

On 28/01/2020 at 16:41, Pillar said:

[Edit] - I don't generally use separate flux for electrical work, but I have some ancient brown BICC flux which has worked well for stubborn joints in the past.

 

Nor me, cored solder (or solder paste) for normal soldering operations, but if tinning and then joining two parts a little extra sometimes helps.

 

1 hour ago, NIK said:

I may have interpreted the datasheet wrong but I think the UMH4 has integral 10k base resistors:

 

They are the little boxes in the base in the schematic that RAF96 posted.

[NIK]

12 minutes ago, Crosland said:

 

Nor me, cored solder (or solder paste) for normal soldering operations, but if tinning and then joining two parts a little extra sometimes helps.

 

 

They are the little boxes in the base in the schematic that RAF96 posted.

Hi,

 

Well spotted re the little boxes. I thought it was worth pointing out the resistor value in case Liam has to use an alternative component.

RS don't seem to stock UMH4s.

 

Regards

 

Nick

[RAF96]

My experience of soldering smds is:

 

Method 1 is to apply solder paste over the leg pads area of the pcb, plant the smd on this splodge and apply heat using a hot air solder gun. When the solder paste melts capillary action puddles the solder onto the pads and settles the smd into place. Care must be taken not to have too much airflow or heat or other devices will melt off the board and be displaced.

 

Method 2 is to apply the solder paste s before then press the smd into place using  metal probe. A fine bit soldering iron is then drawn across one set of  legs which melts the older as above. Repeat on the other set of legs. My iron of choice is a 15W Antex with a 0.5mm bit.

 

Magnifying glasses are essential.

 

I have managed to solder these little spiders in place using both methods, but found the soldering iron easier than the gun.

 

 

 

 

[RAF96]

https://www.ebay.com/itm/10PCS-UMH4NTN-TRANS-DUAL-NPN-50V-100MA-SOT-363-UMH4-UMH4N-UMH4NT/192707230087?hash=item2cde3ef987:g:RAsAAOSwzINb2L0u

 

Closest I can find 10 K resistor.