Sympatico 751D Switch Modification

[regme]

Hi

 

I bought a bunch of 751D switches to control (twin coil) Hornby point motors (R8014). The problem is what I recieved was the 751K (single coil).  Before someone mentions it, I have sent an email to Ken, however being on the other side of the world I didn't fancy my chances.

 

The second problem is I'm not an electrical engineer.

 

So based on that, is there a way of converting the 751K to a 751D.  From the parts list it seems I have all the parts to create a 751D, but I think the circuit boards are different.

 

Does anyone have a picture of the back of the 751D so I can compare it and maybe modify it work, or do I just cut my losses.

 

Want I liked about this switch is that I could also run an LED dwarf signal to indicate the position of the point.

 

If needed I can post up the circuit boards.

 

Cheers

[Gordon H]

Looking at the two circuit diagrams here:

http://www.classixaudio.com/pdf/751D.pdf

http://www.classixaudio.com/pdf/751K.pdf

it looks like all you have to do is remove the 751K's P Channel FETs (IRFU90140) and optionally their 1k Gate resistors, then connect as per 751D.

Not clear whether the 'Coil Com' arrangement is meant to be slightly different between the two versions, but you might have to bypass the 1N4003 diode to make them the same.

I suspect the 'Coil Com' connection is shown incorrectly on the 751K diagram.

I would also suggest that 24AWG wire is a bit thin for driving solenoid coils over longer distances, due to voltage drop. The resistance of the connecting wires needs to be insignificant compared to the coil resistance to ensure maximum power transfer.

[regme]

Thanks, I thought I had to connect the "Coil Com" on the 751K.  The big issue is the circuit board I don't have the 751D to compare it to, unless I make my own up.

[Gordon H]

It should be fairly easy to follow the tracks to see what is connected to what. You could also probe it with a multimeter for continuity testing.

[regme]

Thanks I'll check that out