Changing the doghouse on an ST head

Let’s face it – the doghouse that comes on most Chinese ST heads is just a disaster waiting to happen.  The metal is flimsy, the connections look like they were soldered by a two year old and the circuit board (for want of a better description) is prone to breaking.

What to do?

Well, with a little work you can change out that piece of Chinese junk for something much more substantial and while you are on, put in a good, North American rectifier.  Now this will take some concentration and whatever you do, don’t start the job if you are short on time or you have important work to do for the better half.

To begin, you will need

Parts:

• A common, 8” x 8” empty electrical junction box (you may need a larger one for bigger heads, depending on your local electrical code)
• Three multi-station electrical connectors
• A solid state rectifier
• 4 x 1” ¼ x 20 bolts
• 4 x ¼” spring washers
• Optional – coloured electrical tape, red and white

Tools: 

• drill (drill press if available),
• wire cutters/strippers,
• screwdrivers,
• ¼ x 20 tap,
• magnet and clean white cloth 
• three coloured pens capable of writing on wire insulation are also handy, as well as a piece of paper.

O.K., now you are ready to start:

1. Take off the doghouse cover.  Have a look at the innards, noting where the two “hot” wires and the two neutrals terminate (you will only see one neutral bus bar on top of the yellow board, there are two wires connected to the underside)  These wires should be marked U1 and U2 (hot) and U5 and U6 (neutral).  Also, look at the black rectifier, you will see four wires, two labelled as Z1 and Z2 going to AC connections on the rectifier,, and two labelled U3 and U4 (one is marked positive and the other negative, make a note of which is which)).
   
   
2. Check the wires going to the rectifier, voltmeter, switch and light.  You have too many here, some will be removed as described below.
   
   
3. Have a look at how the yellow board is fastened to the top of the generator, there are 4 bolts.  Start by removing these bolts, making sure nothing drops into the genhead through the holes in the top.
   
   
4. Now the board is semi-free, it is being held by the wires.  First, mark U1, U2, U5 and U6 (I use coloured lines on each wire and keep a record on the paper)  Now, cut those 4 wires where they are soldered into the bottom of the yellow board.
   
   
5. Trace the wires coming from the rectifier The negative lead (U4 in the example) goes to R1 on the board. This is connected to FR2 via the bus you can see on the top of the board, the wre from FR2 goes into the head. Ths is the wire you need, cut and mark this as negative. U3 goes from the rectifier to F1, then out from F1 into the head, again, it is this second wire you need - mark that as positive Z1 and Z2 should go directly into the genhead, so no messing is needed with these wires. After all this, you will be left with two extra wires that went to the switch and light. These are unecessary and can be capped - mark them white to remind you. Please mark all wires despite them having nice little metal tags labelling them as U1, U2 etc - Mr Murphy states that if you do not mark the wires, then the tags will fall off!
   
   
6. Now you should be able to remove the yellow board and the base of the doghouse frame, leaving 10 wires coming out from the holes in genhead.
   
   
7. Next you need to prep the junction box.  Measure the distance between the mounting holes where the old doghouse was, transfer these measurements to the bottom of your new box at one end.  Drill 5/16” holes at these locations. (I always leave a bit of wiggle room in case I don’t get it just right)
   
   
8. Next, you need to drill a couple of 2” holes kind of central between the four locating holes you did previously.  The trick here is that you want these holes to be immediately above the large holes in the genhead when you have the new box mounted.
   
   
9. O.K., time for a check – do all of your holes drilled so far line up?  If so, things are going well!
   
   
10. Next thing to do is to prepare the head for the new bolts.  Tap the four old threaded holes with the ¼ x 20 tap.  This is where your magnet and white cloth come in – wrap the magnet in the cloth and place it under each hole you are tapping – that way you will catch all the metal shavings and not drop anything into the works of the generator.
    
    
11. Now to mount the connector blocks.  Dry fit them into the box, making sure you have space between them (you don’t need much)  Mark the holes and drill.  Fasten the connector blocks to the box with suitable nuts and bolts.  I use red loctite on mine to ensure they don’t come loose.  Note I’m not a big fan of self tapping screw for this job, I worry about them working out.
    
    
12. Pick a spot on the side of the box, drill a hole and mount the rectifier.
    
    
13. O.K., now to put the whole thing together. (The one illustrated does not have 2" holes, it would be better if they were so the wires can go through easier)
    
    
14. Place the box on top of genhead, pulling the wires through the large holes (note,      you will need to seal the edges of these later).  After you have all the wires pulley through and you are happy with the placement, bolt the box down to the head using the ¼” x 20 bolts.
    
    
15. Mark your U1 wire with the red tape, then install it in one of the connector blocks.
     
16. Connect U2 to the farthest connector from the U1 block (I like to use the centre block for neutral)
    
    
17. Mark both U5 and U6 with white tape and connect them to the last (central) connector block.
    
    
18. Install female spade connectors to the 4 wires that will be going to the rectifier.  Note on the rectifier that there is one pin marked + and one marked AC ~.  The other two are unmarked, but the second AC is diagonally opposite the first.  Same for the negative - diagonally opposite the positive.  Connect up the 4 rectifier wires – you are just about done.
    
    
19. Check the tightness of your connections, double check everything.

If you decide to install the voltmeter,

you can drill an appropriate sized hole in the top plate of the junction box (make sure it is not directly above the connectors), install the meter and lock it down.  Run SMALL wires from the two hot leads (U1 and U2), one to each side of the voltmeter.  Give yourself enough wire so you will be able to take off the top plate and set it aside if you need to work inside the box.

Now it is time to prove your work. 

Leave the top of the new box off and fire up the engine.  If it is not lugging or running slowly, then you have the wiring correct.  To check, put a voltmeter across U1 and U5/6, it should show 120 volts or so.  Repeat across U2 and U5/6, again it should show about 120 volts (the two reading will most likely not be the same, there is some fluctuation in these heads).  Lastly, check across U1 and U2 – you should have 240 volts.  If all is well, shut the engine down and make your external connections.

Clean everything up, tape and insulate as necessary and close the box up – you are done!

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