I have a JM and all the gi on the same strand is not working. When testing for power at j115 it has 6.3v, but at j121 I have no voltage. Fuse is good.
From my understanding power comes in at j115 and goes through a pre driver and a couple resistors then into the "triac" which looks like a transistor attached to the large heat sink, from there power goes out to the connector
At j121.
The predriver tests the same as the others .8 so I assume it's good. I am unsure of how to test the triac. It is labeled as bt138 500e.
Is it possible to test it, and is it the same part as this one for sale at marcos.
http://www.marcospecialties.com/control/keywordsearch?SEARCH_STRING=bt138
Is there anything else that would affect this gi strand. Fyi it is the brown and white/brown strand.

I have ac power at the board j115.
The gi is working on every strand except the lights with the brown and brown/white wires connected to them.

I have continuity from j115-2 to j121-11. And continuity from j115-3 to j121-7. Those are the brown/white wires.

The solid brown wire at j121-1 goes to the triac at q18 the trace goes to the middle leg. It has continuity. The left leg of the triac at q18 has continuity to j115-8. Which is oddly an orange wire.

I hope this helps.

I posted a link above with one available from marcospeciallties. Is that one compatible?

I replaced the triac and the 5401 to no avail.

Here's some new info that may help.
With connectors j121 and j120 unplugged I get around 6 volts ac on the pins associated with the brown and white/brown wires. If I plug either j121 or j121 in the voltage goes to zero.

I got it and I'll do my best to explain. The solution was simple and LTG pointed me in the right direction but I didnt look hard enough.

Their was a hack job done to bypass the connector from the transformer. They eliminated the connector for 2 of the wires literally clipping the wires at the connector and crimping them together. The wires are actually pairs of wires and 1 of the wires wasn't connected. The unconnected wire was still in the connector but nothing was connected to it.

I figured it out the hard way. I was looking through the schematics and realized the orange wire (the aforementioned culprit) actually goes to the triac at q18. This is the triac associated with strand that was out but this strand is on the brown and brown/white wires.
Why does the orange wire feed the brown brown/white strand? I don't know but once I saw that in the schematics I focused on the orange wire checking for continuity from the board backwards and finally leading down to the transformer connector.
This is more of a ramble than a repair guide.

Long story short.. problem solved. All g.i is working.