I've done a lot of searching for a solution to the problem of no 12V trigger for the main amp on an Onkyo A/V receiver (although it has a mostly worthless Zone 2 trigger), and found a lot of workarounds on avsforum, but nothing simple and straightforward. So I decided to go to an old standby--hacking the hardware. Finding the circuit to hack took several hours, as the service manual for the SR702 consists of 131 pages and is not easy to get through. But it turns out that this is an easy hack, so I thought I would post a quick description of what I did here and, if anyone is interested, I will post a more complete "how to". Here goes:
Proceed with caution and at your own risk. But be aware that, at least on my SR702, the PCB that has the circuit to be hacked is very easy to get to and remove, and provides no other function than supporting the Zone 2 trigger and the IR In jack. Actually, during the time I was doing the hack with the PCB out of the receiver, the receiver was on and continuing to fill the house with music. So this PCB is not even needed for any other purpose.
Download the TX-SR702 service manual from here
or elsewhere. Print pages 28, SCHEMATIC DIAGRAM 10 I/O termianl sections (their spelling, not mine) and 46, NAETC-8367 U3 Control terminal PC board (your page numbers may be different, so you may have to search on the text).
On the schematic, note that Z2, the zone 2 12V trigger jack, is turned on when Q174 is switched on by signal 12VTRGC, which in turn switches on Q171, which provides protected TRG12V to the trigger jack. 12VTRGC is actived when Zone 2 is switched on. The object here is to have Q174 activated by TRG12V directly, which is output from the amp's PS whenever the amp is on, and is not active whenever the amp is totally in Standby.
So what you need to do is cut the 12VTRGC land pattern near where it enters board U3 at connector P122-3 (counting front to back). Use the PCB board view you just printed for a guide. BTW, remove the PCB carefully. There is a tab on the connector that has to be released to get it out. To be sure you are cutting the right land, physically trace it from the connector to the base of Q174, located at B4 on the PCB view. Probe the land where you plan to cut it and make sure you have 0 ohms to the base of Q174. Be careful to not cut adjacent lands.
12VTRGC, which supplies Q174, when active is +4.66V. I could not find any specs on this transistor, KRC102S, that provides max base voltage, but on others in the area it is limited to 10V. TRG12V is 12V, so just for safety I made a splitter consisting of two 1K ohm resistors, one connected to ground and the other to TRG12V, which provides a base voltage of 6V to Q174. (I used 1K ohm resistors because they were on hand; chances are lots of other values higher than 1K would also work--just do the math and make sure that the splitter provides an appropriate voltage to Q174.) After soldering one end of each resistor to ground and TRG12V respectively, twist and solder the other ends together and solder a single lead of that end to the Q174 side of the previously cut land pattern at any convenient point. You can either solder directly to the base of Q174 or, like I did, clean a piece of the copper etching for that land and solder there. Use a low wattage iron with a very clean and pointed tip, as things get close in this area.
Examine and meter your work carefully. A short could pop a PS fuse or even worse take it out. Hacking the wrong land and/or transistor? Who knows?
On my Onkyo this PCB is mostly unpopulated, and uses only one of the six possible back panel connector outlines etched thereon. So it is possible that this PCB in various implementations are used on many Onkyo receivers. These hack instructions may, therefore, be applicable to more than just my receiver.
As I mentioned, if someone is interested, I would be happy to provide further detail with a couple of illustrations. Good hacking!