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Is there anything else connected across S06? It may not be that device, but something else across the same terminals/traces. Is it still 20 ohms when disconnected?
By the designations of other "S" components, it seems they call their diodes "S". It could be that this was a zener or some other diode that is shot, and shorted.
Without a schematic, hard to tell for sure. Where do the pins of S06 go to? Does one go to the power supply directly? Does it look like S07, S08, etc? Other diodes look like regular 1N4148 types.
Something not right with the schematic. S06 shows it setting a voltage across C3 (negative?)between ground and the -ve side of the cap. But, that line goes off to the R01 network where you show it going to ground.
I also doubt that transformer pins 1,2 and 5 all go to ground, though it may be possible if each is from a separate secondary winding.
Try to check those traces, If S06 is indeed across C3 like in the diagram, it would be creating (somehow) a negative voltage.
Now, if C6 was also a 47uF like C3, then I would say S06 is a 7.5V zener like S10, and both are used to create negative voltage supply.
This all said, the fact that the part sheets clearly labels Zeners, but not S06 as such, I tend to lean back to assuming it is just a regular small signal diode. This all depends on how accurate that parts list is...
If you figure the circuit draws 1.3W, then a 1.3W zener may not be big enough - you don't have enough cooling to keep it properly cooled (based on what you state). Also, since the voltage drops to 15V, it is entirely possible something else is drawing too much current. You have to look for other failed components.
It also sounds like the supply voltage, 200V pk-pk is a bit too high. The zener has to draw that down, even at 7ma, you are looking at a 160V drop at 7mA. That is a 1.12W power dissipation.
Hi I just repaired an Astrowars with this exact same fault I too had a working unit for comparison. The working unit was a v3 board and had an additional diode (SO11) in series with SO6. I wasn't able to ascertain the zener voltage from the good board either, so I ended up using an S06 and SO11 from a scrap Grandstand Scramble. I connected soldered the two zeners in series, then soldered the anode of one and the cathode of the other back into the board where the old SO6 had been. I imagine So6 was troublesome in the past so Grandstand made a design change to drop the voltage in stages by adding SO11. Unfortunatley since I didnt have a suitable DC source I didnt test the parts to determine their value. It then occurred to me I could perhaps have used a few 9v batteries in series to get a high enough DC voltage then slap a resistor in line and measure the zener voltage that way.... perhaps if I have a chance next week I might do that. I think I can get a few cheap PP3s at Poundland. If you figure out the value in the meantime let us know.