Pins 3 and 8 on the modules should be almost zero volts. (0.000)
Pins 1 and 0 on the modules should have (1=+1.4v), (0=-1.4v). No signal.
This post below, is for a SX-1980 so some interpretation is needed.
One of Marks technical posts:
MTF, circa 2017
https://audiokarma.org/forums/index.php?threads/sx-780-dead-pa3004.717168/
https://audiokarma.org/forums/index.php?threads/pioneer-pa3004-ic-required.760886/
PA3004 troubleshooting
That PA3004 chip rarely goes bad, it's usually the support circuitry around it that gets cranky.
It's function is the protection sensing, then controlling the relay appropriately.
It's not all that rare, it's pretty widely used in MANY Pioneer models of the era,
so a salvage chip from a donor unit is best.
Just for grins I will do a rundown on the PA3004, perhaps your service guy can read it, and say BTDT...
There's nothing worse than digging up a unique chip, putting it in, and getting the same thing happening.
Many have mistaken the PA3004 for the problem, and Pioneer does many strange things with and around the PA3004 in the SX-1980.
First there is the +13v power for the PA3004, on Pin 1, it's taken off the +34v supply, which depends upon the +80v supply.
I will grant that if the zener diode D301 (13v) opens up, the PA3004 could be fried through the 2200 ohm 1/2 watt resistor if the +80 gets onto the +34v rail.
edit: 5/28/2021 johnhoef has proven that 10v on pin 1 is inadequate to run this chip.
Pin 3 is the output to the base of Q302, a NPN relay driver transistor, I usually use a higher voltage (120+) to-126 instead of the to-92's they usually use.
The Q302 driver transistor and relay (and 3.3k 2W resistor) circuit is run off of the +100v raw dc, so if Q302 shorted or had an inadequate voltage rating, the PA3004 could be fried. Q302 sees +100v on it's collector when the relay is not on.
Pin 4 is the DC sense, and it LOOKS pretty straightforward, Left and Right channel power amp output through the 75k ohm summing resistors R304 and R305 but they had a SURPRISE!!! PNP transistor Q301 is ANOTHER DC input to the "summing node" and if leaky can drive someone nuts. That's not all because if IT is in good shape, yet the TWO off-board transistors that feed it (Q34 2sc869, one on each amplifier card) have a leaky one - it will still misbehave. The BOARD's pin 7 input for the protect sits near +13v and pulling it down some is the protect trigger. I just replace Q301 with a 2sa / ksa992 transistor and be done with it. The two sensing transistors (2sc869) on the gwh-109 / gwh-110 amp cards get 2sc / ksc1845
Pin 5 is the ground for the PA3004.
Pin 6 is another power supply for the PA3004 to operate, -7.6v derived from the -34v supply through R309 3.6k 1/2W. No zener, just the resistance... (yech)
Pin 7 is the AC power on/off sense for the whole unit: The 100v or more AC from the transformer secondary goes through a voltage divider R311 620k and R310 1.5 meg AND neglecting the current drawn by this pin 7, WOULD have about 30v AC at low current there. How much is actually there depends upon the PA3004's current draw on that pin.
Pin 8 timing capacitor to ground, 47uf 16v
How do I test that pin 4 junk if voltages look ok? Subtractive troubleshooting. Requires de-soldering so I pull Q301 and see if protect behaves, then a new Q301 goes in and I pull D302 to see if it is coming in from outside of the PA3004's board.
a successfully executed 6 way diode test. you are on your way.
, we look at the meter reading... and .... oops!!)