Hi Ken,
I am guessing the GS 4 schematic is close enough that it can be used for troubleshooting. Hopefully, you can read a schematic and have a simple digital voltmeter. I have uploaded a GS4 schematic that is relatively accurate. Since the GS 4 has the same controls as your unit, I am guessing the schematic will work fine.
If you look at the attached schematic, you will see an IC3 that has 4 internal IC's. I suspect your iC will have a different IC number. The last two internal IC's will be the ones we will focus on. Also, notice the reset switch. That will be used also. You will need a small wire with alligator clips on both ends to short the reset switch. You might be able to do this with a single alligator clip across the switch.
Now, note the threshold pot both on your detector and on the schematic. One leg of that pot connects to U3 on the attached schematic. Since the three leads go to different places, this shouldn't be that difficult. Once you have figured out which lead is which, then short the reset and measure that lead going to the opamp. It should read 0.00V oh the lowest voltage scale on the digital meter. This voltage should read 0.00V on the 200mv scale.
Let me know what this voltage is? I am guessing there is a negative voltage. Also let me know the IC number if it is on the pc board. Measure pin 14. It should have the same voltage as that on the lead going to to the threshold pot. Now, with the gain pot an minimum, the voltage should be .00 and should be the same on PIN8, which is the previous opamp output. If not, try to find the trim pot that connects to pin 9. It might be RV7. This trim pot center leg connects to a resistor (470K) and then to pin 9. Measure the voltage at the center leg of this pot and write it down. Let me know what it is also.
When measuring voltages, I am guessing the chassis is a ground connection so you can connect the negative lead of the digital meter to the chassis.
When checking voltages, make sure to note where the voltage was taken and the measure voltage. This makes it possible to put things back if necessary. When in doubt, take pictures also. Send me clear close up pics of the pc board if possible.
That is enough for now.
Reg