Well, Did some diging.
F104 must use a 6amp fuse, because the magnet has peaks of 10amp.
And when f104 blows, its normal that coil #8 fires.
Found this interesting article.
Fuse F104 investigation
After playing our first few games on the newly restored machine, F104 blows as soon as the ball passes the magnet ramp (when the magnet is energized). This fuse does not blow in test mode. In investigating this problem on RGP, I found some interesting questions being raised about this fuse. They were:
What is the proper part to use for F104? Depending on where you look, it can be specified as a 2.5 Amp SB (slow-blow), 4 Amp FB (fast-blow), or 4 Amp SB. The fuse I was using was a 4 Amp FB.
When F104 is blown, and the Ramp Magnet (Solenoidis tested, it is the Ramp Diverter (Solenoid 21) that fires! Why is that?
When F104 is blown, and you disconnect the Ramp Magnet, when the Ramp Diverter is activated, it stays pulled in. Why?
Here are the results of my investigation:
Question 1: What is the proper size for F104? Answer: Using a clip-on DC current probe, the current pulse (in test mode) through the ramp magnet is shown below.
Current trace through the Ramp Magnet (Solenoidin test mode.
Note peak current of about 10 Amps, and the
ripple of the full-wave rectified 60 Hz line power.
Looking at the blow curve of fast blow fuses, and slow blow fuses we can understand how long the fuse will sustain this 10 Amp current. The data is summarized in the table below.
Type of Fuse
Average Blow Time @ 10 Amps (sec)
4 Amp SB
4 Amp FB
5 Amp FB
6 Amp FB
I would prefer to use a fast blow fuse if possible. That sentiment is shared by Martin Reynolds, another RGP member with electronics experience. After trying a 5 Amp FB (which was not successful), I installed a 6 Amp FB. After a few games, I realize that the Ramp Magnet is energized for as long as 2-3 seconds (!). It remains to be seen if the FB fuse will survive. Updates to come in the weeks ahead.
Question 2: Why does Solenoid 21 fire when F104 is blown? Answer: The schematic below was obtained by tracing the wiring in the machine and consulting the schematics of a WPC-95 machine. Note that the protection diode D41 is not directly connected to +50V, but is connected via F104 (!). When this fuse is blown, and the Ramp Magnet is activated, the two Solenoids are put in series and connected to +50V. This is the reason why Solenoid 21 fires when F104 is blown. The current path in this case is marked by the dotted line.
Current path when F104 is blown and the Ramp Magnet is activated. It causes the Ramp Diverter to pull in.
Question 3: Why does Solenoid 21 stay pulled in when the Ramp Magnet has been disconnected? Answer: Note that in the circuit above, that the coil collapse current is passed through the ramp magnet. If this magnet is disconnected, a potentially very large voltage can be created at the anode of D41. This may cause damage to Q26, and is probably why S21 stays energized if it is pulsed when S8 is missing.