Stern Pinball (Stern, 1977) coil q14

[gchase59]

Anyone know which coil is Q14 on a Stern "pinball" game

 

[pinballdaveh]

The stern pinball machine called pinball. The pinball database schematic shows that q14’s output goes to jack j5 pin 11 which says it drives the 3 left drop target reset coils. It’s rare that 1 output transistor would drive 3 coils at once, so if there is any drag or problem resetting these targets the extra current surge might blow out this output transistor.

 

[gchase59]

Great thanks, I had kind of figured out that it was the targets. I have a ultimate Solenoid driver board and it has LEDs that show red if something is wrong. Q14 is red. I tested all three coils with one wire un-soldered, they each read about 16 - 18 ohms which is what I would expect. Since all are in parallel I get 5.5 to 6 when connected. I connected a jumper to the wire that actuates them and tapped ground. They all actuated but something else happen. The MPU rebooted. I have had a problem with the MPU rebooting randomly as well. So now I am wondering if they are related or if I just caused it by doing something stupid. Three coils cause quite a draw. I am open to any thoughts you might have.

Thanks for your help
Gary

 

[pinballdaveh]

Does the q14 led stay lit all the time, briefly pulse when the coils fire , or constantly pulse? Have you tried hooking up a dc amp meter to find what each coil draws and the total amp draw with all 3 coils connected? Try resolder the coil wires one at a time and check for mpu reboot. Has anything been done to the power supply rectifier board as far as reflowing it’s connections on its back side, and is it the original board? When a mpu reboots it’s usually a loss or a lowered of voltage usually 5volts. When a coil amp surge happens it might pull down the 5 volts causing the reboot. Has the rectifier board had its female connections been repinned? You can try calling alltek tech support to find out what they recommend.

 

[gchase59]

Q14 LED flashes rapidly. I have resoldered the wires while doing my tests. I replaced the rectifier board when I put in the ultimate MPU and Solenoid boards. I have not measured the current draw. If I leave any one of the coils unhooked the LED does not come on. I will check the connections to the rectifier board. I will also measure the 5 volts to see if it is low to start. I will give alltek a call. Thanks again for the help.

 

[gchase59]

Quick update. I turned on the machine and was looking at the solder joints and measuring voltages. when it started to reboot every few seconds so I measured 5V TPI on the MPU. it was stable at 4.7volts. At one point I code hear a hum for a couple seconds. The sound you get when you put a heavy load on a power supply or a coil is stuck on. So something is loading it down the voltage. I had not start a game or done anything other then turning the machine on. After about 5 minutes it stopped rebooting.

 

[pinballdaveh]

What do read at the 5volt test points on the rectifier board and the solenoid driver board? 4.7 volts is border line low. Does the voltage increase after the mpu stops rebooting? Ask alltek if the solenoid driver board has a under and over voltage protection in its regulator circuit to cause this rebooting.

 

[gchase59]

OK, on the rectifier board the 5.4 VDC reads 6.2 which seemed pretty high to me. so I check the 5 volts on the solenoid board and it reads 4.9 Which made me feel better. I still get 4.7 on the MPU test point. So I started checking other voltages. 43 VDC was 45.7 VDC, 230 VDC was 239 VDC, and 11.9 VDC was 15.5 VDC Yikes.

So I checked the AC feed for the 11.9 coming from the transformer and it was 13.1 VAC. Since the bridge can't amplify I got a short somewhere. I checked the wires from the transformer to make sure I had them soldered to the correct points and actually took a reading at the solder joints and got 13.1 VAC. I checked all the other wires to make sure they were in the correct places(for the third time) and they were. I was thinking maybe I will pull the fuse (F3) to kill the AC and see if I still get a reading at the 11.9 test point. I can only assume something is shorted to the circuit.