Fixed: Slingshot not working...

[Mfresh]

Just been shopping my Earthshaker, and now the left slingshot doesn't work.

I've checked power to the coil, and its getting a good 75V. I've grounded the earth lug, and that causes the coil to fire. And I have grounded its TIP122 (Q71), which causes the coil to fire. So the coil itself is working fine, as is its main transistor.

The slingshot switches both register points when they are closed, but the coil doesn't fire.

Anyone have any ideas what the problem might be? 2N401 pre-drive transistor?

Thanks,

Paul

 

[Jay Walker]

Hi, Paul,

Even though it isn't driven as a 'Special' solenoid, the slingshot uses one of the Special circuits, so the drive signal will be the Cpu controlling one of the 'Special Solenoid Trigger' lines, gated through one of the small chips (U 45) in that corner of the pcb to a pre-driver transistor, Q 70.

Grounding the tab of the TIP 122 shows that the return circuit from the coil to the transistor is okay, but not necessarily that the transistor is. I'd check the 122 by momentarily touching its base terminal to 5v, or using a meter to check if the drive pulse is arriving from the pre-driver. A handy spot to measure from is the base resistor of Q 71, R 99. This is grounded at one end, so be sure to use the other, especially if applying 5v.

 

[Spandangler]

Damn it... Jay beat me to it

 

[cooldan]

Yeah I was gonna say that

 

[Mfresh]

Thanks Jay. I'll investigate as you suggest.

 

[Mfresh]

So the Tip 122 is working as verified by touching 5V to the base - it triggers the kicker solenoid as it should. Next step seems like replacing the U45 chip or the pre-drive transistor. The U45 is socketed and Phil sells them for 45p or so, so I will replace that next before messing about replacing that tiny Q70. Just waiting for a delivery from Phil now....

 

[DRD]

When dealing with stuff like this I put my meter into diode mode. I then test the various readings it gives across the three legs of the two transistors that sink the relevant troublesome solenoid. I then do the same checks on a known working solenoid's pre-drive and power transistors. If there are any material discrepancies then I replace the relevant transistor (s) - this usually sorts it

You can usually do the testing with the board in situ in the backbox.

When it doesn't I then assume it is beyond me and I send the board to Andy Netherwood.

 

[Mfresh]

So I have confirmed that the drive transistor Q71 is working, and I have replaced both the tiny predrive 2N4001 pre-drive transistor Q71, and the logic chip U45. Still no luck. I think the problem must be upstream - resistor pack or capacitors perhaps? Coil is not locked on though.... Any suggestions what to check next before I bother @pinballmania ?

 

[stoptap]

So I have confirmed that the drive transistor Q71 is working, and I have replaced both the tiny predrive 2N4001 pre-drive transistor Q71, and the logic chip U45. Still no luck. I think the problem must be upstream - resistor pack or capacitors perhaps? Coil is not locked on though.... Any suggestions what to check next before I bother @pinballmania ?

Have you checked R99 ? The 2.7k resistor.

 

[Mfresh]

Not yet. But I will now....

 

[Mfresh]

Well got to the root of the problem in the end after a quick phone call with @philpalmer and, following his advice, the purchase of a £7 logic probe from ebay. The U45 logic chip had been socketed at some point by someone, and the track between one of the pins on the chip socket and the zener diode was broken somewhere under the socket. A wire bridge between the pin and the zener diode restore continuity and it works again.

A bit of a Dennis Wise fix: ugly but effective...