Blowing TIP102

[robm]

Looked at a T2 today that was working fine, then the plunger coil locked on, blowing Q58 as well as the associated fuse.  So i took the power driver board home, replaced Q58 (TIP102), plus its 5401 predriver, as well as the diode.

I put the repaired power driver board in a Dr Who at home, and it worked fine, with all coils firing and no lock ons.

So this morning i took it back to old mates place, put the repaired PD board back in, started a game and the transistor was blew straight away.....

So then i looked at the coil itself - resistance was around 3.5 ohms, so prob OK. I then got the kickback coil (same one - i think its 23-800), and put it in the plunger, put another diode, TIP102,5401 in PD board, and same problem again.

Any ideas - its obviously not the coil, the coil lugs are clear of any earthable objects, so i'm leaning towards a wiring short - although seems very unlikely as it happened out of the blue when the bloke powered the machine on.  Power driver board problem seems unlikely as i thought i would have picked it up in Dr Who?

Any ideas on what more or how to test?

Thanks

Rob

[Marty Machine]

First thing comes to mind is to check if the plunging mechanism is actually moving?
Even though the coil is fine it might not be repelling the plunger causing a greater current draw in the coil and killing the driver.

[robm]

To follow this up, i decided to take another power driver board over. Checked the switch in shooter lane, and all good. Fired it up and it works perfectly. So i am suspecting that U4 or U5 which control the TIPs has been destroyed. Although i'm still scratching my head why it worked in Dr Who - and it does use the same Q58 transistor for one of the slings... Anyway, i left the working board with him and will hopefully repair this one soon