solenoid board woes....help needed

[Sinbad79]

Hi guys
As you may have gathered by now, im working on repairing a stern galaxy.

Rectifier board has been rebuilt and tests OK
New Alltek MPU board installed and boots OK

I am now having issues with the solenoid board, whilst voltages test out OK, it keeps blowing the same transistors, Q3, Q5, Q10, Q11 and the under playfield fuse.

I am at a loss as to why this is happening. All i can think of is that something is wrong with an IC on the board itself.

Short of replacing with a new board, I am at a loss....any help appreciated.

[Ballywannabe]

Blowing the under playfield fuse usually indicates a short somewhere.  Q3 is the knocker, Q5 top thumper, Q10 lower left thumper, 11 one of the drop target coils.  First check all of these areas for shorts (particularly the drop target coil), or incorrectly wired coils (the common wire should go to the same lug as the  banded side of the coil diode).  I have never had a board that blew transistors on its own, but it would be simple to check, just dont, put any of the large plugs on the SDB, and disconnect the knocker.

Hope this helps...

[Sinbad79]

ok noted, i think i need ot go on a bit of a discovery mission with this and see where it takes me.
Coils resistance test out OK. As you say, i need to double check the wiring of the coils and perhaps the diodes too.

I also seem to recall Nino mentioning something about one of his boards blowing transistors so he had to replace some of the IC's.

Something has gone very wrong with this machine at some point, the MPU had blown components and so too did the solenoid board.
Wasnt able to repair MPU board so i've replaced with an Alltek, so solenoid board issues are all which remain.

[Boots]

I am now having issues with the solenoid board, whilst voltages test out OK, it keeps blowing the same transistors, Q3, Q5, Q10, Q11 and the under playfield fuse.

How is it blowing the transistors and the fuse?
The fuse is there to prevent the transistors blowing.
Are you over fusing it?

[Sinbad79]

no, its a 1amp slow blow fuse.
I'm not sure whats going on to be honest... you're more than welcome to come take a look :)

[Homepin]

The fuse is there to prevent the transistors blowing.

That is not quite correct. The fuse is there to PREVENT THE MACHINE FROM CATCHING FIRE and nothing else.

A fuse is a mechanical device and a transistor is a semiconductor - excessive load on the semiconductor will fry it in micro-seconds where the fuse will take MUCH longer to go open circuit. By the time the fuse blows, usually damage has already been done to the semiconductors.

[Sinbad79]

So i've done some reading on the net regarding the purpose of diodes across a coil. If anything this journey so far has been very educational.

It seems that the voltage spike created when the coil is activated and subsequently released is opposite in polarity. The end of the coil connected to the collector of the transistor will be positive with respect to the coil's other end and unless something were done about it (ie the diode), the collector voltage could rise so far above the supply value that damage might be caused.

A bad diode across the coil would explain why a transistor would fail - definitely, however, would this explain why the transistor is failing PRIOR to even activating the coil, ie on first powering up the machine?

[Homepin]

If the diode was wired the wrong way IE the banded (cathode) end connected to the ground side or (usually) the switched side it could cause the problems you mention - this is a VERY easy mistake to make by simply soldering in a new coil with the wires reversed on the two terminals. 

[Sinbad79]

OK, so what is an idiots way to verify the wiring of the coils to the harness and diode across the coil is correct ?
Thanks

[Boots]

OK, so what is an idiots way to verify the wiring of the coils to the harness and diode across the coil is correct ?
Thanks

The fat daisy chained wire is the power - to the banded side of the diode.

[Sinbad79]

sure, but are the coil lugs denoted as +/- anywhere, or does this not matter so long as the diode is orriented correctly according to the daisy chained lug?

[Boots]

The fuse is there to prevent the transistors blowing.

That is not quite correct. The fuse is there to PREVENT THE MACHINE FROM CATCHING FIRE and nothing else.

A fuse is a mechanical device and a transistor is a semiconductor - excessive load on the semiconductor will fry it in micro-seconds where the fuse will take MUCH longer to go open circuit. By the time the fuse blows, usually damage has already been done to the semiconductors.

Yeah I see what your saying, coils are locked on due to a board fault, fuse blows to stop the coils overheating and creating a fire.
But if the board fault is repaired and the correct fuse is installed but the coils have a short, surely some protection is provided to the board by the fuse.

[Homepin]

surely some protection is provided to the board by the fuse.

Practically none as the semiconductors will short before the fuse even realises there is an overload - even fast blow fuses are 200 times slower than any semiconductor.

[Sinbad79]

I have a feeling the issues lie with U2 on the driver board, pins 19-23 dont measure out the same resistance as another known good board.
Would this make sense? What is the purpose of the 74hc154p?

[Ballywannabe]

A bit of history on the machine might help.  Did you get it DOA?  Are there signs someone has tried to fix it and failed?  As I said, connect the SDB up with none of the coil plugs (large pin plugs) connected.  If this blows the transistors, you know you have a board problem.  If not, you need to look downstream.  Coils are not polarised, so it is the orientation of the diode that determines which side the common (usually yellow) wire goes on. 

Resistance probably means nothing - diodes and transistors are tested using the diode setting on your multimeter.  Have a read of the PinWiki:  http://www.pinwiki.com/wiki/index.php?title=Bally/Stern

cheers
Ian