Repair Logs x3.(December 2011) - MPU4 PCBs

 

Fault: MUX drives alarm

 

PCB alarms immediately. Fault is related to the lamp drive section.

 

Source of fault is lamp drive transistors. Attaching the sniffer loom reveals that 8 transistor lines are shorted. The PCB will only run if 8 of the 16 drives are turned off.

 

Metering the transistors using the diode test reveals 8 transistors 4 in selects and 4 in the drives section which are shorted.

 

Replace all 8 transistors, the board powers and boots into attract. Leave on test for 4 hours.

 

No further issues. Board has had transistors replaced in the past, so one to keep an eye on. Add fly lead battery and reattach case.

 

Fault: Reel A Setup Alarm, boot halts on alarm

 

Machine refuses to boot on either a MOD2 or MOD4 card.

 

The fault is not related to the reel drive, but is related instead to the optics. As the reels index, the optic on reel A cannot be read. Initial tests on diodes, resistors and transistors relating to the optics don’t reveal a problem.

Start testing, ULN2803's are all shorted on pin 9 and 10, but only for 2 seconds, then the meter shows charging, like a cap is under load.

 

Change IC27 and IC31, seems OK, but when powered same fault, Reel A setup alarm. Re-test, short is back. Sometimes the short is there, sometimes not. When it is there, it meters for 2 seconds and then clears.

 

Cut off all the tants to see if they are shorting the 12v line, C38, C49, C55, C20, C53, C20. No same fault.

Cut off electo C56 and C58, nope same fault.
 

 

 

 

Replace PIA IC4, nope same fault (40 pin IC so not a 2 minute job)

Repace PIA IC6, nope same fault (another 40 pin IC, it's getting tedious)

Replace all 8 ULN2803s, now the short seems to be permanently gone, but the fault is the same, Reel A setup alarm.

At this point, you start to lose the will to live...  

Continue testing comparing results against a good board...

Over protection diode ZD19 meters bad, replace it, it still meters bad in circuit, but good out of circuit.

Went back to RU101 just off PL16 PHOTO, pin 2 is way off, changed RU101. Good out of circuit, bad in circuit. How can that be? It doesn't make any sense?

So the only place that goes back to IC4. But I had changed IC4 for a brand new part.

Change IC4 again.... (bad IC4 to begin with)

She boots!

 

Fault: 12V fuse blows immediately.

 

12v line is shorted. 5v line is not shorted.

 

After replacing the fuse, the LED lights for a fraction of a second and fuse blows again.

 

Source of fault is a shorted 12v component.

 

There are burn marks and signs of arcing around IC30. (ULN2803A) - Reel drive IC.

 

Meter IC30. 3 lines are shorted.

 

IC30 is one of the 5 reel drive IC’s.

 

Cutting it off the board removes the short.

 

Power board, boots OK.