(Topic ID: 217499)

74LS139 getting cooked. Thoughts as to why?

By pb456

1 year ago

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  • Latest reply 1 year ago by pb456
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#1 1 year ago



74LS139 getting cooked.

This is a BlackOut game, with the MPU looking like it's had some work in the past (sockets, excess resin, cuts on the solder side from some sloppy solders I've cleared up). I've cleanly resocketed and resoldered this chip, in the "Address decoder" region of the MPU on this Williams System 6 MPU for the game BlackOut. The jumper J3 is wired with a 0 Ohm resistor, J3 is not connected. Looks like a pin is bridged on the solder side (connected in the data buss section in D4-D8 on pins 12 and 14 on chip ECG6889/P 312 185617KK) and the solder side pin 16's solder pad is gone. Does not look like any traces were connected to it.

Overall the board is not bad after I pulled off the old solder, reflowed new 60/40 rosin core solder on the main interconnect and anything else I saw that was off.

I would like some direction from those more knowledgable than I to narrow down the search. Thanks in advance.

#3 1 year ago

The address decoder IC - snip from the System 6 MPU assembly diagram (from the link above) - it's IC 15 by that diagram.

When I say getting cooked - meaning it's very hot to the touch, system won't boot. No other ICs or parts on the MPU are hot, on the MPU itself two red diag lights won't clear, diag button does nothing, switch DS1 has all positions off. I'll take parts pictures and post them here also. Thanks for the pro-tip!

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#4 1 year ago

Oh, I realize I mistyped my first post. J3 is connected to a zero ohm resistor, and J4 is connected to nothing.

#7 1 year ago
Quoted from creepykenny:

pull chip back out of socket and measure all voltages on each pin of that socket - make sure nothing is over 5v and ground really is ground and nothing is shorted to pin next to pin measured
possible defective chip? an internal short will cause a chip to get amazingly hot with just 5v applied

I think I'm onto something - pulled chip out, measured what I believed to be a short, measured socket, replaced with new chip.

Game starts but errors out - top left display reads code "1495 1" and bottom left display reads code "04 00".

I'm going to look for this code online and thank you all.

#11 1 year ago
Quoted from PinballAir:

Was notch lined up ?

Two of the legs on the 74LS139 was slightly bent under - enough I couldn't see it without pulling the chip and examining it closely.

It's been a lot of work getting to this point, glad I'm seeing the light.

#12 1 year ago
Quoted from Eric_S:

Sounds like you're booting into audit mode. Easiest thing to check is the batteries. Check pinwiki for troubleshooting tips.

Yes, did that - put in batteries and now it's 'stuck' with all displays showing full 0's, with the top left display alternating 600000/000000. Looking on PinWiki etc...

The game does seem to work properly when in "Master Command" mode and when testing either in manual or auto-step mode.

I was able to get into the settings mode and it looked like it was working, allowing me to page through settings and whatnot, much like my BK does.

Just can't get past this 600000/000000 'thing' it's on. I'm not sure what it's doing.

#15 1 year ago
Quoted from stangbat:

Sounds like it is in attract mode and you're seeing it alternate between the high score and no score. System 3-6 games show all zeros or all the leading zeros on the score displays. You should see the playfield lamps lighting up and cycling unless the fuse for the lamps is blown. Add credits and try to start a game, but you should probably do some housekeeping before blindly starting it up in case something is shorted and locked on. And the 2.5A SB fuse for the coils is usually blown on games that haven't been maintained so starting a game may result with not much happening as the ball won't kick to the shooter lane.

Thanks for that - I'll look into that 2.5SB fuse for the coils as well.

#17 1 year ago

Yes, thanks - I've done the bulletproofing on the power supply and the rectifiers.


Something screwy keeps happening - I'm blowing the 2.5A SB fuse for the coils. Checking each, flippers read 1.2 ohms when not energized. Most all of the other coils - kickouts and drop targets - read 4.3 ohms. The one drop target at the top of the board reads 3.8 ohms. I've checked and triple checked.

Anyone have any tips (have looked closely for broken wires, but saw none)...?

#19 1 year ago
Quoted from stangbat:

Now comes the fun part. You need to find the reason for the 2.5A SB fuse blowing. Time to study Pinwiki, other threads, and other repair sites. You need to do the bullet proofing steps such as replacing the 40 pin header and sockets.
Look for a shorted transistor on the driver board. Less common is a diode shorted on a coil (it probably isn't this). The slings and pops are not CPU controlled and if one's switch is stuck it will lock on and blow the fuse. Sometimes the caps on the switches for the pops and slings go bad and short the switch and lock on the coil.
I've also seen blanking issues where when you power on the game all the coils fire at once and blow the fuse. This can be harder to diagnose as there's multiple things that can cause it.
There's a lot of things to look over and take care of. You've just got to read, dive in, and do it.

Gotcha, thanks for the tips! Much to read, having direction is very helpful.

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