My Williams Skylab started to blow the 24 volt fuse (15 amps). I, at first, thought the problem was a shorted score motor coil because the fuse wouldn't blow when I removed the power from the motor (unplugged the service jack to remove power). However, this turned out to be wrong. The real problem wasn't related to the score motor coil, but, rather, the position of the score motor cams. When I unplug the motor jack and turn the cams by hand, I can cause the fuse to blow when the cams switches reach a specific configuration. There are 6 cam switch banks, designated 1- 6, plus an index bank cam and an impulse bank cam, eight total cams. On Skylab, 7 of the cams have a "valley" where the switch follower drops into as it rotates around the cam. The eighth cam, the impulse, has "hills" that raise the switch follower as the cam rotates. The short occurs when the switch follower of cam 1 falls into the cam valley. The cam switch 1 has the following switch functions according to the schematic: 1A: Pulses Game Over Relay. 1B: Pulses Lock Relay. 1C: Pulses Knocker Coil. 1D: Pulses 1,2, or 3 Adjust Position. I assume that one of these functions is where the short is located. Should I start unsoldering wires from the switch until I find the one that does not cause the fuse to blow? A quick look at these locations does not show any obvious problems. It's also worth noting that prior to this fuse blowing, I had been working on the score drums (reels). I took each one apart and cleaned it. This was because my numbers matching function was not working nor the "win a replay" function. Both of these functions go through the score drums. After cleaning, these functions started to working. However, after a few plays, the ball count started counting all the way up to 10 on a new game and the score motor would continue to run for several seconds before stopping. After a few occurrences of this, the fuse blew for the first time. I always tend to think that new problems are a direct result of the work I had been doing at the time they showed up. I could not find any problems in the score reels that should cause a problem. So, back to my original question, should I start unsoldering wires until I've isolated the wire that's causing the fuse to blow? Or, is there a better way to go or something to check before I do that?

Sorry for the long story, just wanted to make sure I get all of the facts on the table.

Thanks.

Wow! Lots of good ideas. I never thought about cardboard on the cam switches. Yes, much better than unsoldering a lot of wires. Also, the circuit breaker idea. Yes, I've already gone though a box of fuses. I like that idea. I'll have time to get back to the machine today and I'll let you know what I find. There is an electronics store near me that will probably have the circuit breaker. I'll try to stop by there today also. Regards and thanks!

So, I started looking at the cam switch with using some paper to not let the contacts touch in order to isolate where the short is coming from and I immediately saw something that didn't make sense, but made me wonder if I was looking at it correctly. Please reference the photos attached. In the middle of the photo is the schematic representation of cam switch one. In the drawing, you see a representation of the cam wheel showing the position of the cam follower on the cam. In the drawing, the cam follower is on "top" of the cam, not in the cam "valley". As I understand the schematic, the photo shows all of the switches in the open position when the follower is on top of the cam. So, when the cam follower drops into the cam valley as the cam rotates, all of the switches on cam 1 should change to the closed position. Note, there are no make/break switches on cam 1. Do I have this correct? if so, then as I'm looking at the switch with the follower dropped into the cam valley, the top switch, position D, is actually open when it should be closed. Not sure this could cause my problem, but maybe???? The other thing that's bugging me is that the wire colors on the schematic do not, in any way, match the wire colors actually on the switch. When I ohm out the path from the cam switch to the relay switches I get continuity, but the wire colors on the relays are correct for what "should be" on the cam switch. Seems like somewhere, there is a junction point where they change wire colors, but I've not found it yet. This is probably not a problem, but it makes me worry I'm missing something. If you guys could confirm that I'm looking at the cam switch 1 open closed positions correctly, I will adjust the 1D switch to be closed when it's supposed to be. Thanks.

Skylab Cam Switch 1 (resized).jpg

Hi Mark,
Thanks for the clarification. Just one comment and my latest finding. I've seen the jumper wires across switches, that you describe, and I understand why they are there. In the case of my cam switch 1D, the two wires go into the bundle and I, so far, have not found where they come out of the bundle and back onto the cam switch assembly. So, I don't think it's a jumper wire. I could be wrong. I suppose it's possible it was just a one off kind of change for some purpose. In any event, other than confusing me, I don't think it's a problem. Ok, so on to my latest "revelation". I used the suggestion from dgAmpGuy to use some card stock between switches in order to isolate which one is causing or related to the short. I did this and I had success when I placed the paper between the switches on cam switch 1A, the bottom switch on the stack. As you can see from the cam switch photo, above, 1A pules the game over relay (specifically, the latching coil). I started looking at possible ways that this switch closing would cause, or enable, the short which blows the fuse. So far, I have not come up with anything that makes sense. I did measure the latch coil on the game over relay and I think it was something like 7.5 ohms, so I figured it wasn't the problem, but I have to find that coil resistance chart and see what it's supposed to be. I had to leave the house and haven't got back to that yet. If anyone has a schematic of Skylab and could make some suggestions of where to look for the cause of the short, I would greatly appreciate it. Oh, by the way, just before the fuse blows, there is something chattering (60Hz rate most likely) somewhere, I think, on the underside of the playfield. At least, that's where is sounds like it's coming from. I only get about 1-2 secs before the fuse blows and I'm usually trying to get the switch off before the fuse blows (no, I have not yet implemented the suggested circuit breaker change out of the fuse). If I could isolate where that is coming from, it may give an indication of what is being shorted. Appreciate any help or suggestions someone may have. Again, thanks Mark.

Mark,
Yes, I've been using that chart. I need to go remeasure, but I think I saw 7.5 ohms across the game over latch coil. The chart says something like 10.6 ohms. Also, as you mentioned, I need to unsolder one side of the coil before I remeasure. With it reading low, it's entirely possible that there is something else in the circuit that is causing that, not just the latch coil. If it still reads 7.5 ohms after disconnecting one side of the coil, then I'm not sure if that is sufficient to be considered a short. As far as a percentage change (against 10.6 ohms), its a fairly large percentage. I know from other areas (power and audio transformers) a small change in resistance can indicate a short between turns and significantly impact the performance. I'll try to make that measurement tonight or tomorrow morning. As far as the chattering sound, I agree, it would be really good to know where that's coming from. Sound tends to bounce around the box, so it's possible it's the game over latch coil and isn't on the playfield bottom at all. I was thinking that I could replace the fuse with not a circuit breaker, but, rather, a power resistor. Just enough to drop the voltage and, therefore the current. Just enough to keep the chatter going but not draw enough current to damage anything. First thing, however, is to unsolder the coil and measure the resistance. Hopefully, more to come. Thanks

Ok, sorry for the delay in getting back. I thought I may have a bad lock relay coil, so I ordered a new one and installed that last week. Unfortunately, that was not my problem, but the plastic that holds the solder lugs was cracked and breaking from all of the heat, I suppose, so it's good that I changed it. So, on to my updates. As mentioned by HowardR, above, I purchased a 15A circuit breaker. But, instead of soldering the end tabs from a glass fuse, I purchased a medium sized alligator type clips, then soldered them on to some 12 gauge wire. This seems to be working nicely. No more blowing fuses. Anyway, this has given me a lot better opportunity to troubleshoot the problem If you recall, the problem (blown fuse) was occurring when the score motor was dropping into the cam valley on cam 1. I then isolated the problem to position 1A. 1A pulses the game over relay. Just to be sure, I unsoldered the coil wire on the game over relay. I did not blow a fuse when the coil disconnected. After that, I started putting in a piece of paper on the game over relay switches to see which one is causing the problem. I found that the short is related to position A on the game over relay. This switch is listed as "open circuits to most playfield switches" in the pin manual. I've included a portion of the schematic showing the switch circled in red. I guess the good news is that I've further isolated the problem. The bad news is that there is a lot of the schematic that is still involved not isolated. My question is what isolation advice do you guys have? I could stare unsoldering all of the coils that are past the game over switch on the schematic until I, hopefully, find the bad one (of course, I'm assuming that it's a coil, but it could be something associated with a switch associated with the coil). Or, is there a better way? I've been playing with the sequence of events that leads to the short. However, I've not been able to do that, yet. It seems that I'm starting from a game over status, and when I push a new game button, the pin goes through it's reset process and gets most of the way through it up until and just before, the ball should be ejected. I start out with the game over relay in the unlatched position. After going through the reset process, it turns the relay "on" which closes the switch circled in the schematic and, should, enable the rest of the playfield and eject the ball. As soon as the game over coil is energized and the switches move (circled switch closes), the fuse blows. Does this give anyone a thought about where to start looking?

Appreciate any help. Thanks.

Schematic showing switch (resized).jpg

I have not checked the credit unit coils, but it does decrement. I did check the ball count up and down coils. They were good. I will check both the credit coils and the match coil. Also have some addition al information. I thought that I had found the problem, but it turns out I did not. However, I may be one step closer to finding the problem. I was kind of going nuts looking at the schematic trying to figure out which coil could be shorted based on what coils are energized during the a new game start. That game over switch on the schematic page I attached covers a lot of territory on the machine. I kept following the lines down the schematic from the switch when, near the bottom, I see that the 24VAC line goes into a full wave rectifier block that supplies DC to the 2 pop bumpers and the 2 side kickers. I unhooked one of the wires on the rectifier block and turned the game back on. It worked! It even kicked out the ball from the drain (even though the ball was not actually in it, but I heard the solenoid fire). When I hooked the wire back up, it shorted again. I figured that the rectifier block must be shorted and that's exactly what it was, completely shorted. So, I head down to my favorite electronics store and pick up a replacement. Unfortunately, after installing the new block, the problem didn't go away. In this case, I was still getting a short but the rectifier block was still good. As part of checking the rectifier block with my ohm meter, I discovered that there was a short between the + output and one of the AC inputs. This short exists even when I remove the wires off of the rectifier block. In other words, the rectifier block is not causing the short. The AC input to the block comes from both sides of the power transformer. There is a yellow wire and a red wire. The red side is the side associated with the game over relay switch in the schematic above. The short is only between the rectifier + lead (orange wire on the playfield bottom) and the red wire (24VAC). I can't find anywhere that these 2 line even come close to each other, unless there is a short somewhere in the harness bundle. They come together at the rectifier block. Also, please note. On the schematic, the orange wire is associated with the - (negative) rectifier output, but on my machine the orange wire was connected to the + output. It's been that way since I've had it and it's always worked ok. Not sure if it should make a difference with the operation of the bumpers or kickers. I've attached a marked up schematic section showing where I'm seeing a short. The question is, where or how are these to points getting shorted? Any thoughts?

schematic showing short location (resized).jpg

Any thoughts?

To answer your question, yes, I've looked at all of the coils and none appear to be black or brown. I'm not really understanding your comment on disconnecting and "resetting" the playfield. Not sure how to do that. The problem "appears" to be a short between the 24 VAC red wire and the DC orange wire. These lines don't have any wires or components in common that could cause the short. When you remove the diode bridge, they are, effectively, isolated yet, they are shorted. I must be missing something. Perhaps, there is a short in the harness where a DC wire is shorted to an AC wire. I have found it yet. I think I'm going to put the ohm meter on with the short and start wiggling the wires to see if I can affect the short. Not sure what else to do.

Hi everyone. I wanted to let the people who have followed this post that I have, finally, resolved the problem. First off, I want to thank everyone that took the time to read my posts, try to understand what I was trying to say, and then respond with some suggestions. They were very helpful. Also, a shout out to LeChuck to say that I think you had a pretty good understanding of what my problem was. What you said was exactly what I was thinking and your suggestion to keep doing what I was doing was a good one. I'll try to give a brief explanation of the problem, as it's somewhat unique, well, to me at least. The Skylab machine, basically, has 2 systems. The main AC system and a much smaller DC system. The DC system powers the 2 jet bumpers and the 2 L&R kickers. Curiously, they don't power the flippers. I assume this is a common design for Williams EM machines of the 70s. The only common power between these 2 systems is the bridge rectifier, which provides the DC for the above mentioned devices. So, when you remove the rectifier out of the circuit, there should be no connection possible. In my case, however, there was. I also should mention that my post, above, describing what was shorted is incorrect. I show a short between the red (24VAC) bus and the orange negative (-) DC bus. Actually, the short was between the - DC and the yellow (neutral) bus. I call it the neutral, but that may not be the best name. (just as a side note, the schematic shows the orange wire as the - rectifier ouput, but on my machine, the orange wire is actually the + rectifier output. So, actually, the short is from +DC to neutral AC) I assume this is a mistake in the schematic. Sorry, for the confusion. To find the short, I started unsoldering the 4 DC coil locations, one at a time (2 jet bumpers and 2 kickers). After I unsoldered one location, I checked the ohm meter to see if the short was gone. It tuned out that the last location I unsoldered was the one that had the short. It was the Right Kicker coil. As a test, I temporarily reconnected the orange wires, one at a time, to see if the short returned. It did not, so I had the location narrowed down to the right kicker. However, I still had not found the actual short location. I checked several items around the kicker and didn't see anything. I found that a harness bundle, right next to the kicker, actually went over the bottom of a playfield lamp, so I thought that, maybe, the wire had worn through and caused the short. That turned out to not be the case. To make a long story short, the problem was one of the 2 switches that fired the kicker. The problem tuned out to be that one of the kicker switch leaves, where the switch leaves stick up through the playfield and are hit by the ball, had been pushed completely back to the rear of the hole it was in. Think of a hole with the leaves sticking out. They stick out on one side of the hole. This one leaf was pushed to the back of the hole (not down, just across the hole to the opposite side). Right next to the hole was a playfield lamp. Remember that one side of a lamp is at 6 VAC and the other side is on the neutral bus. The leave was hitting the lamp that was coming out of the hole for the lamp. The outside of the switch is at neutral. So, the switch normally switches a DC voltage, but, in this case, it was shorted to the neutral bus coming from the side of the playfield lamp. All I had to do was to push the switch leaf back in position and the problem was solved. Of course, I had to solder back all of the orange wires I had removed. After that, no more shorts and no more fuse blowing. I have no idea how that switch leaf was bent back, but it happened. I played a few games, tonight, and all seems to be working normally. So glad, I've been pulling my hair out over this one.
Thanks again for all the help I received. It's a really great forum.