I'm grateful to find a community like this! Thanks in advance for sharing your expertise and experience.

I've got a 1978 Nugent that I've owned since 1979. It got a lot of use back in the day, and since then I've been very careful moving it from house to house over the decades. But I guess the years have caught up. I've been dabbling on the repairs for some years now, and finally want to focus and get it working again.

The MPU had the expected battery corrosion, which I've repaired. I had high hopes that would be it, but no. After more online research, I decided to replace C23 and C26 on the solenoid driver board, which went very well. I've made no other board mods so far.

But unfortunately I'm still stuck at the self-test LED lock-on problem - no blink or flicker. I recently started my diagnostics over from the beginning.

Power supply: The DC voltages at test points 1, 2, 3 and 5 are within specs. TP4 is low at 5.9 vAC, but if that's just for "general illumination" then that seems less important for now. Fuses and fuse holders are in great shape, and all connectors look clean and normal.

Solenoid driver: Ground differential to power supply is ZERO. TP1 show 5.4 vDC or so, and I've read that over 5.2 can inhibit booting. I'm also concerned that TP1 also shows 11 vAC, and I've read that less than 0.25 vAC is expected. TP5 matches TP3 on the Power supply (11.9 vDC).

All measurements are with all connectors in place.

Suggestions for next steps?

If I had to guess, I'd suspect maybe an old BR2 on the power supply of leaking AC to the input of the 5v regulator. Or it could be the LM323 itself (but hope not, since that's not sounding like a fun problem to solve). I'm wondering what my DMM can tell me to help narrow things down.

A bit about me: this is my only digital pinball. Growing up I helped my dad repair and resell dozens of EM machines of all types. I had a job in high school building circuit boards to get money for college. I can read schematics, but I'm not an EE. This is my first post here, thanks for being gentle.

Thanks for the link, balzofsteel. Something to spend quality time with for sure!

Quench> How much AC voltage are you measuring at TP5 on the solenoid driver board? it should be around 0.3VAC

I suspect that 25 to 58 VAC is likely not a good sign on TP5 there.

> If you disconnect the J3 connector from the rectifier board how much DC voltage do you measure at TP3 on the rectifier board? With no J3 connected it should measure around 11.9VDC.

With J3 disconnected from the rectifier board, I measure 10.98 VDC on rectifier TP3.

With J3 connected, I actually measured (and confirmed again) 13.1 VDC on TP3. I transcribed it wrong in my OP (typing the expected rather than my actual value). So that's below the range you mentioned.

I confirmed that F3 is 4A. Closer inspection in the vicinity of BR2 shows a crappy solder blob at E12. That whole board is OEM with no mods. I'm hesitant to repair that blob because access to the back of that board is a big hassle with the BRs bolted to the mounting plate / heat sink.

> It might suggest a problem with the connections to the C23 capacitor not filtering that DC rail.

I pulled the solenoid board to carefully re-inspect my work replacing C23. It looks good. I reused the cap lugs and wires, so there was no resoldering. The lug screws were tight. I ohmed from C23 - to the pin on J3, and from C23 + to the J3 pins and C24 +. They were all solid. Here's what I ordered for the C23 replacement:

https://www.digikey.com/en/products/detail/cornell-dubilier-electronics-cde/500C123T025AA2B/953299

I doubt it's related, but I also replaced C26 at the same time with this:

https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B43547A9187M000/4942233

I suspect that 25 to 28 VAC is likely not a good sign on [solenoid board] TP5

You bet! My trusty Micronta analog measures 29 VAC there. Rechecking with my DMM confirms 28 VAC on TP5 now with all connectors in place.

Rectifier TP3 is giving me 13.3 VDC with all connectors in place. Removing J3 on the rectifier, TP3 started at 11.35 and worked its way slowly up to 11.8 VDC over about 5 minutes.

All connectors in place except sound board J2 and J3 gives me the same values (13.3 VDC and 28 VAC).

IMG_9516 (resized).jpeg

On the (-) lead of C23 on the solenoid board.

Yup! I always do that before any power-up.

Ok. I decided to begin at the beginning, using the PinWiki link that balzofsteel offered in response to my OP.

Specifically, I'm using section 3.21 for expected voltages at test points, and section 5.2 for what to connect and disconnect for the most basic Rectifier (-18 type) and Solenoid baseline tests.

With ALL backboard connectors unplugged (displays were plugged in) and only Rectifier J2 installed, my Rectifier measurements were:
TP1 = 5.72 VDC (expected 5.4 +/- 0.8) a bit high
TP2 = 143 VDC (expected 230 +/- 27) WTH?
TP3 = 10.56 VDC (expected 11.9 +/- 1.4) at the very low end
TP4 = 6.35 VAC (expected 7.3 +/- 0.9) at the very low end
TP5 = 42.0 VDC (expected 43 +/- 5.4) OK
GND = Pin 1 on Rectifier J3

I then connected Rectifier J3 and Solenoid J3, and measured on the Solenoid board:
TP1 = 4.12 VDC with 8.2 VAC (expected 5 +/- .25) too low
TP3 = 4.10 VDC with 8.2 VAC (expected 5 +/- .25) too low
TP5 = 9.45 VDC with 19.8 VAC (expected 11.9 +/- 1.4) too low
GND = Pin 1 on Rectifier J1

I also noticed my displays flickering digits erratically during the J3 & J3 test.

Again, the displays WERE connected, and I didn't disconnect anything in the lower cabinet. But all other circuit board connectors were off, except as noted above.

Does this tell us anything useful?

Spoiler: some progress seems to have been made.

In the 44 years that I've owned this machine, I've never opened the transformer cage, nor removed the rectifier board. We played this silver ball for many years in the '70s and '80s. So I'm pretty confident that the line-to-transformer wiring isn't messed up. You did make me wonder if the outlet I'm using might have hot/neutral swapped, so I checked. My outlet tester shows "Correct" wiring.

My bad. I thought I had all the MPU connectors off, but J1 and J4 were still connected. Dammit. No flickering with them disconnected now, but each display does have one illuminated orange dot (hidden below the plastic mount). So still getting power, but the seven segments are quiet.

In removing my test lead from C23 (-), I noticed that that wire to the circuit board was no longer attached to the cap lug. I'm now wondering if that might have been a bad connection to this point, perhaps down to just one strand if it was that fragile. I stripped a new end section and resoldered it to the lug. All subsequent measurements have improved (as shown below).

I'm not sure where I read to use C23 (-) as a GND point, but it seems legit. My schematic says C23 (-) goes to solenoid J3 pin 10, then to A2J3-16 [W-BRN]. That doesn't seem right, since W-BRN runs to pin 17 on rectifier J3 (not pin 16). Typo? I'm quite sure I'm right on tracing that W-BRN, so with both J3s connected, C23 (-) is a reasonable GND. However, I'm more than happy to switch to the braid from now on.

I'd love to be vindicated on that typo in the schematic. Anyone care to verify?

I measure no resistance from the ground braid to: rectifier J1 pin 1, backboard backplane, solenoid GND TP, solenoid C23 (-), MPU TP4, and lamp driver TP1. Those last two boards had no connectors attached. So I think my ground distribution is verified?

Redoing the solenoid board measurements with the C23 lug wiring fixed, most everything disconnected (just J2/J3 + J3 engaged), and using the braid for GND:

TP1 = 5.32 VDC with 10.8 VAC (expected 5 +/- .25) OK
TP3 = 5.28 VDC with 10.7 VAC (expected 5 +/- .25) OK
TP5 = 13.7 VDC with 28.8 VAC (expected 11.9 +/- 1.4) a pinch high

The AC is still excessive, but I'm much happier with the DC values.

For this test I reconnected everything except the sound board.

MPU LED still shows solid lock, no blinks or flickers.

As long as literally everything (except the sound board) was connected, I checked the solenoid board measurements again:

TP1 = 5.3 VDC with 10.7 VAC - OK
TP3 = 5.27 VDC with 10.7 VAC - OK
TP5 = 12.65 VDC with 27 VAC - OK
GND = braid.

Essentially no change, although TP5 is better likely due to load.

Moving through the PinWiki steps up to section 5.5.1, I connected only rectifier J2 & J3, solenoid J3, and MPU J4. LED is locked on, no flicker.

Pin 40 of the 6800 is 5.3 VDC with 10.7 VAC, as expected given the numbers above. Shorting pins 39 and 40 briefly did nothing.

Any ideas where all that AC is coming from? That's likely my next concern, yes? Thoughts about what I can do about it?

I'm not an EE, but it seems like AC and DC can not only be in the same circuit, they can be in any combination. The DC is simply an offset from zero, and the AC varies around the offset. Here's what I read about it:

https://electronics.stackexchange.com/questions/327643/can-dc-voltage-and-ac-voltage-flow-on-same-conductor-at-a-same-time

Exactly how a meter will measure the AC or DC when both are present apparently can depend on the meter circuitry. I found this article on the subject:

https://us.flukecal.com/literature/articles-and-education/electrical-calibration/papers-articles/making-accurate-dc-voltage-

The latter article suggested that using higher ranges can produce different readings due to the meter circuitry involved. So I tried using all the higher ranges on my two meters to see if the readings changed. They didn't. I feel this simply verifies that one range on one of my meters isn't wonky. As for the readings being exact (the true DC offset considering the presence of AC, and the actual AC amplitude given the presence of DC), I simply can't say.

In my case, I have an inexpensive 3.5 digit DMM and an old-school analog multimeter. They both agree on the values I'm seeing on all the relevant range settings. So I'm convinced my reading are not a fluke (pun intended).

What I think is meaningful is that both AC and DC are clearly present. I read somewhere that the TPs of interest on the solenoid board should contain less than 0.25 VAC, and I'm clearly well beyond that.

Since I'm getting high AC when only the rectifier (J2 & J3) and solenoid (J3) boards are connected, the state of my MPU isn't relevant yet. I see no reason to even plug it in at this point. I would expect nothing to work if my +5v bus also contains twice as much AC.

Again, I have only rectifier connectors J2 and J3 installed along with solenoid J3. All other back box circuit board connecters are unplugged. However, the displays and bottom cabinet connecters are in place (connected).

There is very little in the circuit running to the input of the +5v regulator -- just BR2, C23, and C24 (plus its little helper). C23 is brand new. I measure 0v on the (-) sides of C23 and C24, and the same values as solenoid TP5 (and rectifier TP3) on the (+) sides.

I find it hard to believe that a BR2 failure would provide the right DC. The presence of AC here suggests C23 isn't doing its job, but it's new. I disconnected one lug and did the standard resistance test, and C23 charged up slowly just as expected. I'm stumped.

Should I be unplugging all the display boards, and all the cables to the bottom cabinet? Could anything there be in play?

I'm also wondering if someone else could measure the AC on these TPs in a working game and see what you get?

Checked all of those, and there is solid continuity from the ground of the line cord to the braid to all the listed places. I'm assuming an ohm or two is not relevant.

Yet that's not what I'm seeing. I would expect that my brand new C23 from DigiKey would be fine. Is there any reason to want to spend another $20 to swap out C23 again? Such as:

https://www.actionpinball.com/parts.php?item=KITC23CAP

I'm not against picking one up, but budget is finite, and it will get limited use (this project, and one other digital effort just getting started on my workbench). Wondering if something like this would be sufficient:

https://www.amazon.com/YEAPOOK-ADS1013D-oscilloscope-Oscilloscope-Bandwidth/dp/B08L3FRKYF

So then I'd be looking at something like this then, yes?

https://www.actionpinball.com/parts.php?item=PWRSUP10

Ok, I'm kind of excited now. Some substantial progress has been made. Here's what I did:

- Ordered a bench power supply (+5 and +12), and bench-tested the MPU board.
- Still showed as locked-on.
- Reseated all the chips in sockets.
- Got 5 flashes out of the LED on bench MPU power-on.

I also noticed that when I removed the MPU board, the corner screws were not tight. Not sure how critical that is for grounding that board, since J4 and J5 includ GND lines.

Put the MPU board back in, tightened the corner screws this time, and reconnected all the cables. Power-on and the non-play bulbs light up. Also:

- MPU LED flickers briefly, then goes on once and off once, followed by...
- 6 blinks, with the Solenoid relay activating on blink 4.
- LED goes dark after the 6th blink.
- Then the speaker emits this quiet warbling sound, continuously.

Multiple tries at this shows sometimes a slightly delayed 7th LED blink, a dark pause, then 6 blinks again. Sometimes the sequence ends with the LED on, but usually it finishes off.

Sometimes some playfield bulbs come on (e.g. Special, multipliers), but usually not.

One time it sounded like one playfield solenoid activated and held.

At no time do the displays show anything (although I can see the . segment is illuminated, so they are getting power).

Test points on the Sound board seem ok:

- TP1 exp=5 act=5.27 VDC
- TP4 exp=0 (GND) act=0.05 VDC
- TP6 exp=? act=5.05 VDC
- TP7 exp=12 act=13.15 VDC
- TP9 exp=? act=9.44 VDC

The push button on the MPU board and the self-test button inside the coin door do nothing once the power-up cycle is complete.

I rechecked the Solenoid board low-voltage test points (TP1, TP3 & TP5). The DC readings look good (5.3, 5.3 and 13.1) but they all still have excessive AC readings (11-18).

I have another brand-new C23 replacement for the Solenoid board, and I'm tempted to swap that in to see if that eliminates the excessive AC. I still wonder if all that AC in the DC is wreaking havoc in various places.

Anyone have any suggestions for next steps?

MPU back and front:

MPU-back.JPG

MPU-front.JPG

It was easier to go from R23 to R18 (which is electrically equivalent, according to the schematic, if you get the correct side of each resistor, which is trivial due to the obvious top-side traces).

TP5 measures 5.18 VDC and TP2 measures 12.8 VDC on my bench.

I get one more flash with the jumper. The LED ends up off, not dim.

Now I'm questioning how I'm counting the flashes. At power-on, there's a very brief flicker, a pause, one solid blink, a pause, and then a series of solid blinks with shorter pauses between them. So do I count that first solid blink? If so, then I have 6 solid blinks without the jumper and 7 with the jumper (and a longer pause between blinks 1 and 2 than the others).

Nope. I put two 1x1 inch wood slats under the board, either along the center or along the outer edges. That allowed me to flex the board both ways. I then repeated that on the other axis, for a total of 4 flexing tests. No difference in LED blinking.

It's 45 years old like everything else in this machine, other than the handful of parts that I've replaced.

All the headers on all the boards look amazing. No darkness or tarnish. I've never stored this outside a heated/cooled house.

Nope, this is my only electronic pin.

I took the time to pull, inspect, and reseat all the ICs with sockets. In doing so, I noticed that some of the battery corrosion had snuck up into socket pins 1, 2 and 3 of U11. Also, pins 11 and 12 of U7 looked darker than the other pins. Those pins are closest to traces where my battery used to be.

I shined the IC pins up with 320 grit sandpaper (inside and out), and tried to clear and blow out the sockets for those pins. I know, half-assed compared to all the fun work of replacing the sockets. After all that, the behavior was the same.

I haven't put the MPU back in to see if it works any better yet. And the "AC in my DC" problem (off-bench) still exists.

More progress! Getting very excited now...

After playing on the bench with the MPU board jumpering as Quench suggested, and getting all the LED blinks, I did a few more things:

- Tightened up the socket contacts for U11 pins 1, 2 and 3. I noticed that when pin 1 wasn't making good connection, I got a locked-on LED. I was able to use a straight pin to ensure that the socket contacts would be tight against the IC pins. Seven blinks on the bench.
- When I ordered my bench power supply, i also ordered another replacement C23 for the Solenoid Board. I put that in.
- Reconnecting everything in the machine backboard, and power on ... 7 blinks ... and ... Attract Mode!
- The start game button reset the scores, I fired a ball into the playfield, and points started racking. Flippers worked.
- No playfield solenoids worked. Playfield fuse had blown, popped in a new 1A slo-blo fuse, bingo.
- Ran through self-check and confirmed that every rollover, contact, and solenoid worked.
- Confirmed that I still have the same AC in my DC as most recently reported. So I suspect swapping C23 didn't actually help (likely it was U11 all along causing my lock-on).

And here's the only thing not working yet:

- Speaker still emits a constant warbling. Now that I think about it, it used to do that decades ago, but at a much, much lower volume. Now the warbling is just a bit louder than the electronic scoring sounds which are also coming through. I'd like to enjoy the electronic sounds again, which means eliminating the warbling.

Other than that, I'm planning on tracking down a new set of rubbers (all 3 flippers, and anything else stretched between 2 or more posts). I'm also thinking about replacing the incandescent bulbs to reduce power usage and heat (esp. backglass).

Suggestions for the audio problem, and recommendations for sourcing rubbers and bulbs?

Ordered from Marco! Some other places offered red bands for the flippers, but Marco offered the same color as my originals. Sure hope Marco remembers this game has 3 flippers.

Siegcraft price: $45, Marco appears to have the exact same thing for $59 (!).

But I also see that Marco sells a $99 kit of the exact LED set needed for my machine (including playfield-matching color bulbs). I need some time to accept the $160 sticker shock for this bulb retrofit.

I've read somewhere that replacing the (non-electrolytic) capacitors on the SB-100 might fix some problems, but the cap kits I can find are all elecrolytics. Next step is to just pull the sound board out and give it a good looking over.

Not simple hum. As a musician, I have 60hz burned into my brain from our big amps and speakers over the decades (ground polarity, all that jazz).

It's a warbling, quickly repeating type of sound. When I run the self-test, I can clearly hear each test sound overlap this continuous warbling, except for the 5th sound. My suspicion is that that one specific sound is misbehaving on the board. I uploaded a recording of it here, including the first part of the self-test:

https://drive.google.com/file/d/1YAIu39ZQhQJjQPEVElXgjcJi5mPg6Lg1/view?usp=sharing

Using diagram 5.13.8.6 on the Pinwiki Bally/Stern guide, maybe I can identify a possible problem with sound 5 being continuously triggered?

Other than the warbling, the sound board seems to work fine. During game play all the sounds fire as expected. But the constant warbling gets annoying in a hurry, so I unplug the connector near the speaker whenever I power on the game for any length of time. The warbling starts after the boot sequence completes when the displays turn on.

The sound board test points measure ok in-game. TP7 is a bit high (12.5 not 11 VDC) and TP9 is a bit low (9.5 not 10 VDC). I've got a partially populated SB-100 and there is no jumper at TP8. I also marked the 3 pots and turned them each back and forth a dozen times before resetting them. I resoldered all the connector pins that ran to board traces. And I did a careful visual inspection - no burned or hot looking components, pins are clean, no damage.

Probably not related: I'm also observing a subtle flickering in just one set of backboard bulbs, perhaps at the same frequency as the warbling? All the other backboard bulbs are rock solid. That one set of bulbs is in the same wire bundle (two wires for ground, and 3 drive lines), which seems rather coincidental. The voltages on those three bulbs circuits (as measured on the backboard wiring) are notably out of whack (both AC and DC measurements) compared with the other backboard lights.

No.

I made the link public. See if that helps.

My new set of rubber rings from Marco arrived. Will be a fun time this weekend installing them.

Here are the pics. This board is super clean with no mods.

Also, I have a logic analyzer on-order, hoping to see it by next weekend.

New rubber rings have this game playing so well! I do miss the sounds, though.

SB-100-back.JPG

SB-100-front.JPG

TopPins.JPG

TopPinsBack.JPG

U19-U4.JPG

U6.JPG

With the sound board reinstalled and game in attract mode, I get:

- U3 pin 9: 0.94 VDC
- U4 pin 9: 1.62 VDC

The base cabinet connector to the speaker and speaker volume pot was disconnected.

U2 pin 8 is 0.22 VDC in attract mode.

However, I just relocated the machine from the basement (now that it's playable) to my home office upstairs. Things worked fine at first boot, playable, and the sound warbling was still there. I did the test measurement above, and then retried the speaker connector just for the heck of it. No warbling! The sound is working fine now, with one small exception...

Sound #3 is not quite the right pitch (I remember them all very well, they are burned into my brain from all the years I played this machine). I messed with all three pots on the sound board when I was troubleshooting. So it's likely I just missed getting one back exactly right. From the schematic it looks like R6 controls the pitch for sound #3? So I should just be able to adjust it there - yes?

Good to know. Keeping the "original" pitches is important to me, as this game has been part of my life for so long. Now that I stop to ponder that - longer than my spouses or children!

The start game tone was in a minor key (to this musician's ears), so I'm looking forward to it sounding happier!

After updating the sounds to the factory frequencies, I'm less happy. I'll have to tweak them back to what I remember. As a musician, my sense of pitch is rather good, and I guess I need the notes that *I remember* from all those happy years in my youth playing this machine back in its prime.

In other news, the Replays/Match/Ball Number display (#5) stopped working. The other four displays were working fine. I had swapped displays 4 and 5, and so I knew the glass and the board were good in all the displays.

Fortunately I had figured out how to use my new mini logic analyzer. So I studied the schematics and narrowed it down to the Strobe and Latch lines driving display 5. I could see the Strobe working fine, but no Latch signals on the display board. So I backed it up to the MPU board. The logic analyzer clearly showed me that pin 2 of U11 was working as expected, but pin 13 of U20 was not receiving it. Another indication of corrosion on my U11 socket from the old battery. Should be a lot of fun replacing the only 40-pin socket in this entire machine. Sigh.

One pic here shows my mini logic analyzer an iteration prior to diagnosing the precise problem. The other pic shows my temporary solution: a stick pin jammed into pin 2 of the U11 socket to force a connection. Not shown: tape around the pin to ensure it doesn't fall out during gameplay and short something on the display driver board.

IMG_9921.JPG

IMG_9927.JPG