Saturday May 18, 2024

Repairing Non-working Solenoids on a Stern Nugent

Author: Wayne Eggert
Date: 7/30/2011

Stern's Nugent Pinball is one of the earlier solid-state games.  It uses the early Stern SB-100 Sound Card which uses tones for sound (no speech).  It's not a very in-depth game, but not many games from the early era of solid state are.  It's fun in its own right, great for a first-time pin owner and a great backglass if you're a Ted Nugent fan.  Anyway, one of the larger remaining issues were 3 non-working coils in the game.

Problem Description
After fixing the sound card, one of the remaining issues were 3 non-working coils.  The upper left and right pop-bumpers were not working, but the switches would score correctly.  The outhole kicker at the top left of the playfield also wasn't kicking the ball out.

Here's a picture of the locations of the non-working pop bumpers and outhole..

Nugent Playfield - Non-working coils
Image: Location of non-working pop-bumpers and outhole kicker on a Stern Nugent

Diagnosing The Problem Further - The "Quick & Dirty" Way
I already new some of the coils were working in the machine.  The bottom pop bumper would fire just fine, as would the slingshot coils.  To diagnose further I checked quick under the playfield to verify wiring was connected and measured voltage at the coils.  Nothing seemed wrong there.

I had a spare *working* solenoid board from another machine I was fixing, and often it's just easiest to swap boards to help isolate the problem.  After swapping boards, the non-working pop bumpers and outhole kicker fired just fine.  So I isolated the problem to the solenoid driver board..

Often you'll see burnt transistors on the solenoid driver board or a good amount of hacks.  This board looked unhacked, at least till I got to it anyway =)  Some of the connectors on the board looked like the solder joints might have been cracked, so I reflowed a few of the connectors and retested the board.  At first none of the coils worked, then the outhole kicker started working.

Since I was working off of a Stern Hot Hand pinball schematic, I had to identify which driver transistors corresponded to these coils.  By grounded the tabs on the solenoid board driver transistors, the coils would fire just fine.. but this doesn't tell you 100% that the transistor is good.  In any case I figured out that Q13 was the outhole kicker and Q9, Q10 were the upper pop bumpers.  I tried replacing the Q9 transistor but it did not make a difference.

Diagnoising The Problem Further - The "More Logical" Way (no pun intended)

Rather than do a shot-gun approach to anything else I used my logic probe to help isolate the problem further.  I probed on thee 74154 IC and inputs/outputs on the U3 IC.  This verified the correct signals were being received on the 74154 and that U3 was sending the correct signals to the driver transistor.  The only thing in-between U3 and the driver transistor are a few resistors, a capacitor and a 1N4004 diode.

Problem Resolution
I checked the resistors and 1N4004 diode corresponding to the driver transistors (Q9, Q10, Q13).  The resistors measured fine when compared to other resistors on the board.  The diode also tested fine, but measurements while it's in-circuit aren't very accurate.  I cut the end of one of the diodes to get an accurate measurement, and sure enough it read 0.6-0.7v so it seemed okay.  I decided that if it wasn't the transistor, wasn't the diode and the resistors measured correctly -- it might very well be the capacitor.  I had one of the disc type capacitors really give me a fit on a Stern MPU board one time -- and you can't physically see when they go bad.  So I stole a few capacitors off of another board and soldered them in.

Now here's my gripe, Stern mounted a lot of components on these boards with leads bent over at a 90 degree angle across the board.  Makes it difficult to get the component off sometimes.  I usually cut the legs and pull the pins from the back of the board.  I have a temp controlled soldering iron, don't stay on the pads very long and despite that had even lifted a few pads as I was repairing this board.  It's always kindof disheartening to take some unhacked board and have to add hacks to it, but it happens and you just press onward!  I am wondering though if these pads are more prone to lifting since they get stressed from the heat/coil current.

Anyway -- after replacing both capacitors in circuit with the driver transistor for the pop bumpers & also replacing the 1N4004, I put the board back in the machine and low-and-behold both pop bumpers worked!   Then one of them stopped working.  Sometimes it's NEVER easy!

I pulled the board and inspected the work.  Looked like one of the legs of one of the capacitors I had soldered in was loose.  I hate salvaging components off of old boards and reusing them, often the legs are too short and it makes it much more difficult than if you had a new component with long leads.  Anyway, resoldered and the pop bumper started working again -- then stopped again.  No amount of wiggling or pressing on anything got the pop bumper working again.

As I thought about it more and looked at the schematic, I realized the pop bumper that was still working was the one I had replaced both the capacitor AND 1N4004 diode on.  I had cut the end of the diode and replaced it with a new one, even though it had tested good with diode test.  So, I decided to replace the other pop bumper's 1N4004 diode on the solenoid board -- this got the non-working pop bumper working again!

Now for the outhole kicker, I decided instead of replacing both the capacitor and 1N4004 diode I would try just replacing the diode.  And what do you know, the outhole kicker worked just fine after replacing the 1N4004

Solenoid Board - Diodes
Image: CR9, CR10 and CR13 1N4004 diodes that were replaced (corresponding with the Q9, Q10, Q13 coils)

Summary / What I Learned
With electronics, sometimes you can get your butt seriously kicked even trying to diagnose something the smart way.  In this case, all of the 1N4004 diodes I had cut off tested fine out of circuit using the diode test.  None of them were burnt, none of them had broken solder joints on the back of the machine.  They looked perfectly fine so instead of suspecting the 1N4004 diodes I suspected the capacitors.  What's also weird is the non-working coils would work once in a while.  I'm willing to bet the 1N4004's were all that needed to be replaced in this case.  I'm not sure of the exact cause, had the 1N4004 broken down and not been passing the current that it should have?  Were the solder joints in-fact bad?  In any case, next time I won't be counting them out of the equation.. that's for certain!

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