Transformers, The Pin (Stern, 2012)

Symptom: Would not power up
Location: Firestone, CO

This is a home model pinball machine.  There is currently very little (none?) technical information about this pinball machine. It uses the Stern Spike system, which is used in some other professional machines and one other home machine (Avengers). I was unable to find any schematics for this machine or the individual boards. (I’ve read that Stern will be releasing some documentation in the near future.)

The machine would not power up. There is a 48 VDC power supply mounted inside the cabinet (you have to remove the bottom of the pinball machine to gain access to it). The 48 volts was working.

On the Spike MPU/Sound board (which is in the backbox), there are 4 LEDs to indicate the status of +48V, +24V, +8V, and +5V.  In this case the 5V LED was not lit. I found that D12 was shorted. Once D12 was removed from the board, I checked that D12 was truly shorted and it was. I also checked the pads where D12 was located, and it was still shorted there as well. This meant that the A8498 regulator chip (U30) was also bad. Once both components were replaced, I bench tested the board and all of the power status LEDs came on.

(Note: if the 5V is working, the +24V and +8V LEDs will only come on once the microprocessor has booted.  If the 5V is not working, then they will light regardless of the microprocessor.)

Stern Spike MPU board, 520-5318-01, from Transformers

The following is a more detailed look at the power section for other repair people who have the expertise to repair surface mount boards.This only covers the power sections and not any other functions such as sound, the microprocessor, or the interface circuitry.

The 48 volts from the cabinet feeds the 24 volt, 8 volt, and the 5 volt regulator sections. Each of the regulator sections utilize an Allegro A8498 chip, which is a 3 amp switching step down regulator. The enable pin (pin 2) for the 24 volt and the 8 volt sections are connected elsewhere on the board.  If the enable pin is not 0 volts, the regulator will be disabled (turned off). The enable pin for the 5 volt section appears to be grounded so that it’s always enabled.

Each regulator section consists of the A8498 (Allegro A8498SLJTR-T), a 68uH inductor, a 60V, 5 amp Schottky diode (Comchip CDBC560-G), and some input and output filtering capacitors (470uF at various voltages).  Near each regulator is a test pad where the voltage can be checked.  The 5 volt test pad is near the reset button. Note that the A8498 has a thermal pad underneath the chip which is soldered to the board. Only a hot air rework station will remove this chip.

Each regulator section consists of the following components. Refer to the A8498 data sheet for details on how things are connected together.

5 volt section: U30, D12, L8, C132, C135, C133, C134, R106, R107, R108.

8 volt section: U10, D4, L6, C55, C56, C59, C60, R24, R23, R25

24 volt section: U6, D1, L5, C47, C48, C49, C50, R125, R16, R18

The output of the 5 volt regulator goes on to power three other regulators: 3.3V (U9), 1.8V (U31) and 1.0V (U32). These regulators are Rohm BD18KA5W, BD18KA5W and BD10KA5W respectively. There are no LED status indicators associated with these regulators, however there are test pads near each one to check voltages.

AC/DC Pinball Machine (Stern, 2012)

Location: Denver, Colorado.
Symptoms: Ball getting stuck at ball flap.

From talking to people who buy brand new machines, it’s not uncommon to spend some time getting it to work right.  This was my second visit to this machine.

The first visit, which I don’t think I posted, involved replacing the canon motor, filing the bushings on the bell, and getting the lock-down bar to lock.  I guess there is not any quality assurance inspection after the pinball machine leaves the production line.  The things I fixed were very obvious problems.

This visit was for a problem that wasn’t very obvious.  The ball would occasionally get stuck at the ball flap in the upper right of the playfield. After unsuccessfully trying to get the ball stuck, I resorted to just using my fingers to discover there was an electrical wire hanging down that was catching on the ball. I couldn’t see the wire, but I could feel it.

I bent the wire out of the way and the everything seemed to work fine.

acdc-0480

Sharkey’s Shootout, Stern Pinball Machine (2000)

Location: Boulder, CO

Symptoms: Ball trough solenoid not working — ball not delivered to shooting lane.

For some reason, this past week I’ve gotten a lot of Stern/Sega pinball machine service calls, and all machines are roughly the same era.

I started a game and no ball was delivered to the shooting lane.  The Dot Matrix Display (DMD) indicated it was ball 1, so the computer thought the ball should have been there.  I checked the flippers and they weren’t working either.  At this point, I suspected a blown fuse.

The flippers, pop bumpers, vertical up-kickers, and ball trough solenoids all are “high current” solenoids and run on 50 volts DC.  I checked fuse F21 on the IO Power Driver board and it was blown.  F21 is a 3 amp slow-blow (time delay) fuse for the 50 volt power supply used on these high current solenoids.

Portion of control board showing F21 and power LEDs (click for full res).

A quick visual check of the various voltages can be made by looking at the bank of power LEDs.

I replaced the fuse and the solenoids began working again.  I did a coil test from the diagnostics menu to see if there were any problems that would have caused the fuse to blow.  I then tested all of the switches.  Not seeing any problems, I concluded it was simply what I call a “fatigued fuse”.

Fuse fatigue occurs if the current in the circuit exceeds the rated value of the fuse for a brief moment.  The F21 fuse is a MDL time delay type of fuse rated at 3 amps.  If 3 amps is passing through the fuse, it won’t blow.  If 4 amps is passing through it, it will blow within an hour. It can handle 6 amps (double) for up to 5 seconds before it blows.  So a fuse like this can handle spikes in current without blowing.  But if coil or a drive transistor shorts out, it will blow protecting the circuit.

During multi-ball, the flippers and bumpers can energize all at the same time causing current spikes much greater than 3 amps.  After years of this, the fuse filament gets weaker and eventually breaks, like a light bulb burning out.

Which fuse is blown? The center fuse. See text.

The fuse on the left sure looks suspect with it’s black residue.  This is a fuse that I took from a high powered coil circuit from a different machine.  It still works.  It still measures good on an ohm meter.  I doubled checked by putting exactly 3 amps through it with my bench power supply.  It’s what I would call “fatigued” and probably will blow in the near future.

The center fuse is blown, but it’s difficult to tell visually.  The fuse on the right is intermittent.  It works for 15 minutes then stops.  If you turn off the power, it will start working again.  It’s the most bizarre behavior I’ve ever seen in a fuse.  It had me thinking the loss of power was somewhere else.

When it comes to fuses in pinball machines, don’t trust your eyes or your meter.  But if the fuse looks like the one on the left, replace it anyway because it’s about to blow.