Silverball Mania Pinball Machine (Bally, 1980)

Location: Arvada, CO
Symptoms: Blows playfield coil fuses; needs new rubbers and bulbs, cleaning.

I started by replacing both playfield fuses since they were blown.  When I started a game, the kicker at the outhole started firing randomly, sometimes very rapidly.  The sensing switch seemed to be fine. I also noticed that some of the pop bumpers weren’t firing correctly.  If I pressed on the left pop bumper skirt, the center pop bumper would fire.  Basically there was something not right with the solenoid driver circuit.

I started with the signal for the kicker coil, and with the oscilloscope, I traced it back through the solenoid driver board. The signal going into driver transistor Q11 was going crazy.  I went further back to the output of the 74154 decoder chip (U2) and the signal (pin 15) was still crazy and random looking. No wonder the fuses blew.

I checked the input signals to the 74154 and the “B” signal was randomly moving between 1 and 2 volts.  This is neither a digital “1” or “0” and it makes digital circuits act randomly.  I traced the signal further back to the MPU board to the output of the 6820 PIA (U11, Pin 11).  Since the 6820 was already in a socket, I lifted it out of the socket and bent pin 11 out, then put the 6820 back in.  This isolated pin 11 from the rest of the circuit to make sure that nothing else was interfering with the signal.  The oscilloscope showed it was still bad.

6820 PIA chip with pin 11 lifted to isolate it from the rest of the circuit.

6820 PIA chip with pin 11 lifted to isolate it from the rest of the circuit.

The faulty “B” signal would also cause the wrong pop bumpers (and other solenoids) to fire.

I replaced the 6820 with a 6821 (they are interchangeable)  Also, I replaced the 5101 RAM chip on the MPU board with an AnyPin NVRAM module and removed the battery from the circuit board. This will save the MPU board from future corrosion caused by battery gasses.

At this point the machine was working well, except it wouldn’t boot about 50% of the time.  It looked like a problem with the reset circuit.  Bally didn’t include a time delay in the reset circuit like most other manufacturers. I added a 4.7 uF tantalum capacitor across R2 to give the reset a little bit of time delay when it boots.  It solved the problem and booted 100% of the time after that.

4.7uF capacitor added across R2 to assist reset circuit.  AnyPin NVRAM module in lower right.

4.7uF capacitor added across R2 to assist reset circuit. The positive pin of the capacitor is connected to the right side of R2.  AnyPin NVRAM module in lower right.

Finally, all of the rubbers were replaced, bad bulbs replaced, and the playfield cleaned and waxed. Some of the light sockets needed cleaning because the bulbs weren’t making good connections.

Black Hole, Gottlieb Pinball Machine (1981)

Location: Littleton, Colorado.
Symptoms: Overall tune up, spinning disk in backbox not working.

I didn’t get many photos of this machine.  The owner was expecting a house showing and I was trying to get everything finished before the realtor came.

The machine needed a lot of little things fixed, everything from loose ball guides, rubber pieces, bad bulbs, broken bumper caps, broken bumper body, and drop targets.  The machine had too many balls installed in it which caused problems with the outhole and the eject mechanism to the shooter lane. This era of Gottlieb machines can only hold 3 balls in the trough.

Lower playfield

Lower playfield after cleaning and replacing broken pop bumper.

This machine needed a new motor for the spinning disk behind the backglass.  The parts needed to replace these motors are (as of 4-4-2014):

Quantity Description Supplier
1 3 RPM Gear Motor, part no. 638158 Servo City
1 0.770” Set Screw Hub for 6 mm shaft, part no. 545576 Servo City
1 0.770” to 0.625” hub adapter, part no. 545456 Servo City
4 Socket Head Cap Screw 6-32 x 1/4” Servo City
or hardware store
1 Machine Screw M3 x 6 mm Servo City
or hardware store
1 #4 lock washer, split or internal star Servo City
or hardware store

The hub mounts to the motor, the hub adapter mounts to the hub with the socket head cap screws, and the motor mounts to the machine with the M3 screw. The screw holes in the front of the motor don’t align with the existing bracket, so I only used a single screw with a lock washer to mount the motor. It seemed sturdy enough.

The existing black flat-head screws that hold the disk to the old gear motor are reused to mount the disk to the hub adapter.

I attached a connector to the new motor and plugged it into the existing connector on the wire harness. I connected the polarity to spin counter-clockwise. Everything worked great!

I was recently at the Texas Pinball Festival and saw two Black Hole pinball machines and neither had working motors. It seems to be a common problem.

Underside of main playfield

Underside of main playfield

Gottlieb System 80B Pinball Machines, Voltage Adjustment

Tip: When adjusting the 5 volt supply on a Gottlieb Systems 80B pinball machine, adjust it to 5.00 volts or lower, but not below 4.85 volts.

The reason for this is the poor design of the Memory Protect circuit, located on the CPU board.  There is a 3V zener diode (VR1) located on the CPU board that will start getting hot and fail if the supply voltage goes above 5.0 volts.

Zener diode with a bulge and crack along right side.

Zener diode (VR1) with a bulge and crack along right side.

Although the failed zener diode shown above was still basically working, I suspect it was acting intermittently, causing the CPU board to freeze up. Regardless, a bulging and cracked diode shouldn’t be trusted.  This was from a machine where the 5 volt supply was adjusted over 5.00 volts  (5.12 volts).

Also, the 5 volt adjustment pot on the power supply should be replaced with a fixed resistor.  The pot will get dirty and become sensitive to vibration, causing voltage fluctuations.   The best thing to do is adjust it for 5 volts, de-solder the pot from the circuit board, measure the resistance, and replace it with a fixed resistor or a combination of fixed resistors to obtain an equivalent resistance.



Spy Hunter Pinball Machine, Bally (1984)

Symptom: Cheap Squeak sound board blowing fuses.

I had another person send me their Cheap Squeak board from their Spy Hunter pinball machine after it was blowing fuses. Compared to last time, it was much easier to identify where the short was located. Both C10 and C22 capacitors were shorted.

Shorted capacitors.

Shorted capacitors.

Tantalum capacitors were used for both C10 and C22, and a variety of other locations on the sound board. Although tantalum capacitors don’t age like aluminum electrolytic capacitors, they have a weakness: they don’t tolerate voltage spikes very well (nor reverse polarity, where they will likely explode).

Both C10 and C22 are located on the unregulated 12 Volt supply.  This supply normally runs a little higher, and since it’s unregulated can have voltage spikes on it.  So if your Cheap Squeak is blowing fuses, replace both of these capacitors.  The original caps were rated at 25 volts, but I use either a 35 volt or a 50 volt for a replacement to make them more resistant to voltage spikes.  The value is 4.7uF.

The other tantalum capacitors on the board should be fine since they are downstream from the 5 volt regulator and it’s very unlikely a voltage spike would get that far. They should last forever.

Also note that aluminum electrolytic capacitors have the minus “-” side identified on them and tantalum capacitors have the positive or “+” identified on them.  The circuit board has only “+” polarity identified for all capacitors regardless of type.  So double check the polarity of the capacitors before soldering them.


Seeburg SHFA4 Jukebox Amplifier

Location: Lakewood, CO
Symptom: Loud hum on speaker outputs, amplifier rebuild.

As usual with an amplifier of this vintage, all of the electrolytic capacitors were replaced. This fixed the problem with the loud hum.  Also one of the electrolytic capacitors was causing a bias problem with one of the preamp transistors, causing that channel to be weak.

After the capacitor replacement, I was testing the amp with the oscilloscope, there was still one channel substantially weaker than the other, and both channels had non-linearity distortion. I traced this to the AVC (Automatic Volume Control) circuit.  The AVC circuit is used to level the volume between different records and different sides.

The AVC circuit uses the resistance through diodes to achieve this. The resistance of the diodes changes with the amount of current flowing through it.  The louder the song, the more current flows through the diodes, the lower the resistance, which lowers the volume.

AVC block diagram (click for larger).

AVC block diagram (click for larger).

The problem with this vintage of amplifiers is they used selenium diodes.  These diodes seem to fail with age.  In this case, diodes in both CR103 and CR104 were bad.  One was nearly open, another nearly shorted.  The resistance across the other diodes was high.  My multimeter couldn’t properly read them, so I relied on just measuring voltages across them.

Small selenium diodes are no longer made.Silicon diodes can be used, however, you have to use more than two in series for each selenium diode you replace.  I started with using two 1N914 diodes for each diode in CR103 and CR104.  The resistance was too low and my signal going into V101 was too weak.  So I doubled them, using four 1N914 diodes for each diode in CR103 and CR104.  That was 16 diodes total.

Old selenium diode pairs.

Old selenium diode pairs.

Rework showing the strings of four 1N914 diodes in the AVC circuit.  One string is exposed, the others are in the green shrink tubing.

Rework showing the strings of four 1N914 diodes in the AVC circuit. One string is exposed, the others are in the green shrink tubing.

I also replaced the CR102 diode pair.  For this, a single 1N914 can be used for each diode in CR102. After taking some measurements, I could have used three diodes for each, instead of four.  But with four, I get a little more input into the V101 tube (about 120mV) without causing clipping on the output.

The amp is now working great!

So if you’re rebuilding an amp with selenium diodes in the AVC circuit, replace them with 3 or 4 1N914 diodes in series for each diode in CR103 and CR104 and use single diodes for CR102.  It ends up being a lot of diodes, but they are small and cheap.


Disco Fever Pinball Machine, Williams (1978)

Location: Windsor, Colorado
Symptoms:  Wouldn’t boot.

The owner didn’t realize there were batteries in the backbox.  And of course they were leaking.  I removed the battery holder from the board and fortunately the board hadn’t been damaged by the alkaline. I replaced the RAM chip with an AnyPin NVRAM module so that forgotten batteries wouldn’t be an issue again.

The machine booted up fine after that.  I did a quick “shop” job on the machine, replacing rubber rings, burned out lamps, and cleaning the playfield.  There is a broken pop bumper cap, but I am unable to find an exact replacement.

Pop bumpers, with target in the center and arrows/triangles around edge.

Pop bumper caps, with target in the center and arrows/triangles around edge.


Spy Hunter Pinball Machine, Bally (1984)

Location: Lone Tree, Colorado

The owner had done some previous work on the sound board because it was blowing fuses.

When replacing capacitors, diodes, and ROMs, always double check the polarity.

When replacing capacitors, diodes, and ROMs, always double check the polarity.

Backwards ROMs.  The notch in end edge of the ROM chip should align with the notch in the socket and silkscreen image on the board.

Backwards ROMs. The notch in end edge of the ROM chip should align with the notch in the socket and silkscreen image on the board.

I determined that the 6803 controller was bad, as well as one of the ROMs.

Seeburg Jukebox Amplifiers, MRA4 and SHFA1

Location: Loveland, CO
Symptoms: SHFA1: one channel not working well; MRA4: generally not sounding good.

Both amplifiers were brought back to my office for bench testing and repair.  It’s really the only way to work on an amplifier.  A known signal, usually a sine wave is injecting into the input.  A dummy 8-16 ohm load is connected to the speaker outputs.  With the oscilloscope, I start at the speaker outputs and observe the signal.  If it looks distorted or weak, I work my way back to the inputs to find the fault.

The SHFA1 had one channel that wasn’t working well.  I found that the output of the first stage 12AX7 wasn’t outputting as well as the other channel at the same point.  The grid of the weak channel had a more positive bias on it of a couple of volts.  I traced it to a leaky 0.22uF capacitor.

Once that was repaired, now the weak channel was much stronger that the other.  I traced that to a bad 12AX7 just before the final output stage.

This amp had some previous work done on it, some capacitors had been replaced throughout, but interestingly, some of the most common ones that would normally be replaced hadn’t been touched, like most of the electrolytics.

The MRA4 hadn’t ever been service.  It still had the original paper and wax capacitors used prior to the 1960’s.

When rebuilding an amplifier, I usually replace every electrolytic capacitor.  If the amplifier is from 1960 or earlier, I usually replace every paper/wax coupling cap that has high voltage across it. I will usually leave tone control and other low signal voltage caps.

Prior to the cap swap, the MRA4 had a weaker output than I normally see.  I traced this to a leaking 0.05uF capacitor in the coupling circuit to the final 6L6 tube.  This caused the tube to be biased so that it wasn’t operating in a push-pull configuration.

This amp still had the original 6L6 tubes installed.  For fun, since I had some brand new 6L6 tubes, I installed those and they didn’t deliver the output that the original tubes did. I put the original tubes back in.  I rarely replace tubes unless there is a good reason to.  And this little experiment proves why.

Both amps are working great!

Diodes across coils on solid state pinball machines

I read so much false information on pinball related sites when it comes to technical electronic information.  Even on sites such as Pinwiki and Pinside there is usually about 50% to 70% misinformation.  These people state these false facts with such authority.  I worry about the people who believe it. (But hey, this is the internet.  You have to take everything you read with a grain of salt.)

I should start a regular feature here called False Facts.

Today’s false fact: Diodes are connected to flipper coils to help them release faster.

Actually, it’s just the opposite.  The flippers would release faster if the diodes were removed.  I’ll explain why in a second.  But first, don’t even think about removing your diodes to improve flipper response.

The diodes are there to suppress the voltage that is generated when the magnetic field of the coil collapses, after the power is removed from the coil.  This applies to coils that are powered by a DC voltage, which would be all solid state machines and a few of the later EM machines.

OK. So here is the slow motion version of what’s happening.  You press the flipper button and power is applied to the coil. Actually there are two windings in the coil, one strong one for “pull-in”, and a weaker one for “hold”, but just pretend for this discussion there is only one.  In most machines, the flipper button is in the ground circuit of the coil. The power supply voltage passes through the coil, through the switch, to ground. The magnetic field builds up and pulls in the plunger.  This takes a little bit of time, maybe 20-50 milliseconds (I’ve never measured it on a flipper coil).

(This is simplified.  Usually there is also a relay involved to keep the flippers off when the machine isn’t playing a game.  And the Williams Fliptronic system adds quite a bit more complexity to the scenario.  I’m going to omit the phase relationships between voltage and current. I’m also going to refer to current flow from positive to negative.)

A regular diode allows the current to flow in only one direction, like a one-way check valve. When the coil is energized, there is no current flow through the diode because the current is going in the opposite direction for which the diode is installed.  If you were to turn the diode around, all of the current would flow through the diode and not the coil, causing a short and likely burning up the diode. So at this point the diode is invisible to the coil.

As long as current is flowing though the coil, a magnetic field is maintained.  When you release the flipper button, power is removed from the coil.  But due to a variety of factors, including the plunger being inside the coil, it takes a little bit of time for that magnetic field to collapse. As the magnetic field collapses, it generates a current in the coil in the opposite direction that was used to create the magnetic field.  So for a brief time, the current starts running backwards.  Since it’s now going in the opposite direction, it goes though the diode to the other side of the coil causing a momentary short across the coil.  This short dissipates the power until the magnetic field is gone and the plunger is released.  But the point is, for a brief time, there is still current flowing in the circuit, through the diode, after the button is released and a magnetic field is still holding the plunger.

If the diode was removed, the magnetic field collapses, but there is no current flow because there is open circuit and no place for the electrons to go. Instead the voltage keeps rising and rising across the coil until it arcs across the switch contacts or sends a big voltage spike into the rest of the pinball machine. (This is basically how sparks are created across spark plugs in your automobile engine.)

But since there is no current flow (very little anyway) the magnetic field goes away faster and the plunger releases faster.  But as mentioned, the downside is a voltage spike, which can cause havoc in a solid state machine.

So that is why the diodes are installed across every coil in the pinball machine, to reduce the voltage spikes.

There are no diodes in EM machines because they are running on AC instead of DC.  The voltage is lower to begin with and the coils not as strong, there isn’t much of a spike.  But it could be suppressed with a resistor and capacitor across every coil (called a snubber).

Now… there is a way to decrease the release time of flippers, but I’ve not tried it.  And I’m not sure how noticeable it would be and don’t recommend it. Replace each regular diode with two zener diodes in series, back to back (opposite polarity).  The diodes would need to be rated at the maximum voltage seen by the flipper coils (about 70V for a 50V system) and a current rating of 2-3 amps. A voltage spike will still be generated, but it will be a bit more controlled and may still cause havoc with a solid state machine.  This is how fuel injectors in an engine are handled (essentially a coil and a plunger, just like a flipper) and those are switching on and off at a very rapid rate.

Medusa Pinball Machine (Bally, 1981), LED upgrade

Most pinball LED’s that I’ve come across are not compatible with the early Bally solid state pinball machines from 1979 to 1985.  These machines use a lamp driver board, where each controlled lamp is driven by a silicon control rectifier (SCR), which is also known as a thyristor. Bally’s Medusa falls into this category.

An LED installed into a Bally of this vintage will flicker or not work at all.  The problem can be overcome with a 1000 ohm (1K) resistor in parallel with the LED.  The reason for the flicker is somewhat technical and is explained below.

Some people opt to solder a resistor across every lamp socket.  This isn’t too much of a problem if the number of lights is not high.  Medusa has over 80 controlled lights and that would be a lot of work, especially on the hard to get at sockets.

Since one side of each lamp is common to all of the others, a pull-up type resistor network can be used.  Also, since the connector pin spacing on the lamp driver board is 0.100″, this is a perfect match for using though-hole resistor networks because the pin spacing is the same.

Rear of Lamp Driver board showing resistor networks (pullups) installed.

Rear of Lamp Driver board showing resistor networks (pullups) installed.  Click for larger.

The resistor networks were laid horizontally next to the lamp output pins on the reverse side of the board (the view from the front of the board is unchanged and you’d never know the resistor networks are there).  The common pin from each network was bent up vertically where a wire connected all of them together (blue wire in the above photo).  The blue wire was routed through a single pin connector to the lamp common on the backboard. The single pin connector allows the driver board to be removed from the backbox.

Another nice thing about doing it this way, as opposed to putting a resistor on every socket, is if the machine is ever sold and the new owner (a purist) wants to switch back to regular #47 incandescent lamps, the resistor networks can easily be removed from the back of the circuit board (though incandescent lamps will work perfectly if the resistor networks remain in place).

On Medusa, there is a light bar at the top of the playfield.  It was decided to leave those as incandescent lamps. A LED can turn off and on faster than an incandescent bulb, and I think with today’s bright LED’s and the fact they are aimed right at the player, the flashing would be a bit too much.  Aside from that, all controlled lamps and general illumination on the playfield and backbox were replaced with LED’s.

Playfield with all LED lighting (except for row of red lights at the very top).  Click for larger.

Playfield with all LED lighting (except for row of red lights at the very top). Warm white LEDs were used for under playfield plastics. New translucent polyurethane flipper rubbers were used on the illuminated flippers. Click for larger.

Backgox LED lighting, with a mixture of warm white, cool white, and red LEDs for the eyes.

Backgox LED lighting, with a mixture of warm white, cool white, and red LEDs for the eyes.

Side LED's from were used in places like the Gorgon rollover switches.

Side LED’s from were used in places like the Gorgon rollover switches.


I’m not totally sold on the idea of upgrading older machines with LEDs, but all in all, I think it’s an improvement for Medusa.

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