IGT VFD Display
We recently got a request for assistance from one of our readers who works in South Africa that needed help on IGT’s VFD Display used on their upright 960 games (75117700 VFD Controller and 75117800 VFD Display). He found the problem. Troubleshot the controller down to the bad capacitor, but didn’t have access to the part. He posted a request for assistance to the Slot Tech forum, and it also got routed to me through Randy Fromm. The part in question was C3 on the 75117700. The confusion was understandable. If you look at the schematic C3 appears to be an electrolytic (polarized) capacitor, but the part on the board is about ¼" square and flatter than a dime. Here’s the scoop. C3 is a Ceramic capacitor, 0.47uF, 10%, 100V, X7R (but don’t buy that guy yet, read on). All of that is self-explanatory except, maybe, the "X7R". X7R specifies how much and in what direction the capacitance will vary with changes in temperature. X7R is pretty good. The capacitance varies only slightly with temperature. "COG" and "NPO" are better, but I doubt if the difference would be of value. It's only used as a filter capacitor for a power supply. In contrast, the really cheap "Z5U" varies so much they can be used as a thermometer. Avoid them except for use as noise filters across power lines on ICs.
Measuring the output of the power supply that supplies Pulsating DC to C3, I measured 70 Volts. That’s 70 V RMS. That makes the peak voltage about 98 V (RMS x 1.414). Right at the 100 Volt rating of the capacitor. It is no surprise that this puppy fails a lot.
Since the initial posting of this article I found out that IGT has a Change Notice out upgrading C3 to a 200 V rating. Thank you IGT for being the high caliber of character we have come to know and expect. You set the standard for the rest. And thanks to Ed Hoffman for bringing that to my attention.
This board has been on my ToDo list for quite a while. I had a box of these boards with no way to even test them. Needless to say this project got moved to a front burner, and I finally got a test setup built and the boards tested. Along the way enough interesting things transpired to make it worth an article.
The VFD controller is a NETPLEX device. This bugger can cause more interesting problems than you can shake a stick at. NETPLEX is one of the serial I/O ports that are built into the game. Not to be confused with SENET stuff, but quite similar in operation. At the controller we have a transmit data line (NETPLX_TXD) that is data from the VFD controller back to the NETPLEX controller on the Logic Board. We have a receive data line (NETPLX_RXD) that is data coming from the NETPLEX controller on the Logic board going to VFD controller. There is a Reset line (RESET\, the "\" means it is an active low signal). And power (+13V).
If the VFD controller has a major failure it can hold down any of these lines, resulting in some wild symptoms that point anywhere but to the VFD controller.
Operation
Let’s take a peak at the workings of this controller and see what makes it tick.
U13 is an LM340T-5.0 (AKA, LM7805T) that drops the +13V coming into the board down to +5 V for the Logic circuits on the board.
U17, T1, CR1, CR2, CR3, CR6, CR7, Q1, and a handful of other passive parts (including which is mister C3), make up a voltage converter circuit. U17 (LT1172) is a switching voltage regulator that drives transformer T1. The output of T1 creates AC to drive the filaments of the Fluorescent display, and the higher +DV voltage (VAND, Anode voltage, about +70 V) that puts a positive voltage on the Anode of the display. The charge ionizes the gas in the tube making the cathodes glow blue when they get pulled to ground. This 70V should be testable across C3. This +70 Volts is dropped down to around +56 V by CR7 and Q1, to become the Grid voltage (a bias voltage). Signals to the VFD display are applied to the Cathodes of the display (Ground turning it on). If you are old enough to remember how vacuum tubes work, all that makes perfect sense.
The rest of the controller shows no real surprises. U10 is the microprocessor, 16-bit Address, 8-bit Data, multiplexed data and lower eight address lines, an 8-bit Programmable Port (P1.0 through P1.7) and a built in Serial port. If all this sounds familiar, it should. This guy is a cousin of the 80C31 and 80C32 used in the older IGT 8032 based games, and described in detail in a previous article.
U4 is a Programmable Logic Device used to decode the upper four address lines and select what device the 80C320 is going to talk to. U1 is the Program memory, a 27C256 EPROM. (IGT Part #, ?, AKA VFD00006, on my board. Yours may vary). U2 is an optional FLASH memory, not present on my boards. U3 is a RAM chip (LH51256L) that holds the message to be displayed.
U7 is the lower eight address bits latch. At ALE (Address latch Enable) the contents of the Data bus contain the lower eight address bits, and are latched into U7.
U6 is a latch used as an 8-bit data port that is control lines going out to the VFD display board.
U15 and U16 are opto isolators that allow communications between the VFD controller and the NETPLEX controller. The inputs to the VFD controller (pin 1, RESET\; and pin 4, NETPLX_RXD) are open collector inputs to the VFD controller board.
Data to the display board comes out of the microprocessor in parallel, and gets converted to serial by the circuits around U8. To load data into the shift register the microprocessor holds the "Shift / Load\" input of U8 low, applies a byte of data, followed by a Clock (CLK) pulse.
Putting the "Shift / Load\" line back high starts shifting that byte of data out U2 on following clock pulses. There are two channels. Channel Select (CHNLS) determines which of the two channels the data will go out. I suspect one channel is dot information and the other channel is control operations, but consider that a guess. I have no information on the display or the Display Driver chips.
U11 generates a timing pulse with a small delay. When GRIDCLK (Grid Clock) goes high it triggers U11A, making pin 4 low and 13 high (for about 5 micro-seconds). Pin 13 going high triggers U11B, making pin 5 go high (for about 7 milli-seconds). Since pin 4 will still be low for 5 microseconds, the effect of this circuit is to turn GRIDCLK into a 7 milli-second pulse, delayed by 5 micro-seconds.
Testing
I assume you have got the problem isolated down to the controller board by replacing the Display board and ribbon cable. The display board has only the display itself and five major ICs. These boards have few failures other than the display tube itself being physically damaged. The display drivers are not too troublesome to change if you play well with large surface mount ICs. The Display Drivers are reasonably priced.
So that brings troubleshooting down to the controller board, itself.
Power on
On Power up the Display should say "Display Passed Self-test", then "Display’s NETPLEX Link is Down" (unless the controller is actually in a game). If it doesn’t check input voltages and Current drain on the power line. Normal current is about 0.200 A. My test fixture is current limited. If the device tries to draw more than 0.250 A, +13V shuts down. I deliberately set the current limit on my test fixtures just above what normal should be. Otherwise the unit under test could draw excessive currents creating smoke and damage. C3 is a popular culprit for pulling down the power. Of the five controller boards I had that were bad, four of them had C3 shorted.
General logic
If you get no display. Power is okay, and no excessive current, it’s time to start troubleshooting the digital portion of the board. With a powered up interface (See testing U15 and U16) holding pin 1 of the 5-pin connector to ground should keep the microprocessor in RESET. All Address, Data, and Control lines of the microprocessor should be in the tri-state condition, and float high (+5V). None of the outputs of U4 should be low.
An unusual voltage on any of these lines would indicate a problem. "Unusual" meaning any voltage not a legit high or low without a good reason. All voltage in a digital world should be close to VCC, or close to ground (especially in a CMOS world such as this board). Usually such symptoms are accompanied by the IC in question getting warm. Replace it.
Counting pins on PLCC devices. Pin 1 is usually marked on the board, or by a indented dot on the IC case. From there counting goes clockwise around the chip.
If all that appears good, remove the Reset\ ground and scope the address, data, and control lines with a scope looking for an unusual signal. The oscillator should be oscillating. The outputs of U4 should be pulsing. The outputs of U6 should be pulsing.
U15, U16
The input optos may be tested reliably. Applying power (+13V) to pin 3 of the connector and Ground on pin 5 should apply power to the Logic Board side of the optos. Pulling pin 1 to ground should put the Controller Board into a Reset. The Display should say "Display Passed Self-test", then "Display’s NETPLEX Link is Down".
Pulling pin 4 of the 5-pin connector to ground should light DS1. You at least can confirm that the opto is working correctly.
The Transmit output doesn’t show activity until it recognized the NETPLEX interface being active. As yet I do not have a NETPLEX emulator to properly test this output of the controller board.
Communications to the display
U11 pin 13 (about 5 uS), and U11 pin 5 (about 7 mS), should be pulsing. U9, U8, U5B, U14A, U14B, and U14C should be pulsing.
If all the above seems okay, you’ve got some serious troubleshooting to do. At a price around $150, it is worth spending a little time on Controller Board repairs. Most of the time the problem is quick and easy (Check C3 first).
Whether or not you want to work on the Display Board is a value judgment. The board costs about $230. The display itself costs about $50. The Drivers / shift registers cost about $6.
5-pin AMP "MTE" AMPMODU connector
Board side – 103669-4 (AMP), 512-2915 (Allied), 21102590 (IGT)
Cable side – 104257-4 (AMP), 512-2656 (Allied)
Pins – 104480-7 (AMP), 512-0016 (Allied)
26-pin ribbon cable connector
Board side – 103308-2 (AMP), 512-2631 (Allied), 21106190 (IGT)
2-pin connector for power
Board side – 103669-1 (AMP), 512-2909 (Allied), 21104990 (IGT)
Cable side – 104257-1 (AMP), 512-2650 (Allied)
Pins – 104480-7 (AMP), 512-0016 (Allied)