Mephisto Chess computer problem

[Mart]

Hi Folks.
I will try and give as much information as I can without it turning into war and peace mk2.

I have an old Mephisto Chess Computer that were very popular back in the day and can still be seen for sale on the bay etc.
It consists of 2 separate units, 1 is the wooden electronic board with all the pcb circuitry and the other is the black control unit which has the processor, input pad and LCD display. (Please see uploaded pictures).

The black computer unit can be either used as a stand alone item to play a game against or it can be connected through the flexible ribbon cable to the wooden board and moves can be input by the magnetic chess pieces through reed switches directly under each square.

When playing just against the black chess computer module the power supply is connected directly just to it.
When playing with the black chess computer module and the auto sensory wooden board the power supply is connected to the wooden board and the black unit to the board by the ribbon cable as per above.

Until recently it has worked just fine in either mode, recently however it will only work when the black chess computer is being used on it's own.

When connected to the wooden board it starts up okay but after about 15 minutes or so into a game the LCD display characters quickly start to dim and either break up or fade out completely and I have tried 3 different power supplies which are all to specification and the same happens. If you leave it for a couple of minutes and turn it off and back on it starts up okay again and just repeats the fault cycle.

This would seem to indicate the problem lies somewhere on the on the wooden board electronics.

Finally the question is, "Where to start looking, out of all the components which would cause the power from the wooden board to the black chess computer unit to fail after 15 minutes and cause the LCD to fade out or lock up?"

I hope this all makes some sense and If anyone can point me in the right direction towards suspect parts it would be greatly appreciated.
Thanks. Mart.

 

[Nisar-von-Voltenbräackter]

Could it be chip failure due to overheating?

Have you tried keeping it chilled for the expected duration until fault has been occurring?

I'm guessing a timing chip gets too warm and then fails, maybe a 555 IC chip, a most popular timer.

 

[kellys_eye]

As with most old electronics, if there are electrolytic capacitors on the board, CHANGE THEM.

 

[Mart]

Hi folks. Many thanks for your replies, please see the additional photos of the board where you can see most of the resistors and chips, some of them the values can be seen and are identifiable and still available with the odd one that isn't.
Which of these would you swap out first, the capacitors (axial first or radial first) or start on the chips as although I can use a soldering iron fine I no little about the electronics side of things.
Thanks again.

 

[Nisar-von-Voltenbräackter]

I'm not seeing a definite timer chip. However, one has to exist for something as complex as a video game, I'm sure.

Analog timing uses RC circuits, both resistors and capacitors. Their values work together to produce timing delays. While hardly as accurate as digital timing, I seem to recall you can find them at work in PLLs as well as oscillators. I surmise oscillation components and transistors operate video screen.

I'm predicting that summarizing the circuit-board/PCB chip identities may help someone with a well-practiced technical background reply.

TC4017BP: decade counter/divider chip
MM74C374N: 3-state octal d-type flip-flop
DM74LS373N: same but adds transparent flip-flop function; probably tracks Chess-piece positions.
TC4082BP: dual, 4-input AND gate
D08-E333: unidentified; obviously handles I/O; probably demultiplexer; Matsushita manufactured.
D08-E391: unidentified; clearly processes and sends data; probably multiplexer chip; Matsushita manufactured.

D-type flip-flops (or D-type latches) require a CLOCK input.

In the meantime, if it were me, I would remove all the transistors and electrolytic capacitors, make a note of their positions, and check them with a digital multimeter, preferably one that can check transistors by plugging all three leads into its slots.

 

[kellys_eye]

I would remove all the transistors

I certainly wouldn't!

The fault symptoms don't point to anything like the transistors being faulty. Testing individual parts by removing them from the board is a 'beginners' short-cut and rarely resolves anything but often creates even more problems.

But that circuit board isn't the 'brains' of the machine - where's the rest of it?

 

[Nisar-von-Voltenbräackter]

Mart,

Oh, I forgot to ask if you know how to do continuity testing with a meter. That can be done prior to testing any specific component and will indicate if a component has lost its internal connectivity.

 

[kellys_eye]

That can be done prior to testing any specific component and will indicate if a component has lost its internal connectivity.

???