Dead Flying Shark

[kyapiko]

Hello guys,

I've recently got a dead Flying Shark original PCB. No picture, no sound...

Checking the 68000 pins with a scope reveals no big activity.
I've checked the reset pin on the 68000 and it is stuck low (not 0V but approximately 0,3V).

I've read that it may be caused by a bad cap but can't really find where to look at. Tracing the reset pin goes to an MS4728 (In-Line Resistor which I don't really know how it works).

Thanks by advance for any help !

[caius]

Well, a resistor network in SIP package is nothing more that some resistors with a common leg (they are called also with commom bus, there are also with isolated bus).Looking at the mark of the your : '472' is value in Ohm so 47 + 2 zero's=4700 Ohm, '8' is the number of pin.Pin 1 is marked with a black band usually and is connected to GROUND or VCC according to its function as pull-down or pull-up resistor.Pull-down and pull-up resistors are used to force the inputs of an electronic digital device in a HIGH or LOW state when these inputs are disconnected or in high impedance state.So, with the PCB powered off, take your multimeter and set it in the appropriate Ohm range, put one probe on pin 1 and the other one on other pins, you should read 4700 Ohm.If you read other values or nothing it can be faulty (usually they break internally)

[kyapiko]

Thank you very much Caius for your explanation. I better understand.

A little correction: the 68000 reset pin doesn't goes to a MS4728 but to a NK1038.

Pin 1 is connected to VCC and all the other pins give 10 kOhm which seems correct (10 Ohm + 3 zero's).
As pin 1 is connected to VCC, should it normally force the reset input to high state instead of low ?

[caius]

Thank you very much Caius for your explanation. I better understand.

A little correction: the 68000 reset pin doesn't goes to a MS4728 but to a NK1038.

Pin 1 is connected to VCC and all the other pins give 10 kOhm which seems correct (10 Ohm + 3 zero's).

That's right, 10KOhm is the most used value for pull-up and pull-down resistors

As pin 1 is connected to VCC, should it normally force the reset input to high state instead of low ?

Yes, but this happens when RESET is not used (tristate).If 68000 RESET line is stuck low, it means this signal is not properly generated so you have problem with this part of circuit.If schematics are not available, I advice you to figure out RESET circuitry, this will help you in your troubleshooting.

[kyapiko]

68000 reset pin goes to a series of TTL chips. Some are simple gates easy to diagnose, some are more complicated (at least for me)...
I will take time later to carefully check and retrace the whole circuit and will let you updated.

[caius]

It seems this kind of boards suffer from the same problem caused by a capacitor.I guess you have read here about :

http://www.system11.org/temp/PCB_repairs.html


Try to replace it (though it's not clear which one is).

[system11]

The one right by the 5v pins. I can't find a photo that shows the cap number, I'll dig one of mine out and correct the repair log next time I update it. I've had multiple dead boards due to that one.

[kyapiko]

Thanks guys.
The 470uF cap near the 5V is C1. I changed it and now get a high state on the reset pin of the CPU, but still a black screen without sound.

[caius]

OK.Check for Fujitsu TTLs.I had similar issue on my board , at the end I replaced two bad 74LS04 from this manifacturer.

[kyapiko]

I checked the board but couldn't find any Fujitsu TTLs on mine. The 74LS04 I found are TI.

[caius]

OK, they used different manifacturers.But, this doesn't mean you cannot have some trouble with them.Have you checked program ROMs and RAMs for first?

[kyapiko]

I tested the signals on every 74LS04 but found nothing suspicious. They're doing their job as inverters.

[Asure]

Time to check out activity on the address and data bus then. Is the cpu doing anything there? Pulsing? Any stuck bits?

[kyapiko]

I'm gonna check that and let you know asap

[frsj8112]

Sorry to wake an old thread, but I've just repaired a dead Flying Shark.

I checked system11's log about the game and desoldered the 470uF 16V cap at C2 and it measured only 20uF.
Replaced it and presto, the game booted

Thanks system11!!

[caius]

Yes, it's a typical issue on this Toaplan/Taito boards.This electrolytic capacitor is involved in RESET circuitry, most of times you will measure increased ESR besides altered capacity.

[frsj8112]

One more thing. The pcb has some scratchy audio, for example when you drop the bomb or when your plane is shot. The music is fine.

I've used the old trick of hooking up a pair of leads to and external audio amp and then to the pins of the audio circuitry.
There is a YM3014 on this which leads pin 2 to an NEC C358C. The scratchy distortion can be found when "listening" on YM3014 pin 2.

And before the YM3014 is a YM3812, and pin 21 on that chip leads to pin 4 on the YM3014. Could it be the YM3812 that is faulty?

[caius]

What can you hear on pin 2 of the YM3014?The only SFXs or music + SFXs?

[frsj8112]

both SFX and Music. It sounds alright except that the explosion sound is distorted.

[caius]

OK.If YM3014 DAC was bad, you would have both music and SFXs corrupted.So I think the problem is before the DAC, most likely there is some problem in data bus of the YM3812.
Anyway try to piggyback a good YM3014 (and a YM3812 too)