"The Invaders" pcb - IC31

[nilfisk]

Hi,

I am trying to identify IC 31 on my "Invaders" board. There are no markings on the IC itself, and the schematics say its a "82S" - however, I am quite sure this does not refer to a 7482. The IC is driving the video output, sending its signal to the BC548. The IC on my board seems to have gotten very hot at some point of time, and as I do not get a video signal atm, I'd like to start going back from there.

Any help appreciated!

Martin

[speleo_de]

Likely a TTL PROM N82Sxxx -> have a look at my page: http://www.andysarcade.de/gen_electronicsspares.html#BIPOLAR_PROM

You need to have the image or a good PROM to read from.

[PaulSwan]

On the board layout diagram it says IC 31 is an 82S130, a PROM, and they often run hot. It's the "colour" PROM. There is no listing for it in MAME for Invaders but there is for Dodgem that shares the same hardware so it may be the same image as that (listed as a 74S571).

I'd start by verifying if the PROM is getting any inputs - video in is on pins 4, 5, 6, 7 (from the 2636 graphics chip), pin 1 from the bitfield, 2 and 3 from a counter (like line counters) and the blanking controls on 14/15.

In the normal operating case, you should see all of the inputs busy doing something if the video hardware is actually generating video.

If you don't have a way to verify this, it's time to buy a logic probe

Paul.

[nilfisk]

Thanks for your help, guys, greatly appreciated.

I do have a probe and (slow, portable) scope at hand, so I should be able to follow on. Also, if the prom is faulty, I've finally found a reason to get my Unipak 2B going 

Martin

[nilfisk]

...

[nilfisk]

Hi,

I've finally found the time to get my probe and take a look at the PROM - having two kids and a job sometimes has a major impact on priorities and hobbies . I can see a pulsing signal on most pins, but have pins 3, 5 and 6 stuck low on the input side. From Pauls post, I understand that 2 and 3 are fed by a counter.

I can trace 3 back to a 7475 (pin 16), on which I find pin 1 stuck high (makes sense, as it's the same, inverted output as on 16), and the input on pin 2 stuck low (makes sense as well, as it's tied to ground via some jumper near the bottom of the PCB).

Pin 5 being tied to low seems fine as well, it's pulled to ground via a 2.2k resistor.

Pin 6 is fed from the 2636i graphics chip (Pin 26) - this being stuck to low I can't explain, but I know nothing about the 2636.

I'll keep going, but obviously appreciate any further inputs from you tech wizzards

Thanks!

Martin

[PaulSwan]

Do you have schematics to follow, Martin?
Paul.

[nilfisk]

Hi Paul,

Well, I've downloaded the manual packet from your site (thanks for hosting that, btw!), but I don't find the main pcb layout in there to be particularly helpful, as it's more about locating components than showing how they are supposed to interact. This may be one of the reasons for me poking blindly (this, and my overall lack of proper knowledge when it comes to fixing PCBs   ).

Martin