Sys 80 Non CPU boot/IRQ issue

[Strangeways]

New CPU in, couple of this resolved but still IRQ high.

From reading the schematic, IRQ begins at pin 4 U1 (6502) goes via R1 & R2 to pin 16 TC1 then to pin 25 U4, U5 & U6.

Is there anything prior  to U1 which has anything to do with IRQ?

Check the 5V at R1 - maybe it is shorted keeping the signal high ?

Can you bend pin 4 (so it is out of circuit) and see if it stays High on the CPU leg, then check the socket (in circuit).

I'm going off a schematic for a SYS80b - but it sounds the same.

[MartyJ]

Getting around 4.8/4.9v on both legs of R1.

Will report back re CPU.

[MartyJ]

With pin 4 bent out, no signal on it at all.

High on socket (pin 4)

[Marty Machine]

I'm guessing if you pull ALL the chips out of sockets that are on the IRQ line, then put them back one at a time, you should find 1 chip will 'hold' the IRQ high, when it was previously blinking wiht the other chips.....tedious elimination.

With all chips out, the IRQ pin might still be held high by a pullup resistor anyway.

Also to note, powering the cpu card on bench might not show the IRQ correctly, you'll need to create some activity on the playfield so the PIA's etc start generating IRQ's...
Typically, a cpu board sitting on the bench powered up wont be pulsing IRQ's at all, maybe this is part of the prob misleading your readings??

If anyone can send me a schematic/link, i'm happy to look thru it.

MM.

[MartyJ]

I'm using the full game power board to boot it, but no playfield load etc.

The TOP video (for BH) and Clay's guide show them being booted on a bench (similar setup to mine) and all documentation states that the IRQ (even with only CPU powered) should stay high for 5 sec, then drop once game boots (around 3.5v I think).

Link here:

http://www.pinrepair.com/sys80/bhcpu.gif

[ddstoys]

Are you using your Logic Probe to test this or your multi meter as im sure this should be pulsing

[MartyJ]

Dmm for voltages as per Nino's req.

You're correct as to it should be pulsing once booted (5 sec).  Mine is stuck at 5v (high).

Yes to logic probe too.

[Strangeways]

I'd be switching to the logic probe for this process - but before you do - R1 should measure 3K. Can you confirm ?

[MartyJ]

2.985 firm on R1

[ajlaird]

Close enough (ie within tolerance).

[MartyJ]

I do have spare resistors if required.

Going back over the schematics I cannot see anything else (other than U4,5&6) which may be causing the problem.  Then in turn it could be any other IC hanging off the RIOTs.

[Strangeways]

I do have spare resistors if required.

Going back over the schematics I cannot see anything else (other than U4,5&6) which may be causing the problem.  Then in turn it could be any other IC hanging off the RIOTs.

Process of elimination - remove all the ICs involved and install them one by one until the signal remains high.

Each hour you spend on the repair will be an hour closer to the solution and you will learn as you go along.

[Marty Machine]

I NEED a schematic   heheheheehe  

[MartyJ]

I NEED a schematic   heheheheehe  

Its on the link 4 posts up.  Not a fantastic one, but all thats around.

[Marty Machine]

Thanx got the schematic, silly me 'selective reading' again, so i missed it hehehe

Umm, looking back at the start of post, although you have 'no displays at all' does the pin powerup or do anything? can u play a game, and flippers, bumpers, rollovers work and make sound etc??
(sorry if i missed this before).

I'm thinking maybe the master clock (Z3-7404 & Z2-7474 into U1-6502) might be dead? which would mean no bootup therefore no IRQ 'flash' that you expect to see.
The 'E2' link might be open?
Of course if the game runs, then ignore this, obviously the master clock is ok into the cpu...

Also, U1-6502, pin 39, which is a cpu clock signal going out to all PIA's, i'd be looking around there too, in & out of that Z3 etc....

The only bummer is you'll need a Cro or frequency display on your MM to measure the presence of the clock signal, capable of reading say 2-4Mhz ?? ??
You *might* be lucky that your logic probe has an inbuilt frequency-divider and will show a blinking pattern....try it i guess.

Also look at U5 pin34 (reset) and see what levle it is held at, maybe chip is held in a reset state??

I'd also look at U5 pin15 (Z26 pin12), and see what that signal is doing, it looks like the 'slam switch' does something to the score driver, blanking it? or something...if it's held in a state, maybe U5 is simply locked out, hence no IRQ??

Hope this helps.
MM.