Tapper rebooting over and over

ChaosJohn

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So my Tapper now has a problem where it just keeps going through the boot process over and over and never comes up. Just to be clear - it never gets to any screen with real data, just a couple of wipes of the screen then black, then a couple of wipes of the screen, then black. I'll dig out my voltmeter and start getting more details shortly, but thought I'd get some first impressions here.

low voltage, maybe?

Something else?

Anyone ever had a Tapper that would keep rebooting like that.

cheers,
John
 
Without any Tapper-specific knowledge, I would suspect either the CPU is off into the weeds (triggering a watchdog reset), or a power droop is triggering the reset. Are you able to get any self-test results? Or is it resetting before self-test completes? Does it always reset at the same point?

In order to complete self-test, you should just need a functional CPU with access to functional ROM and possibly RAM (if the CPU is using it for a stack). If you have self-test results, that would obviously help isolate the failing subsystem. Barring that, there are typically only a few circuits that can trigger CPU reset; if you have the means, it would help to probe these and isolate which one is triggering. And since correct voltages are a prerequisite, you can also start by getting measurements of all power rails at each subsystem.

If it turns out the power is good but the CPU is watchdog resetting, it would be good to verify the ROM data paths next (contents and decoding). What tools do you have available?

LeChuck
 
If it turns out the power is good but the CPU is watchdog resetting, it would be good to verify the ROM data paths next (contents and decoding). What tools do you have available?

Doesn't make it to the self-test, it's rebooting before that.

As for tools I have a hammer, screwdriver, plyers... no, I'm just joshing you.

I've got a voltmeter and a logic probe if you're talking about primary tools for debugging PCBs (though I'm not particularly good at it). I'll check the voltage getting to the board first - that was my first thought. But if the voltage is good I'll try swapping out the CPU (I think I have a spare Z80) and then if neither of those is bad then I'll try for harder issues.

Thanks,
John
 
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Do the couple wipes of data on the screen correspond to the start of the self-test (if you're unsure, try comparing against the self-test in MAME)? It may help narrow down the issue somewhat, since that could indicate the CPU is initially executing from ROM successfully, and going off into the weeds later.

I'll have to check the schematics later, but if you know where the watchdog circuit is located, you could check that with the logic probe to verify that's what's causing the reset. Swapping CPUs is another data point to gather, although I suspect your problem is in the ROM data paths.

Does the Tapper board have any kind of progress code LEDs to help troubleshoot failures before self-test completes?

LeChuck
 
Doesn't make it to the self-test, it's rebooting before that.

As for tools I have a hammer, screwdriver, plyers... no, I'm just joshing you.

I've got a voltmeter and a logic probe if you're talking about primary tools for debugging PCBs (though I'm not particularly good at it). I'll check the voltage getting to the board first - that was my first thought. But if the voltage is good I'll try swapping out the CPU (I think I have a spare Z80) and then if neither of those is bad then I'll try for harder issues.

Thanks,
John


1. Check your voltages first.

2. Your CPU is probably ok, because you have activity.

3. If you have the original ribbon cables between the boards, I would start with those. The originals suck. I believe Bob Roberts has replacements.
 
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