Amiga 3000D doesn't boot

OK, first post here.

Just got an Amiga 3000 with a rev 8.9 mobo. Doesn't boot, no video.

Tested:

o Voltages - look good
o ROM pins - no activity, stuck low or high
o CPU clock (68030 pin 6) - logic probe indicates pulse train, looks good.
o /RESET (68030 pin pin 64) - stuck low - bingo!

So I need help determining how reset is generated.
Tried looking through the schematics but got a little lost...
A nudge in the right direction would help.

Toshi

Found a readable schematic and now I can read the part numbers.

_CPURST is connected to U713 pin 9, which is a 7407 hex buffer with open-collector outputs.
The input (pin 8) is low, so it's working ok.

U713 pin 8 is driven by U712 pin 6, which is an OR gate output. I dunno why they needed a 74F part for it, but anyway...the inputs (pin 4 & 5) are low, so it seems to be ok.

So it seems like the inputs to the OR gate (_KBRST and _EHLT) are both low. Doh!

Toshi

[jimbob]

Hello, sounds like you know what you're doing. I for one will enjoy hearing more about your troubleshooting.

Only thing I might suggest is could a faulty keyboard assert a reset continuously? I don't even know if the A3000 keyboard controller is on the mobo or in the keyboard. Tried booting with no keyboard?

I guess only _KBRST needs to be low in order for both inputs of the OR gate to be low. So probably only it is being held low.

_KBRST is generated by the Gary chip pin 36. Hmm...why is it holding it low?

And yes, I am booting with no keyboard. The mobo is sitting on a wooden desk connected to a power supply. Only things plugged in are a daughtercard and a VGA monitor.

Toshi

[jimbob]

Here's a link to the hardware reference manual. This section can maybe help you rule out a keyboard problem.

http://amigadev.elowar.com/read/ADCD.../node0172.html

According to this there are pull ups for the KCLK signal on the mobo, but if KCLK is held low by some fault, I think the amiga would reset continually.

Originally Posted by jimbob

Here's a link to the hardware reference manual. This section can maybe help you rule out a keyboard problem.

http://amigadev.elowar.com/read/ADCD.../node0172.html

According to this there are pull ups for the KCLK signal on the mobo, but if KCLK is held low by some fault, I think the amiga would reset continually.

There is no keyboard connected and KBCLK (Gary pin 44) is high, so it looks fine.
CPUCLK (Gary pin 43) has looks fine.

Will check the rest of the Gary inputs later when I have time.

Toshi

Few more data points:

o Swapped Fat Gary. Same problem.
o CLK90 (Gary pin 70) looks good
o _KBCLK verified not shorted to ground

At this point, the simplest explanation is that U713 (7407) has gone bad and the outputs are stuck low. This explains why both _KBRST and _EHLT are both low, because they're connected to U713 outputs.

I'm planning on desoldering U713 (and testing it) when I get time.

Toshi