MegaST1 stuck in reset state

[Tophe38]

Hi,

I'm investigating a MegaST1 (Rev1, IMP GLUE) stuck in reset state.
When I press reset, no video sync switch.
I check :
- Powersupply (but remove it, and use a safe 5V/12V external one, safe to probe motherboard, and reset part under power supply)).
- Reset swich.
- TOS (switching with another one; 1.02)
- Test (continuity) Data and addresses between TOS and 68000.

So, I take the schematic, and look at reset part :

img1.png

To make it short, when I press the reset switch, I can see 5 going down, and return to High.
U1 Pin1 & 3 are then High,
But Pin 2 (Reset) is 2.7V, and Pin 4 (CPU HALT) is low (almost 0V)

So, Is 7407 dead, or CPU or MMU/GLUE ?
(I try power up without U5 / Blitter, nothing change).

Thanks.
Chris

[stephen_usher]

You could try removing U2, fit a socket and see what the voltages are at it's pins 2 and 4. Seeing as 2 has a pull-up that should be at 5V. If it's not then the issue is further down the line. If it is then it's possible that U2 is at fault. You can test this by putting it into the socket with pins 2 and 4 raised and then see what voltages they get to. Give there are no pull-ups they will probably be a floating voltage for '1' but pulled down to ground for '0'.

I suggest U2 first as that's a smallish chip and easier to desolder.

[Tophe38]

Hi Stephen,

Thanks for your suggestion.

Also, searching similar issue over the net, I found that both pins (Reset and Halt) are bidirectional.
So, it appears the CPU is not happy, and raise them, preventing the machine to start.

Digging the service manual, it gives some tracks to follow.

I'll keep posting when I have time to investigate.

Chris

[stephen_usher]

Yes, they are bidirectional, but the CPU will halt after a little while, not immediately and it doesn't assert /RESET if it's just halted.

The 2.7V on the /RESET line is just wrong. The pull-up will bring it up to 5V, the chips merely ground this when they assert it so it should never be 2.7V.

[Tophe38]

Hi,

So, I have a good news, the MegaST1 is back to life !!

Following Stephen's hint, I de-solder, U2, replace it and test it, but no change. :(

I keep probing with my oscilloscope around the CPU.
I notice D0 stays high, whereas D1 and next go down while holding reset button, and go up on release.

I decide to replace the 68000 (and remove the blitter patch for testing too).
And it works !!
The 68000 was defective.

Thanks for your help.
Chris