Almost dead STE

[kodak80]

I have an STE board which I purchased as dead and I am using this to learn how to fix things. It is showing a white screen only when powered on.

I have identified that the CPU is in HALT state (pin 19 is showing 0v) and chased this back to the reset circuit. The P100 1.2k pull up SIL resistor is not showing 5v on pin 2 which is connected to pin 4 on the 7406N chip. Pin 3 on the 7406N seems to be working correctly but pin 4 is not changing to the opposite of pin 3 when I push the reset button. Given P100 SIL has 5v on all pins apart from pin 2, am I correct in saying that the P100 SIL is faulty and needs replacing? If so, do I have to replace it with another 1.2k SIL? I only have 1k and 2.2k replacements at the moment.

When I hold the reset button, the 7406N pins are as follows:
Pin 3 - 4.3v
Pin 4 - 1v

When I let go of the reset button:
Pin 3 - 0v
Pin 4 - 0v

I have checked the CLK on the CPU and can see that this is working using my scope.

[PhilC]

Hi Kodak,

It could be that the reset is ok but the glue is pulling Halt low so that's worth checking out.

Try removing the CPU and then test the reset citcuit. If it works then then my guess is check the glue, mmu, shifter and DMA for correct operation.

But that will obviously depend which chipset is in the STe, ie combined glue, glitter etc.

Good luck.

[exxos]

You would need to check on a scope with deep memory and record the reset and halt pins from power up to verify they are switching as expected.

Though as PhilC says, after power up, reset will change state, but if the "GLUE" is getting a bus error from something, then it will halt the CPU about 100 cycles or so later. I don't remember if the GLUE will keep the system halted even after a reset.

[kodak80]

Thanks PhilC

I have removed the CPU and reset circuit is working fine with 5v on 7406N pin 4 so as you say, something else is pulling it low to cause HALT.

My STE board has a separate glitter chip and socketed GST shifter with soldered GST MCU chip (model CA4003290). Will keep looking now that the reset circuit is ruled out as the cause.

[kodak80]

Have put the CPU chip back and tested with the GST Shifter chip removed and the CPU does not HALT.

Does this then mean that the GST Shifter is causing the CPU HALT or do I still need to look at the GST MCU chip?

I have previously replaced the GST shifter chip with a known working chip so I know it is not the chip itself which is faulty.