1040 STE with bad clock?

[leonk]

I have an Atari 1040 STE that only shows me a white screen. According to the schematics, pin 3 of the GSTMCU generates the 8Mhz clock signal (CLK8) for the CPU (pin 15). I don't believe there's anything else on this wire.

When I check this frequency on my oscilloscope, it looks like jumping noise/garbage. To make sure there's nothin wrong with my oscilloscope, I checked pin 4 (CLK4) and I get a solid 4Mhz. Does that mean my CPU is potentially dead or bad? Anything else might be causing this behaviour?

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IMG_7497.JPG (362.58 KiB) Viewed 950 times

[leonk]

I lifted pin 3 of the GSTMCU - it shows the same junk on that lifted pin. The other output clocks, and even the input clock on pin 2 are all good. I think it's a dead GSTMCU.

[rubber_jonnie]

What clock do you see at the CPU?

Are any of your ICs socketed as I'd suggest a clean/tweak of the socket pins and IC pins if they are.

[leonk]

The clock at the CPU is identical to that at the MCU (there's a direct wire connecting them) Both the MCU and the CPU are soldered to the board / not in socket.

I also verified the clock to the MCU (pin 2) is good.

[rubber_jonnie]

The clock at the CPU is identical to that at the MCU (there's a direct wire connecting them) Both the MCU and the CPU are soldered to the board / not in socket.

I also verified the clock to the MCU (pin 2) is good.

Hmm, I think your diagnosis may be correct...

[leonk]

UPDATE: NOS MCU received from eBay and installed into the 1040STE. It fired right up and works great. Check your clocks using an oscilloscope if you got one.