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STFM with no CPU clock

Gary C

Gary C

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May 26, 2018
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Location
Lancashire, UK
Got an STFM with the C070789 Rev C motherboard showing no signs of life

The CPU doesn't have a clock. Looking at the circuit diagram, I believe its 4mhz is derived ultimately from the 32mhz through the shifter which produces 16mhz and while the IC has 32mhz in, nothing comes out.

Now that might have been due to the fact the IC was the wrong way around :) but put back and its dead as a dodo.

So, my question, with 32mhz on pin 2 of the shifter, I should get 16mhz on pin 39 and the 32mhz is from the video oscillator, correct ?
 
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My confusion was, I could see the crystal on the board, but it didn't seem to provide any clock to the CPU, didn't notice the two inside the RF shield that actually provide the clock.

Doh !

Wonder why Atari went to the trouble of putting the tracks and pads for a Blitter, then didn't fit it until the Mega ? Was it too expensive or could they not get it to work.

I think this machine had an upgraded shifter at some point and then thrown back together for clearing out. The RF shield is all bent at one corner as if to let a cable out and I'm not sure the single sided drive is original for this motherboard revision
 
Gary C said:
My confusion was, I could see the crystal on the board, but it didn't seem to provide any clock to the CPU, didn't notice the two inside the RF shield that actually provide the clock.
Click to expand...
Sorry should have chimed in sooner, but memory fades...
Gary C said:
Doh !

Wonder why Atari went to the trouble of putting the tracks and pads for a Blitter, then didn't fit it until the Mega ? Was it too expensive or could they not get it to work.
Click to expand...
I think a bit of both. From what I remember very early Mega's didn't have the blitter either, as it wasn't ready. I used to own a Mega and I am sure it had had its Mega added or perhaps just patched.

Gary C said:
I think this machine had an upgraded shifter at some point and then thrown back together for clearing out. The RF shield is all bent at one corner as if to let a cable out and I'm not sure the single sided drive is original for this motherboard revision
Click to expand...
I think the single sided was common on many STs. My first was an ST-E which was a nice machine with a blitter and double sided drive but wouldn't run some games. I got it for the MIDI interface, and the fact it had a free "C" compiler....
 
The only reason I wonder about the single side drive is that the interface from the standard internal Shugart 34 pin connector was just thrown on top of the shielding with nothing to stop it shorting out but after reading i see the single sided drive has the big eject button which matches the case. Best bet I imagine would be to replace with a Gotek. Would it need new ROMS to operate as a double sided drive ?

Probably won't work even with the shifter replaced. While the general condition is good, its obviously been in bits and then thrown back together (it only had two screws and they were holding the PSU down !) so I can quite imagine there are other faults, but one step at a time.

but hey ho, the fun is in the fix :)
 
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Yay, shifter replaced and now I have a 16mhz clock into the MMU :) but no 8 or 4mhz clocks coming out of it so the CPU still isn't running.

Suspecting this machine is a vehicle for all someones failed components.

So MMU next.
 
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