Atari ST and HBS 640 (28Mhz) accelerator problem

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Retro freak
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Atari ST and HBS 640 (28Mhz) accelerator problem

Postby Sonix » Thu Apr 08, 2004 10:11 pm


I've got a HBS 640 accelerator from a friend - he was unsure if it works or not. I installed it in my ST 260 - 4MB. Running with 8 MHz is no problem (so at least the CPU of the accelerator board works). But if I switch the clock to 28Mhz the system won't boot - either the screen stays white or you get a weird flickering. I already installed the RESET pullup resistor as mentioned in the manual and checked the voltage - 5.06 V (which looks ok to me).

So - is anybody else using this accelerator? If so did you have similar problems - and is there a solution for it?

Another question: on the lower side of the PCB are two contacts with VCC and GND - I think this could be an additional power supply. Since the correct power supply seems important it could be a try to supply power here too. But this is not mentioned in the manual - anybody knows something about this.

Any help appreciated...


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Atari Super Hero
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Postby Ragstaff » Fri Apr 23, 2004 8:56 am

Are you able to run it at a lower speed that 28mhz? I mean, the thing probably just has a 28mhz oscillator, but if you can get a 16mhz source form somewhere.... it may be the chip just doesn't like 28mhz?

Retro freak
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Postby Sonix » Fri May 07, 2004 10:36 pm

it may be the chip just doesn't like 28mhz?

That's what I expected from the very beginning. The accelerator board contains a 68k 16Mhz. So perhaps ~10 years ago it could be that it would be able to run at 28Mhz. It's was a bit tricky to remove the 28Mhz oscillator (I didn't want to ruin the board nor the oscillator). It seems to be a 4 layer board and via's are connecting the layers at least at two pins of the oscillator. But I managed to remove it and took the 16MHz output of the shifter to provide the clock.

With 16Mhz it works!




Postby simbo » Fri May 14, 2004 12:45 am

the 28mhz oscillator sounds like one of the can type's

add the connections vcc and gnd as what will be happening is the boards 12v is being supplied instead from these connections by the pull up resistors and other interconnections

this will alter the signal path integrity and bias
and would stop the oscillator cold
as this package is a ttl oscillator and has also an enable pin {three connections} gound enable and output
put it back and make the other connections to the psu

recheck your work
and retry the test

and it will run the 28mhz clock and boot

so add them
vcc = +12v psu blue wire
gnd = ground 0v psu black wire
vdd = +5v psu red wire

as a general note and reminder

always purchase a volt meter {simple lcd one} from your local electronics store
as its handy to confirm BEFORE connection

any info you get
however accurate it seems {im not always 100%}

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