exxos's Atari, Amiga & retro forum

I have been rebuilding one of the near empty 520ST boards that I received from Frank about a year ago. A board that was used for parts probably had assorted problems with it to begin with but it seems a shame to have multiple derelict boards when perhaps one of them could be used again.
It isn't complete but I got far enough to give it a try at booting. It white screens. There is no clock to pin 2 of the Shifter. The clock crystal is stuck high, whereas the Y3 crystal to the right of it pulses nicely on my logic probe. Is it likely that it is simply a defective clock crystal or can it be something else? I have never encountered this problem on any other ST.

Check the 74HC74 inputs and outputs.

Inductor near the top looks burnt..

I have to agree with @stephen_usher

U41 is a potential problem point, so an easy one to look at as its socketed.

As per the schematic:


There are a few transistors in the circuit too and one or more could be dead, but it would be worthwhile checking all the connections point to point as well to be sure all the connections are as they should be.

Also, do you see a signal with the Shifter removed? If it's bad it could be holding pin 2 high.

All STM mainboards are for 6 chip TOS only isn't it ...

frank.lukas wrote: 04 Mar 2024 10:26 All STM mainboards are for 6 chip TOS only isn't it ...

I think you're right. IIRC on mine, there were pads to select mask ROMS or EPROMS, but nothing to select larger ROMS. I fitted a 6 chip set in mine.

But would that affect the oscillator circuit? Surely that just requires power to provide the clock signals and nothing from the OS?

rubber_jonnie wrote: 04 Mar 2024 10:40 But would that affect the oscillator circuit? Surely that just requires power to provide the clock signals and nothing from the OS?

No but boot up with a white screen this is because no 6 chip TOS is installed. I think because of the white screen the clock on the shifter is ok ...

frank.lukas wrote: 04 Mar 2024 11:08

rubber_jonnie wrote: 04 Mar 2024 10:40 But would that affect the oscillator circuit? Surely that just requires power to provide the clock signals and nothing from the OS?

No but boot up with a white screen this is because no 6 chip TOS is installed. I think because of the white screen the clock on the shifter is ok ...

I definitely agree with that statement, but the OP did say there was no 32Mhz clock at the Shifter either, which IIRC should be present regardless.


though doing some more digging, there is this on the site: ST 2 Chip TOS

It looks to be the same REV as the OP, so there may well be jumpers/pads on this variant to allow 2 ROM sets.

rubber_jonnie wrote: 04 Mar 2024 11:18 It looks to be the same REV as the OP, so there may well be jumpers/pads on this variant to allow 2 ROM sets.

Yes, there is a 74LS11 for the 2 chip Eprom config ...

frank.lukas wrote: 04 Mar 2024 10:26 All STM mainboards are for 6 chip TOS only isn't it ...

Nope. My 520STM came with two ROMs. That was purchased in mid-88.