No clock signal to the Shifter

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russellnash
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No clock signal to the Shifter

Post by russellnash »

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.
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20240301_152523.png
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stephen_usher
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Re: No clock signal to the Shifter

Post by stephen_usher »

Check the 74HC74 inputs and outputs.
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Re: No clock signal to the Shifter

Post by exxos »

Inductor near the top looks burnt..
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rubber_jonnie
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Re: No clock signal to the Shifter

Post by rubber_jonnie »

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:

Screenshot 2024-03-04 at 10.12.46.png
Screenshot 2024-03-04 at 10.12.46.png (490.37 KiB) Viewed 309 times

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.
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frank.lukas
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Re: No clock signal to the Shifter

Post by frank.lukas »

All STM mainboards are for 6 chip TOS only isn't it ...
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rubber_jonnie
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Re: No clock signal to the Shifter

Post by rubber_jonnie »

frank.lukas wrote: Mon Mar 04, 2024 10:26 am 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?
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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frank.lukas
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Re: No clock signal to the Shifter

Post by frank.lukas »

rubber_jonnie wrote: Mon Mar 04, 2024 10:40 am 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 ...
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rubber_jonnie
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Re: No clock signal to the Shifter

Post by rubber_jonnie »

frank.lukas wrote: Mon Mar 04, 2024 11:08 am
rubber_jonnie wrote: Mon Mar 04, 2024 10:40 am 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.
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800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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frank.lukas
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Re: No clock signal to the Shifter

Post by frank.lukas »

rubber_jonnie wrote: Mon Mar 04, 2024 11:18 am 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 ...
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stephen_usher
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Re: No clock signal to the Shifter

Post by stephen_usher »

frank.lukas wrote: Mon Mar 04, 2024 10:26 am 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.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
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