16k Issue 2 board with short.

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Erm
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16k Issue 2 board with short.

Post by Erm »

Hello. I've just been trying to fix up an issue 2 16k (it's never had 48k by the looks of it).

Initially there were RAM probs, so I've removed the RAM and socketed them. TR4 was shorting , so changed that. It was still not right, so I tried a known good ULA - same. I was getting a jumbled copyright message but am now getting thick black horizontal stripes with white border. I removed the Z80, tried another and the black blocks are still there but with some Z80's the border is also black, that's as far as it gets. Voltages are all correct on the RAM chips.

I had a little breakthrough and discovered that the emitter and collector of TR3 were shorted, so removed that. The TR3 seems fine - the pads are shorted on the board. I have no idea how. Looks like that in turn means pin 6 (CLK) on the Z80 is grounded. Just wondered if anyone had an idea on where/what to check?
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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1024MAK
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Re: 16k Issue 2 board with short.

Post by 1024MAK »

Without a valid clock signal on pin 6, a Z80 will just site there doing absolutely nothing useful, and worse, it’s control outputs will not be in a valid state.

For it to have been working, and now not working means something has been disturbed.

The circuitry between the ULA CPU clock output pin (IC1 pin 32) and TR3 base has been subject to revision, but as far as I know the TR3 collector / R25 circuit to IC2 pin 6 has not changed. Having said that, at least one person has had problems with a short circuit in this area before.

I’ll go and have a look at two of my issue two boards, be back soon.

Mark

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Erm
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Re: 16k Issue 2 board with short.

Post by Erm »

Mine has the revision consisting of a capacitor where D14 once was and a lower value resistor for R24. There is also a resistor across the right sides of R24 and R25. I have added sockets to the 2x 74LS157's (there are just those logic chips, as it's a 16k so nothing else there) and I've also socketed the Z80. I suppose I'm going to have to remove those sockets and see if something that runs along there is shorted. I expect it's my fault! It did nearly work originally.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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Erm
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Re: 16k Issue 2 board with short.

Post by Erm »

Think I've found the problem! A blob of solder under the socket that was just big enough to short the connection between pin 8 of IC3 (one of the 74LS157's) and the trace that runs right under it which goes to the clock pin 6 on the Z80. Just putting it all back together now....
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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Erm
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Re: 16k Issue 2 board with short.

Post by Erm »

Yup, unsurprisingly, that is exactly what the issue was! All perfectly fine now.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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1024MAK
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Re: 16k Issue 2 board with short.

Post by 1024MAK »

Well done on unshorting it! :lol: :D

Mark

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