Live ROM board exit and dead QL result

[KINGCHEF]

Hi,
I made a mistake.
The ROM port on my QL was having trouble seeing the TK2 rom I had plugged in. I tried another board and it was O.K.
I then started swapping various boards and promptly forgot to turn it off once.
That's it dead. No screen image at all, just black.
It says in the manual not to remove the ROM when the QL is live but not what to expect if you do.


What have I damaged?

Any help much appreciated.

Dave

[Chr$]

Black screen is usually a sign that the ZX8301 has died (- in my experience and usually in relation to pulling the RGB cable out when it's on). That's the first thing I'd attempt to swap. If that doesn't work, no doubt the real experts will have suggestions!

[mk79]

Yes, definitely ZX8301, it's not just vulnerable regarding the RGB lines, it's all lines, including the data and address lines that are on the ROM port. Killed one or two while developing QL-SD. If you're quick enough removing the power it can survive (<10s or so) before it suffers a fatal heat death.

[tofro]

Yes, definitely ZX8301, it's not just vulnerable regarding the RGB lines, it's all lines, including the data and address lines that are on the ROM port. Killed one or two while developing QL-SD. If you're quick enough removing the power it can survive (<10s or so) before it suffers a fatal heat death.

Yep, in my experience, it's also the first chip to die when something on the expansion port goes wrong - like a bent pin, for example (I even had it go glowingly hot - but survive with the checks below - when I had a Gold Card with a bad solder joint).

I've made it into a habit to put a finger on the 8301 and check for excessive heat when switching on for the first time after I have changed anything in the QL setup and the case open.

[KINGCHEF]

Black screen is usually a sign that the ZX8301 has died (- in my experience and usually in relation to pulling the RGB cable out when it's on). That's the first thing I'd attempt to swap. If that doesn't work, no doubt the real experts will have suggestions!

While at it, you might want to solder some protection diodes or simply 4.7V zeners on ULA signals ( RGB, H/V/SYNC etc) on both ULAs. ULA2 has couple of signals that go to microdrive port etc.

Hi,

Is there a circuit diagram anywhere to help me?I'm pretty good at soldering but need a design to work to.
Thanks

[Chr$]

I personally only bother adding diodes to the ZX8301 when it's a German QL (or US one, but not seen one of those yet) as on those Samsung models it was a factory fitted mod. Others, inc all the UK ones, never had them. And I've destroyed ZX8301's with or without diodes, so I don't think it's a magic cure to make them indestructible.

Here's a pic of a German chipset showing the 1N4148 diode arrangement on the ZX8301.

[Peter]

While at it, you might want to solder some protection diodes or simply 4.7V zeners on ULA signals ( RGB, H/V/SYNC etc) on both ULAs.

Those zener diodes would be wasted work. Too slow and not limiting voltage sharply enough.

[1984]

Is there a circuit diagram anywhere to help me?I'm pretty good at soldering but need a design to work to.

If no one has answered your question and if you have not found a source already: There are all kinds of circuit diagrams on Dilwyn Jones' website in the QL hardware documentation section [1]. Best to start with are the circuit diagrams of QL board issue 5 [2] and issue 6 [3] (see notes in [4]).

You probably do not have to use the circuit diagrams and your solder iron, the (most presumably defective) ZX8301 is socketed, and it is worth giving a try to replace it with a new one.

Good luck! – George


[1] http://www.dilwyn.me.uk/docs/hardware/
[2] http://www.dilwyn.me.uk/docs/hardware/QLissue5.pdf
[3] http://www.dilwyn.me.uk/docs/hardware/QLissue6.pdf
[4] http://www.dilwyn.me.uk/docs/hardware/QLschem.txt

[Derek_Stewart]

Hi,

I have never had a QL with this type of modification fitted.

The mod. was the rexistors on the 8302...

[KINGCHEF]

Hi,

Update.
Swapped the 8301 for another known working and the display is no longer black. It runs through the multi coloured memory check and then goes blank white.
It doesn't do this every time. About half the time the QL starts normally. If I turn it off it might restart O.K. or not. I did think the 7805 regulator was loose but it doesn't seem to be and I've removed it and tinned the legs just in case. I did think the power socket was dodgy but I've turned the QL off and then on without touching the computer and it's still 50/50.

5 mins later
Just went to check and now cannot get the fault to re-appear so I guess I should be happy. At least I didn't need to spend £20 replacing the 8301. I didn't break it after all.
Incidentally, I have 3 QLs and not one has 8301 marked on the chip. Also the chips I have match exactly those in my OPDs so I don't expect I'll run out, got a few of those.

Any conclusions about the mystery fault would be appreciated.