Issue 6 VM12 & resistors confusion

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Pr0f
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Re: Issue 6 VM12 & resistors confusion

Postby Pr0f » Fri Jun 26, 2020 11:53 am

Those old intel Microconrtoller chips are quite robust - a touch screen company i worked for years ago used them on one or two of their controllers (well the ROMLESS version), and they are very tolerant of abuse. Simple resistor diode protection on the inputs was all we used. The QL has resistors on the row drivers as pull ups, and the column reads as pull downs, plus diodes on most lines to prevent unwanted key detection - it's very basic, but unless you are poking about inside with a substantial curreny source with a much higher potential than the QL supply, it should survive static induced inputs.


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Ruptor
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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Fri Jun 26, 2020 2:40 pm

My programmer doesn't support the 8049 or 8749 so I can't burn a replacement. :( I have 8051 but the pins are all different. :roll: How many more faults is my QL going to have? Gone off in a huff.


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Re: Issue 6 VM12 & resistors confusion

Postby Pr0f » Fri Jun 26, 2020 3:21 pm

I have a spare 8049 as I recently put a Hermes chip in one of the QL's i have, so you are welcome to that if you want it. Just PM me. But I suspect the issue is more likely the keyboard matrix...


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Ruptor
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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Fri Jun 26, 2020 3:46 pm

You are very kind but as you say I need to know what is wrong for sure before sticking good parts in and blowing them up. I shall have a rest and look at the circuit to contemplate what to do.


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Re: Issue 6 VM12 & resistors confusion

Postby martyn_hill » Fri Jun 26, 2020 5:00 pm

Hi Ruptor

I don't recall if you ever checked/replaced the ZX8302 - bear in mind that any comms with the IPC (and this the keyboard) must flow through the 8302 first.

One clue, when using Minerva, is whether you get a consistent 'time/date' shown after repeated pressing of Reset - it'll be a nonsense value at first, but should just increment as expected after each press of Reset.

If it varies wildly between each press of reset, then comms with the 8302 is suspect - so anything to do with the IPC will be equally screwed.


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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Fri Jun 26, 2020 7:54 pm

Thanks for the suggestion the time moves on sensibly between resets. I tested the keyboard pickup by the 8049 and it looks like Prof is right with it being the matrix or the membrane connections. Shorting different pairs of pins I could get characters out consistently the same for each pair of pins shorted so the membrane or connections are at fault. Does anyone have an original membrane still working? If the connector ends can tarnish can the actual pads of the membrane do it as well? I wasn't expecting this mechanical problem. I will try cleaning the membrane ends as suggested but if that doesn't work it is toast.


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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Sat Jun 27, 2020 12:12 am

After cleaning the ribbon ends the keyboard still does absolutely nothing and I officially class it as toast. Minerva automatically loads an MDV if you wait after reset so I know the first drive works. I tried a few cartridges and it looks like 30% are corrupted from print through as I was expecting since the tape hasn't moved in over 30 years. At least I know the whole computer works. :) If nothing comes out of the keyboard it can only be that the pads don't make contact otherwise some keys would give something. I wonder if the two membranes can be separated to clean the contacts?


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Re: Issue 6 VM12 & resistors confusion

Postby tofro » Sat Jun 27, 2020 7:51 am

Ruptor wrote:After cleaning the ribbon ends the keyboard still does absolutely nothing and I officially class it as toast. Minerva automatically loads an MDV if you wait after reset so I know the first drive works. I tried a few cartridges and it looks like 30% are corrupted from print through as I was expecting since the tape hasn't moved in over 30 years. At least I know the whole computer works. :) If nothing comes out of the keyboard it can only be that the pads don't make contact otherwise some keys would give something. I wonder if the two membranes can be separated to clean the contacts?


After some decades, the membranes develop a massive amount of micro cracks along the bent part you would only see under a microscope, and, apart from cutting a few mm of the ends that might be broken, there's no lasting way to repair an old brittle keyboard menbrane. I'm afraid you're beating a dead horse - better buy a new one.

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Re: Issue 6 VM12 & resistors confusion

Postby Pr0f » Sat Jun 27, 2020 8:48 am

I'd second what Tobias says, a new keyboard. Repairs? - even a small amount of silver conductive paint doesn't hold up for long.

I just replaced one keyboard where it had failed on the bend where the keyboard tail tucks behind the metal backing plate - this is such a tight turn - the plastic had gone brittle on one edge and a crack was working along the wider ribbon - it basically means you close contact with a bunch of columns - if those happen to be your Fn keys and you don't have Minerva ROM, then the machine will hang on that boot screen waiting for a key press.

Microdrive working and the clock behaviour suggest the ZX8302 is working fine.

It's worth just checking the sprung contact strips are not pushed too far back in the connector - it's a cheap fix after all.


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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Sat Jun 27, 2020 10:52 am

Pr0f wrote:It's worth just checking the sprung contact strips are not pushed too far back in the connector - it's a cheap fix after all.
The pins make lines on the membrane so I didn't see this as the main problem since not one character or any key press does anything. The plastic is brittle like you guys say because it broke on pushing it in to the connector so I don't think there is much hope for it. :roll: What bugs me is why doesn't one key do something they are in effect isolated circuits so to have them all not working is odd unless every single keypad was not making contact due to corrosion.
Thinking about ways around it and getting a descent keyboard I could alter Minerva to look at a serial port for keyboard input and translate a serial keyboard to the QL key codes if they are not ASCII. Not sure about the extra keys it is just in the melting pot so don't jump on me and tell me it can't be done although you can list any major problems I need to know. :)
Another problem is my Minerva EPROM mod is too tall so I can't screw the keyboard top back on the base but without a membrane it will fit otherwise I have to remove a ROM socket to get the EPROM closer to the board. :roll:



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