Issue 6 VM12 & resistors confusion

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Ruptor
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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Tue Jun 09, 2020 4:36 pm

Hi Prof
Pr0f wrote:I looked at doing that - you could put SRAM in one of the ROM sockets and wire the missing address lines to give you static ram in the address map - but you need to make RAM available for the ZX8301 which is producing the video display - and that's engineering to work with Dynamic RAM - fast page mode capable Dynamic RAM. You could use 4 bit wide DRAMS of larger capacity - but that would involve a lot of rework!
I think we are talking at cross purposes. See this ZX 16K replacement that is SRAM but use 128K to replace the whole bank of 4164s where they are shared with the ULA. The refresh signals would be irrelevant since static memory doesn't need it. It was just off the top of my head not having considered the board layout or the signals but seeing it has been done on the ZX it could be a more reliable option.
http://zx.zigg.net/LRR/

Pr0f wrote:If you get the DRAMS out cleanily - there's a good chance most of them would be ok - I have a few spares if you only need a couple.
That's nice of you I shall bare the offer in mind. The pins on my DRAM chips have been bent over before soldering probably to hold them in the board but that has made it more difficult to get the solder out. I think I will have to straighten them up first or if that doesn't work go back to chopping the pins off on the chip destroying them. :roll:


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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Wed Jun 10, 2020 4:08 pm

The desoldering machine can't free up the RAM pins completely due to a lot of solder on the top side so I had to cut them at the chip. It made clearing the holes very easy so a lot of time saved there with no need to push wires in and out just press the button and suck. :) The memory test didn't make much sense with a chip missing perhaps due to a floating line. :?
My TI RAM chips are -12 that is 120 nS I believe but what do they need to be? Could I fit -15 or 150nS?


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Re: Issue 6 VM12 & resistors confusion

Postby Derek_Stewart » Wed Jun 10, 2020 4:40 pm

Hi,

You might have to resolder the joints with fresh solder then suck it out.

This is a standard way to resolder old chips.

But if you do not need the chips cut the chip legs at the IC body and try heating each leg portion left to extract the leg.


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Re: Issue 6 VM12 & resistors confusion

Postby Pr0f » Thu Jun 11, 2020 7:54 am

I've seen boards with 150ns and 120ns chips in - so 120ns should be fine. I've more often than not needed to put a dab of new solder on to 'free up' the existing stuff. Also - it pays to carefully bend up the leg with a pair of fine tweezers or a scalpal blade, once the solder is flowing - normally this is only needed on the diagonal opposite pins where they are bent to hold the chip in place - unfortuantly these are usually the power pins and on the largest copper areas, so some heat needs to get into that area around the pin to allow it to be worked easily - I find spending time doing this helps with the removal.

Not every chip will come out - and for those, Derek's method is always a winner - just looks brutal.


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Re: Issue 6 VM12 & resistors confusion

Postby Martin_Head » Thu Jun 11, 2020 10:12 am

You can also use solder flux like https://uk.rs-online.com/web/p/solder-flux/1462856/ instead of fresh solder, to get the old joints moving.


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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Sat Jun 13, 2020 10:52 am

Hi Guys
Don't worry, as you can see from my pictures of removing the TTLS buffer on page 5 of this thread using a plunger solder sucker I manage OK and the desoldering machine has made clearing the holes very easy. :)
To replace all the dynamic RAM with an SRAM would require a lot of extra logic to handle the row and column buffering you might as well redesign it with dual port ram so not a viable option to get my old box going. :roll: I have ordered some chips after looking at all the different numbers and searching for them. I found this site of useful information about RAM that you are all probably aware of but I wasn't since I am new to old tech. https://www.c64-wiki.com/wiki/RAM
The site points out that 256x1 RAM can be used to replace our 64x1 so it is worth remembering for when 4164s run out and prices rise to extortionate levels.
While I wait for the chips I shall play with my RPi and see what it can do for my LBB (Little black box) :lol: It seems the dynamic ram, decoding and video all controlled by the dreaded ZX8301 is the part of the LLB that needs replacing. What can we do? :?


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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Wed Jun 24, 2020 6:49 pm

Cue the thunderbirds music! Dum delly dum. After fitting a home made EPROM, fixing the random clock and replacing one dead 4164 RAM chip I have a standard colour reset screen followed by some text and Minerva in orange then a magenta cursor on a blue screen. I don't remember that screen because it originally had JM ROMs so I am going to fit my EPROM switch and Load up 4 ROM images to give a choice and hope one jogs my memory. I also don't remember the modulated TV picture being that bad but I suppose the 14" tube TV is thirty years old like the computer. We have been spoiled by the VGA monitors & HD TVs I think. In the back of my QL manual I found a returns letter where I had sent the QL back for repair under the guarantee that I have no recollection about. I wonder how long it will continue to work. Do we have any figures on the average failure rate?


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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Thu Jun 25, 2020 10:47 pm

Maybe it doesn't work. Having put it back together I get this screen and in the white box it says press F1 for TV or F2 for monitor but it doesn't do anything with either key so does it mean my keys are not working?
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Re: Issue 6 VM12 & resistors confusion

Postby Pr0f » Fri Jun 26, 2020 9:15 am

a fracture in the keyboard tails is one of the most common 'failures - particularly if you've had the lid off a few times, - it's also possible that the sprung contcts have moved too far back in those long connectors the keyboard plugs into - if you have a fine pick or some dentists tools, or a fine tweezer, you may be able to gently push them forward to close the gap so that the sprung contacts make a good contact.

I've noticed the contacts on the keyboard tails also tarnish badly over time, but there is so little material there, any attempt to clean them may remove more and make the problem worse - I borrowed a friends guitar jack plug cleaner fluid - it's about £15 for a tiny bottle - but you need so little of it - a gentle daub on a cotton bud and then wipe the contact.

BE CAREFUL to test what ever you clean the contacts with against the overhang part of the ribbon tail that has no contacts on - to make sure it is not attacked by your cleaner!

Alsternatively a new membrane for about £15-20 - available on sell my retro...


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Re: Issue 6 VM12 & resistors confusion

Postby Ruptor » Fri Jun 26, 2020 11:43 am

On the Minerva ROM it automatically switches to a single screen where I could test other keys but none work so it is unlikely to be keyboard connection problems. I went around all the keys and occasionally crap must have been received on the keyboard receive pins because once "not complete" came up and another time a whole string of rubbish was displayed from one key press. Pressing the same keys again did nothing. Thinking about all the faults found so far it seems like the QL has been zapped all over like a lightening bolt and the weakest chips have been toasted. The LSTTL buffer for instance wasn't damaged and yet on one side of it the RAM chip went and on the other side a ROM is toast so maybe the keyboard reading pins on the 8049 have been toasted. OH! maybe I can test the 8049 on my Universal programmer. I will report the result later.



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