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Dot crawl and keyboard membrane.

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  Dot crawl and keyboard membrane. philicorda <philicorda@sorrynospam.com> - 2014-01-24 21:26 +0000
    Re: Dot crawl and keyboard membrane. "Brian Gaff" <briang1@blueyonder.co.uk> - 2014-01-25 05:07 +0000
      Re: Dot crawl and keyboard membrane. philicorda <philicorda@sorrynospam.com> - 2014-01-26 22:36 +0000
      Re: Dot crawl and keyboard membrane. Robert Newson <nothanks@bullet3.fsnet.oc.ku> - 2014-02-09 16:39 +0000
        Re: Dot crawl and keyboard membrane. greenaum@gmail.com (greenaum) - 2014-05-21 18:59 +0000
          Re: Dot crawl and keyboard membrane. "Brian Gaff" <brian1gaff@gmail.com> - 2014-05-25 23:58 +0100
            Re: Dot crawl and keyboard membrane. Hanno Foest <hurga-news2@tigress.com> - 2014-06-01 20:35 +0200

#2749 — Dot crawl and keyboard membrane.

Hello.
I have recently got my ZX spectrum 48 working with a new keyboard 
membrane. Unfortunately, it has dot crawl like a protest march of 
infuriated gnats. Tweaking the tweakable capacitor doesn't make much 
difference. It seems to be better sometimes when the computer isn't 
thinking too hard, if that makes any sense.

What I'm wondering is if I have routed one of the ribbon cables from the 
keyboard the wrong way, and it's interfering with the video circuitry. I 
have some vague memory that the left side one was bent under the 
keyboard, rather than towards the back of the computer, before I replaced 
it.

Anyone know what the correct way is? I don't really want to take it apart 
to experiment without knowing as it was a bit tricky to put together, and 
the cables don't look like they should be flexed much.

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#2750

Personally, I doubt if that is the reason. Are you saying it did not do this 
before? Sounds like an issue 2 as they did seem to be worse. However, Most 
of this is interference  getting into the video and can be reduced with new 
decoupling capacitors  in various places, but its been a long time since i 
could see enough to recall where they are. The effect occurs after the 
machine has not been used for a few years, strangely machines in constant 
use do not have the issue too much. Another thing to try, is that some tvs 
tended to let the crud into themselves via the aerial or video  lead shield. 
One of those wrap the wire around ferrites can work wonders if this is the 
case.

Brian

-- 
Brian Gaff....Note, this account does not accept Bcc: email.
 graphics are great, but the blind can't hear them
Email: briang1@blueyonder.co.uk
______________________________________________________________________________________________________________


"philicorda" <philicorda@sorrynospam.com> wrote in message 
news:gUAEu.7$_25.4@fx18.am4...
> Hello.
> I have recently got my ZX spectrum 48 working with a new keyboard
> membrane. Unfortunately, it has dot crawl like a protest march of
> infuriated gnats. Tweaking the tweakable capacitor doesn't make much
> difference. It seems to be better sometimes when the computer isn't
> thinking too hard, if that makes any sense.
>
> What I'm wondering is if I have routed one of the ribbon cables from the
> keyboard the wrong way, and it's interfering with the video circuitry. I
> have some vague memory that the left side one was bent under the
> keyboard, rather than towards the back of the computer, before I replaced
> it.
>
> Anyone know what the correct way is? I don't really want to take it apart
> to experiment without knowing as it was a bit tricky to put together, and
> the cables don't look like they should be flexed much. 

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#2754

I'm not sure if the dot crawl was so bad before I replaced the keyboard 
membrane. I'd only really fired it up long enough to find out the keys 
were not working. 

It is probably an issue 2, as it has the shiny circuit board traces 
without solder resist. I also upgraded the ram to 48K by putting chips in 
the sockets. I changed the video out to composite too, so it's probably 
not interference from the monitor. The monitor is a Sony PVM, and they 
are normally quite well behaved.

Replacing some decoupling capacitors sounds like it might help. It does 
look like the interference is quite related to what the computer is 
doing, rather than being the slow steady crawl that I remember from my 
Spectrum when I was young. 

I bought this one second hand, so it might not have been used for years. 
The other spectrum I posted here about is still not working. (I got the 
power supply and inverter going, but I think ULA or lower 16K might be 
broken.) Oh well, I've enjoyed fixing it that far anyway. 

Thanks for the ideas. 

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#2765

On 25/01/14 05:07, Brian Gaff wrote:
> Personally, I doubt if that is the reason. Are you saying it did not do this 
> before? Sounds like an issue 2 as they did seem to be worse. However, Most 
> of this is interference  getting into the video and can be reduced with new 
> decoupling capacitors  in various places, but its been a long time since i 
> could see enough to recall where they are. The effect occurs after the 
> machine has not been used for a few years, strangely machines in constant 
> use do not have the issue too much.
This may not be as surprising as you think...I was told once (by my
brother, an electrical engineer) that the "doping"* inside chips and
things tends to weaken if not used, ie [old] electronic devices need to
be fired up (turned on) regularly, even if not in use, to keep the chips
and things healthy.  So the dot crawl being worse after a machine has
not be in use for a while makes perfect sense to me.

I have a Star Chess game machine I haven't switched on for a while.
Last time I switched it on it was like one of those old Valve TVs - it
took quite a while after being powered up for it to "warm up" and work
properly: the effect of being powered up "re-set" the "doping" on the
chips (and possibly other components); I have a suspicion it may not
work now ;'(

[*I use the word "doping" as I can't remember exactly what he said it
was, but in the back of my mind I seem to remember the word "doping"
having some reference to transistors and [old] ICs mostly were
transistors of some kind.]

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#2887

On Sun, 09 Feb 2014 16:39:04 +0000, Robert Newson
<nothanks@bullet3.fsnet.oc.ku> sprachen:

>[*I use the word "doping" as I can't remember exactly what he said it
>was, but in the back of my mind I seem to remember the word "doping"
>having some reference to transistors and [old] ICs mostly were
>transistors of some kind.]

ICs still are transistors, billions of 'em in complicated chips.
Doping is adding chemical elements to silicon in order to give it a
tendency towards a positive or negative electrical charge. It's how
transistors work, and how they're made, including the very tiny wee
ones inside microchips.

Eventually some of the doping ions can migrate. Spectrums use old
chips of course, where the transistors etc were much bigger physically
than they are now. This means, with more material in each transistor,
they're more robust than smaller ones (tho that doesn't account for
other improvements made in chip production through the decades). I
dunno if your brother's right about it being bad enough to actually
stop chips from working, but it's at least plausible IMO.

OTOH the thing that's most likely to go, and arcade game collectors
are experienced in this, is capacitors. Particularly electrolytic
ones, but also the small ordinary ones. This would explain the screen
noise, too. You can just hold a capacitor in parallel with another,
with the legs touching. If it's just there for decoupling, this would
solve the problem while you do it. Might help you narrow down which
ones to replace.

-- 

--------------------------------------------------------------------------------
"There's nothing like eating hay when you're faint," the White King remarked to Alice, as he munched away. 
"I should think throwing cold water over you would be better," Alice suggested: "--or some sal-volatile." 
"I didn't say there was nothing better," the King replied. "I said there was nothing like it."
Which Alice did not venture to deny. 

                    

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#2888

Yes, all my VTX 5000s are now junk, as all the tantulum caps have gone 
leaky, indeed when you power them up there is usually a sizzle and a nasty 
smell of smoke.

On Transistor death. Many early devices of the OC17x and af 11x variety 
succumb to the material they are packaged with leaching a nasty conductive 
substance with age that effectively shorts them out. Some larger sililicon 
Diodes, often coloured a kind of blue/grey made by Mullard  have a problem 
with the body shrinking away from the wires, and heat cycling then turns 
them intermittent where the wire joins the device.

In Specttrums, the older membranes in  Sinclair rubber keyed, plusses and 
the Sinclair 128 all break down and  go brittle. Luckily the newer ones 
around now seem more resiliant, but often getting the remnants of the old 
one out of the connectors can be a challenge.
 In 128s, the plug that fits onto the regulator device suffers from 
corrosion of the legs of the device and the connector causing a 128 to turn 
itself into a kind of vibration detector. Best fix is the ditch the 
connector and solder the leads on to the regulator.
On dot crawl, often as mentioned this can be decoupling issues, but anyone 
trying to use a coomposite output from the edge connector will get dot crawl 
as the earth is poor, always get it from the input to the modulator.
 Its also worth noting that the modulators drift like mad and  most recent 
analogue tvs will not stay tuned in.

Another thing that can happen is that a signal travels down the earth of the 
cable and into the set, so a ferrite ring is a good idea aand wind the lead 
to the tv or monitor  round it at least three times. Most atari computers 
had a lead made this way for the same reasons.



Always make sure any socketted chips are seated correctly. I've seen Z80 
chips which have had their leads so corroded that removing them from the 
sockets leaves some legs behind. These  cheap chips  were unbranded, 
goodness knows where Sinclair got them from!

 Brian

-- 
From the Sofa of Brian Gaff Reply address is  active
"greenaum" <greenaum@gmail.com> wrote in message 
news:537cf609.7332625@news.eternal-september.org...
> On Sun, 09 Feb 2014 16:39:04 +0000, Robert Newson
> <nothanks@bullet3.fsnet.oc.ku> sprachen:
>
>>[*I use the word "doping" as I can't remember exactly what he said it
>>was, but in the back of my mind I seem to remember the word "doping"
>>having some reference to transistors and [old] ICs mostly were
>>transistors of some kind.]
>
> ICs still are transistors, billions of 'em in complicated chips.
> Doping is adding chemical elements to silicon in order to give it a
> tendency towards a positive or negative electrical charge. It's how
> transistors work, and how they're made, including the very tiny wee
> ones inside microchips.
>
> Eventually some of the doping ions can migrate. Spectrums use old
> chips of course, where the transistors etc were much bigger physically
> than they are now. This means, with more material in each transistor,
> they're more robust than smaller ones (tho that doesn't account for
> other improvements made in chip production through the decades). I
> dunno if your brother's right about it being bad enough to actually
> stop chips from working, but it's at least plausible IMO.
>
> OTOH the thing that's most likely to go, and arcade game collectors
> are experienced in this, is capacitors. Particularly electrolytic
> ones, but also the small ordinary ones. This would explain the screen
> noise, too. You can just hold a capacitor in parallel with another,
> with the legs touching. If it's just there for decoupling, this would
> solve the problem while you do it. Might help you narrow down which
> ones to replace.
>
> -- 
>
> --------------------------------------------------------------------------------
> "There's nothing like eating hay when you're faint," the White King 
> remarked to Alice, as he munched away.
> "I should think throwing cold water over you would be better," Alice 
> suggested: "--or some sal-volatile."
> "I didn't say there was nothing better," the King replied. "I said there 
> was nothing like it."
> Which Alice did not venture to deny.
>
> 

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#2897

Am 26.05.2014 00:58, schrieb Brian Gaff:

> Yes, all my VTX 5000s are now junk, as all the tantulum caps have gone
> leaky, indeed when you power them up there is usually a sizzle and a nasty
> smell of smoke.

Got any of them left? It might be fun trying to fix them.

Hanno

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