Low quality RF output - 48K speccy

[Tony]

I've got a couple of Spectrums, one has significant keyboard issues (ripped
membrane), but the output from the RF to the TV is crisp and solid.

I have another where the output is flickery and the TV struggles to keep
the image stable. When it's mostly stable, it's still not a solid block of
grey. I had assumed the issue was the rust on the RF connector, but I've
cleaned that and I'm using a brand new RF cable. That has improved things
slightly, but not to the clarity of the other machine.

I'm considering converting this Speccy to composite anyway, but I don't
want to bother if the problem lies before the RF circuitry, anyone got any
suggestions of where to look?

This is the image from the one with keyboard issues,

http://perceptionistruth.com/wp-content/uploads/2012/09/P1000742.jpg

and here's the comparison one that I'm trying to fix the output for,

http://perceptionistruth.com/wp-content/uploads/2012/09/P1000738.jpg

No amount of fine tuning of the TV signal improves the second one.
--
Tony Evans
Saving trees and wasting electrons since 1993
blog -> http://perceptionistruth.com/
books -> http://www.bookthing.co.uk/
[ anything below this line wasn't written by me ]

[Brian Gaff]

Is it an issue 2 or earlier?
Could be that the video is dodgy. I had quite a few issues with those,
including loss of or wrong colours and flickering unstable video.
Brian

--
From the Sofa of Brian Gaff Reply address is active
"Tony" <to...@darkstorm.invalid> wrote in message
news:k3i1pe$thn$2...@matrix.darkstorm.co.uk...

[Volker Bartheld]

Hi Tony!

On Fri, 21 Sep 2012 16:41:41 +0100, Tony wrote:
> I'm considering converting this Speccy to composite anyway, but I don't
> want to bother if the problem lies before the RF circuitry, anyone got any
> suggestions of where to look?

The "ZX Spectrum Composite Video Mod" (as Womble's Retro Repair Shack likes
to put it) is not that hard to do and it can be used alongside the original
RF modulator if you don't mind some coax cable hanging from the backside of
the Speccy:
http://womblesretrorepairshack.blogspot.com/2008/11/zx-spectrum-composite-video-mod.html

> http://perceptionistruth.com/wp-content/uploads/2012/09/P1000738.jpg

Looks like some interference issue and might improve if you take off the
modulator's lid and tweak the RF circuitry a bit. However, I think you
should just try to get at the composite signal. Worked excellently for me.

Cheers,
Volker

--
@: I N F O at B A R T H E L D dot N E T
3W: www.bartheld.net

[Tony]

In comp.sys.sinclair, Volker Bartheld <dr_ve...@freenet.de> wrote:

>Hi Tony!
>
>On Fri, 21 Sep 2012 16:41:41 +0100, Tony wrote:
>> I'm considering converting this Speccy to composite anyway, but I don't
>> want to bother if the problem lies before the RF circuitry, anyone got any
>> suggestions of where to look?
>
>The "ZX Spectrum Composite Video Mod" (as Womble's Retro Repair Shack likes
>to put it) is not that hard to do and it can be used alongside the original
>RF modulator if you don't mind some coax cable hanging from the backside of
>the Speccy:
>http://womblesretrorepairshack.blogspot.com/2008/11/zx-spectrum-composite-video-mod.html

Yep, that's the page I would be following if I did do the mod.

>> http://perceptionistruth.com/wp-content/uploads/2012/09/P1000738.jpg
>
>Looks like some interference issue and might improve if you take off the
>modulator's lid and tweak the RF circuitry a bit.

Any advice on that, or references / links on where to begin? I don't mind
doing the Composite mod, but if possible, I'd like to learn a little bit as
I go as well, and maybe fix the problem (if it's doable, it may not be).

[Volker Bartheld]

Hi!

>>> http://perceptionistruth.com/wp-content/uploads/2012/09/P1000738.jpg
>>Looks like some interference issue and might improve if you take off the
>>modulator's lid and tweak the RF circuitry a bit.

On Sat, 22 Sep 2012 10:55:35 +0100, Tony wrote:
> Any advice on that, or references / links on where to begin?

No, sorry. My issue 2 Spectrum's (the one having the transistor across its
CPU, see [1] vs. [2]) signal also looks a bit "busy" - I always took this
for granted. Composite video looks a lot better though, but still the
occasional color errors. [3] is a D-SLR image taken from a little LCD
monitor [4] connected to the composite output. Whilst the monitor isn't
exactly what I would call "top quality" and for sure introduces a bit of a
moiré pattern by itself, it's acceptable and probably as good as it gets
with issue 2.

Issue 3 (not having the variable caps on the PCB) is supposed to be a lot
better.

Cheers,
Volker

P.S.: The software, you're seeing on the screenshot is mdv2img and its
purpose is to transfer raw images of Microdrive cartridges to PC via the
Interface 1's serial port for later use in an emulator. I discussed that
project in comp.sys.sinclair and WoS earlier.

[1] http://www.nvg.ntnu.no/sinclair/computers/zxspectrum/issues/issue2.htm
[2] http://www.nvg.ntnu.no/sinclair/computers/zxspectrum/issues/issue3.htm
[3] http://bartheld.net/temp/spectrum_mdv2img.jpg
[4] http://www.amazon.de/Zoll-Mini-Monitor-NTSC-Rückfahrkamera/dp/B004VQD966

[Theo Markettos]

In uk.comp.vintage Tony <to...@darkstorm.invalid> wrote:
> In comp.sys.sinclair, Volker Bartheld <dr_ve...@freenet.de> wrote:
>
> >The "ZX Spectrum Composite Video Mod" (as Womble's Retro Repair Shack likes
> >to put it) is not that hard to do and it can be used alongside the original
> >RF modulator if you don't mind some coax cable hanging from the backside of
> >the Speccy:
> >http://womblesretrorepairshack.blogspot.com/2008/11/zx-spectrum-composite-video-mod.html
>
> Yep, that's the page I would be following if I did do the mod.

Don't know the Spectrum video hardware, but on Beebs you could get
bad-but-usable composite by simply touching the telly composite cable to the
input to the modulator (with it still wired in). It helps if the composite
cable screen is grounded. Might be worth this as a 5-second try to see if
the modulator is on the blink?

Theo

[Tony]

In uk.comp.vintage, Volker Bartheld <dr_ve...@freenet.de> wrote:

>Issue 3 (not having the variable caps on the PCB) is supposed to be a lot
>better.

<snippety>

Thanks for the response and the links. That monitor gives me an idea ...

[Tony]

In uk.comp.vintage, Theo Markettos <theom...@chiark.greenend.org.uk>
wrote:

Thanks, interesting idea, I'll check around and see if that's an option!
I'm pretty sure it must be, because you can have both RF and Composite
working at the same time with the Speccy, if you're willing to drill a few
holes, where-as the link above is a switch from RF to Composite using the
same connector, to keep the case tidy.

[Theo Markettos]

In uk.comp.vintage Tony <to...@darkstorm.invalid> wrote:
> Thanks, interesting idea, I'll check around and see if that's an option!
> I'm pretty sure it must be, because you can have both RF and Composite
> working at the same time with the Speccy, if you're willing to drill a few
> holes, where-as the link above is a switch from RF to Composite using the
> same connector, to keep the case tidy.

I think in theory it's supposed to be capacitively coupled (because there
may be DC voltages on the modulator input) - something like a 100nF cap
(ceramic is good) between the modulator input and the composite signal
output would do the job if you have such a thing lying around.

Theo

[Guesser]

On 23/09/2012 23:13, Theo Markettos wrote:
> I think in theory it's supposed to be capacitively coupled (because there
> may be DC voltages on the modulator input)

I'd be very surprised if your TV doesn't have a blocking capacitor on
the input already though :)

[Dylan Smith]

On 2012-09-21, Tony <to...@darkstorm.invalid> wrote:
> I'm considering converting this Speccy to composite anyway, but I don't
> want to bother if the problem lies before the RF circuitry, anyone got any
> suggestions of where to look?

You can test the composite video without actually doing the mod - if you
look at the TV modulator, you can see there's a signal line that goes
into the modulator. This carries composite video. If you take an RCA
video cable you can actually touch this wire to the centre pin of the
cable while holding the outer conductor to the casce of the modulator
(which is grounded), and see if you get a good picture on a composite
input to a TV. Tested many machines this way :-)

[Tony]

Perfect thank you.