ZX Speccy +2A - B&W & snowy RF output + other issues

Claudius · October 2016

Hi All,

I am new to this forum and I am back to the Spectrum world after 25 years. Recently I've bought a second hand +2A (+2B board - Z70833 Issue 1) as a DYI project and managed to do following mods and improvements:
1. Cleaned the whole machine including keyboard, data recorded, PSU and interior.
2. De-coupled audio (C24) from TEA2000 and connected directly to the TR3 emitter. - No improvement to the RF video signal at all.
3. EAC modified for an external MP3/tape software loading. - Loaded some software from my phone and it works very well.
4. All electrolytic capacitors replaced within the PSU and on the datacoder PCB. - All voltages are stable and solid.
5. New drive belt fitted to the datacoder.
5. Adjusted the datacoder head and loaded a few programs without any troubles.
6. Built a proper composite output board based on the +128k design (3 diodes, 1 transistor and 2 resistors) but awaiting for a few components and not tested yet.

All voltages are fine in every point of the main PCB. All transistors checked and working, no hot IC chips including TEA2000. BUT the Speccy still suffers from following issues:
1. I've tuned my TV to Ch36 again. However the image is very snow-ish with some ghosts on the right hand side to any character or graphics. It looks like watching analogue TV from a very remote broadcasting centre. De-coupling audio didn't help at all.
2. Frequently the RF output video is black & white only. A few resets help for a while.
3. The datacoder works, but only play and record functions. Forward and backward rewinds do not work at all and it seems to be a mechanical issue.
4. A few function keys don't work (Break, Edit, Delete, Graph). Keyboard ribbons looks fine and worked before. I hope it was my mistake when I re-assabled +2A other day.

I hope someone on here would be able to point me in a right direction with the RF output issues.

Claudius

1024MAK · October 2016

When using UHF, unless the tuning of the TV exactly matches the analogue modulation from the Spectrum's UHF modulator, the image you described is typical.

The modulators used by 1980's computers are analogue technology and do not operate at the exact spot frequencies called for in TV transmitter specifications.

First up, try fine tuning. Because some modern TVs will only correctly lock to signals at the official frequencies, try another TV. But you will be far better off using a SCART connection (if you have a suitably equipped TV). The next best option is composite video.

Mark

Your Spec-chum · October 2016

A simple thing to check, but a faulty RF cable would cause these symptoms also.

Claudius · October 2016

The RF cable is fine and the TV works well.

But why B&W not all the time, but sometimes only, on the same cable, with the same TV tuning.

1024MAK · October 2016

I can't tell you the exact reason, as there are loads of variables. I can only speculate why, but there is a reason that UHF is the least preferred connection method.

One problem is the modulator frequency drifts as it warms up. The monochrome image and the colour information are carried by seperate signals that have been mixed together. If the TV is slightly off tune, it will be unable to decode the colour signal leaving a monochrome image...

Mark

1024MAK · October 2016

It's also possible that the UHF oscillator in the modulator is unstable.

Mark

Claudius · October 2016

Thanks 1024MAK. I have got a spare UM1233, so I will replace it.

Claudius · October 2016

Hi,

A short update, I've managed to replace RF modulator and now its output is useable however still has got a ghosty artefact to the right to everything on the screen. There are also yellow and blue ghost in the back of some displayed characters or lines. A few photos below.

1. RF output image

2. Zoomed RF image

I've finally connected the composite output PCB and the video is now much much clearer, but artefacts and the yellowish background to some charters is still there.

3. Composite video board

4. Composite output image

5. Zoomed composite video

Is there any chance to eliminate all ghosts and adjust colours and make black characters black on grey background, no yellowish or blueish bits?

1024MAK · October 2016

What you are seeing is mostly the limitations of the composite colour video signal and the limitations of the way the Spectrum generates the composite colour video signal. So it's normal.

The UHF / RF output has all these limitations as well, as it is generated from the composite colour video signal. Plus the modulator (in the Spectrum) and then the demodulation (in the TV) mush the signal up a bit more :-(

Some of the "ghosts" are actually due to the signal processing in modern flat panel TVs. I assume that is what you are using? If yes, see if you can turn off any contrast enhancement options, or picture clarity / enhancement settings.

Still, it sounds like an improvement compared to what you had before :-)

Mark

dasteph · October 2016

Hi Claudius,

I wasn't happy with the normal composite mods for the +2a either, after a bit of frustration I ended up making an internal RGB to s-video adapter based on a Sony CXA1645 RGB IC. The CXA1645 also does Composite video. I pretty much just copied the reference design in the CXA1645 data sheet.

Hopefully you can see from the picture, that some good quality pictures are possible, without all that yellowing etc. Not sure if the Sony chip is still available, as this mod was from around 10 years ago now, but there are similar chips out there.

The adapter was mounted inside the Spectrum and added I an extra port at the back of the case.

I posted about it at the time I made the adaptor s-video-success.

Unfortunately I sold +2a when I moved a few years ago now due to some imposed space constraints.

Claudius · October 2016

Thanks a lot.

Anyway I am going to do further mods on this +2 and add a band-stop filter (LC circuit) to LUMI (pin 8) of TEA2000 and see if it works. The filer will be tuned to 4.43MHz or so.

In mean time I bought a proper RGB cable and now the video is as it should be:

By the way, all mods I've done so far improved the RF output a lot on this unit. For comparition I powered up second Speccy which I have got and the RF video is below.
%2B3RF_zoomed_no_mods.jpg

The pic is from an unmodified +3 model with a broken floppy drive.

schneil · December 2016

Claudius wrote: »

In mean time I bought a proper RGB cable and now the video is as it should be:

By the way, all mods I've done so far improved the RF output a lot on this unit. For comparition I powered up second Speccy which I have got and the RF video is below.

The pic is from an unmodified +3 model with a broken floppy drive.

I remember the RF output on my +3 being similar to yours. The start menu had noticeable "fuzz" on the letters and some blue bleed on the selection bar.

IMHO my 48k Speccy + had a much better RF output!!

Peter Mulholland · February 2017

Honestly, RF output was always crap. The ghosting you see there doesnt surprise me at all.
RGB is the way to go for using retro computers with modern TV's. If you can't get RGB (ie you live in the US where there's no SCART), the next best thing would be to make an S-Video output using the AD724 or something like it.

ZX Speccy +2A - B&W & snowy RF output + other issues

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