Spectrum plus composite signal degrading.

[SpaceFlightOrange]

Hi.

My issue 6a Spectrum+ has the transistor-based composite mod installed (BC549, Base connected to Composite, Collector to +5v and emitter to the centre pin of the phono connector in the modulator, and the resistor and all other connectors from the modulator removed).

This has been working fine but this week I noticed a kind of flaring over to the left and the picture doesn't seem as good as it is from my issue 3 board with the same mod (using the same PSU, cable and TV) See the video.

https://youtu.be/ByBLecOTV4k

Its getting worse so I was wondering what to do.

My thoughts where to replace TR1 and TR2 with BC549's as the originals are still installed, but I thought I might also replace the lower ram caps while I'm at it (jail bar fix) since I have the caps to do the job.

Beyond that I'm not sure how to diagnose this.

[1024MAK]

I’m not sure what to suggest. Was the border supposed to be yellow?

I very much doubt replacing TR1 and TR2 will make any difference whatsoever (the original ZTX313 transistors in U.K. made machines have much quicker switching times compared to BC549 types). Bipolar transistors are fairly hardy, and should be fine as long as the output signal line has not been shorted to 0V/GND or to the positive supply line.

From what I can see in that very short video, that picture is still far better than a UHF output. Also keep in mind that whatever composite mod variation is used, they are all ‘hacks’ because they all use very simple circuitry, not broadcast circuit design.

Can you post a high resolution photo showing the whole screen. Maybe also the same for your other machine as well.

The composite video output from different issue boards, and even the same issue board, but a different manufactured batch will vary, because we are dealing with analogue circuitry.

Mark

[SpaceFlightOrange]

Hi.

Apologies. I haven't gotten around to replying to this. mainly because I never got around to investigating it further, but I have managed to improve the situation quite a bit.

How?

I replaced the cable.

I've been using a shielded composite cable, and was happy with it, but it was very long and I had it bunched up and sitting in between my two Beebs.

I've since switched to a much higher quality OFC short cable, and there is a noticeable improvement in picture quality.

[1024MAK]

Hmm, you do understand that OFC is just marketing bull$h!t don’t you?

Also a lot of cheap “screened” cables are lap screened (meaning they are not fully shielded, as there are spaces between the wires that form the screening). Proper co-ax cable has a much better braided screen and sometimes a foil screen as well.

Mark

[SpaceFlightOrange]

Hmm, you do understand that OFC is just marketing bull$h!t don’t you?

Regardless of whether it is or not, the picture quality is far better, and I already had the cable (it was an interconnect on a HiFi I no longer own). It's actually a RadioShack cable I bought from Tandy here in the UK, so that will give you an idea of how old it is, and I specifically bought them because they were very cheap and they got a 5 star review in What HiFi back in the day due to the build quality and performance.

It wasn't "sold" to me by some bloke in a tie, so this point is irrelevant.

Also a lot of cheap “screened” cables are lap screened (meaning they are not fully shielded, as there are spaces between the wires that form the screening). Proper co-ax cable has a much better braided screen and sometimes a foil screen as well.

Mark

The one I was using was a thickish cable so I assumed it was braided, rather than a simple twin core, but I couldn't tell because the plugs are moulded. It was packaged with something (can't remember what). The replacement one is fully screened.

So no fettling in the Spectrum needed.