48K dark output

[mtullock]

Hi folks,
I repaired my old 48K spectrum a long while ago, needed a lot of work from putting new tracks on the board to replacing RAM, new caps, new transistors etc.... and it is up and runing well. At the same time I changed the molulator to have a component out rather than RF, but since then the display is really grey. I have a +2 as well and it is not like this so its not the component cable or the TV. Is this a known consequence of switching to component or would there be something else at play? I have goggled it but cant see much in the way of a grey overlay on the screen but plenty on other graphical glitches.

cheers
Magnus

[redgatemodels]

First question - how did you do the composite modification?

100uF capacitor?

[mtullock]

Hi,
Yip, just the standard fix with the 100u cap.

Cheers
M

[1024MAK]

I know your +2 works okay with this TV, but I still strongly recommend trying the 48K ZX Spectrum with another TV.

The composite video from a 16K/48K/+ ZX Spectrum even with a coupling capacitor is not exactly to the normal composite video (standard) levels. Hence each TV may perform slightly differently.

Mark

[mtullock]

Thanks, tried on my other TV it's still pretty dull and lacks brightness. I guess I can live with it. Just would like the whites to be a little brighter

[roganjosh]

You didn't say which transistors you replaced. Changing TR1 & TR2 with higher gain transistors can make the composite output brighter - but make sure you get them the right way round.

From the retroleum site:

If you find the brightness is a bit dark, you can boost it by replacing TR1 and TR2 with BC549C transistors, but note: they must be orientated 180º compared to the originals, i.e. the shape of the transistors should be opposite to the footprints marked on the PCB (ignore the text on the transistor bodies). Nothing will be damaged if you get it wrong, but the picture will be very dark. Bear in mind this brightness mod should should only be done on composite video modded boards, as it tends to create white flaring in the original RF output.

[mtullock]

Ahhh haven't read that. I only replaced TR4 and TR5 out if necessity. Will give that a go, thanks!

[redgatemodels]

You didn't say which transistors you replaced. Changing TR1 & TR2 with higher gain transistors can make the composite output brighter - but make sure you get them the right way round.

From the retroleum site:

If you find the brightness is a bit dark, you can boost it by replacing TR1 and TR2 with BC549C transistors, but note: they must be orientated 180º compared to the originals, i.e. the shape of the transistors should be opposite to the footprints marked on the PCB (ignore the text on the transistor bodies). Nothing will be damaged if you get it wrong, but the picture will be very dark. Bear in mind this brightness mod should should only be done on composite video modded boards, as it tends to create white flaring in the original RF output.

That's very interesting, I've got some BC549C's in stock to do the transistor version of the comp mod. My series 2 has a simple cap mod, might swap tr1 and 2 to see if it makes a difference

[1024MAK]

I do recommend keeping the transistors you remove in a safe place.

Twiddling the contrast and brightness controls on the TV is the easiest option.

The other things that can be tied are:

Doing the transistor version of the composite modification, or using one of the PCB modules that does the same thing.

Or fiddling with the values in the circuitry around TR1 and TR2. Sinclair made various changes to some of the resistors here.

It also would help if you can confirm if the problem is poor contrast or poor brightness.

How does the BRIGHT white look? How does the BLACK look? What about the colours?

Mark

[mtullock]

Replaced both transistors. Did help a bit, but not massively. Also the socket for the RF now component cable is a bit "shoogly" and the cable connection is not reliable. I can't see a replacement part on retroleum as there is for power and audio. Anyone know of a suitable source?