Passive Attenuator

[Peter F]

Someone with a smidgen of electronics should be able to suggest a good way of doing this:

 

I have a domestic video source (sky digibox) where the composite output (on the SCART) is too hot.

I would like to make a passive box or hopefully bodge lead that just drops the video level slightly.

 

Any suggestions what value components I need to use? Chuck a small resistor in to a bnc lead perhaps?

 

Current solution involves using a domestic VHS as an intermediate step to degrade the signal just enough so whites don't look oh so wrong.

 

All elegant and neat solutions appreciated.

 

Thanks,

 

Peter

[bruce]

RS and Farnell do inline BNC attenuators, in 3/6/10/20dB versions. Silly prices though - 10-20 pounds each.

 

You could try bodging something with a couple of resistors to make a potential divider....

 

Edit: coax ones, intended for aerial circuits, are far cheaper. I think Maplin sticks them at about a fiver, and most "proper" (as opposed to high st) suppliers will do them for about a pound. Wrong sort of connectors, but probably very similar inside...

[paulears]

How hot? If you have access to a scope, I'd have a look and see what sort of level you have. Two resistors, adding up to about 70-80 ohms in series accross the output, taking your reduced level from the point between the two resistors. Two identical values will give you 50% as an output. If you can find a low resistance pot, then you can vary the output by tweaking and watching the result.

[pumphouse]

It sounds to me like whatever you are feeding it into (which you don't say - TV/Plasma screen/monitor/projector?) isn't terminated properly. a 75 ohm resistor across the line at the input would sort it out.

 

Have you tried turning the brightness down on the display (assuming it is a display you are connecting to)? This will cause white clipping if set too high.

 

Jason

[Peter F]

You could try bodging something with a couple of resistors to make a potential divider....

And paulears made a similar suggestion.

 

Cheers, I'd figured it was perfectly reasonable to just use a couple of resistors but was curious to see if anyone was going to come up with a reason why the simple solution was a really bad idea.

I have had a scope on it in the past but can't for the life of me remember how bad it was. Have a hunch it was something like 1.3V peak white.

 

 

It sounds to me like whatever you are feeding it into (which you don't say - TV/Plasma screen/monitor/projector?) isn't terminated properly. a 75 ohm resistor across the line at the input would sort it out.

 

Nope, as I said the source is too hot. It has, at various times, been fed into an mx50, mx70, sony dfs500, gvg200, direct to a monitor and straight to a scope. Trust me, the source is too hot!

 

Tally ho,

off to the box of little resistors I go...

 

Peter

[Tomo]

The bit of kit you want is a "PI" or "T" passive attenuator at 75 ohms.

 

It's just three resistors, and there is a quick tutorial here

This attenuator is identical at all frequencies*, and is dead easy to build.

 

*Not exactly true, but the construction is what makes the difference and it's not really that significant until you get near to a GHz

[Jivemaster]

Take ten 10 ohm resistors in series across the hot output and connect the screen to screen then try the line on different taps as appropriate. There should be room inside some connectors the hide the thing, or make a lead with an inline box.