75 Ohm question

[Bripilot]

I'm going to attempt the Supervideo 2.1 mod on one of my 800XL's this weekend, but I have found 75Ohm Resistors to be somewhat hard to find. Can I pair up a couple of other resistors to make the 75Ohm, or is it going to be ok to us an 82Ohm or 68Ohm instead?

 

I've checked with Radio Shack and they don't have any of the 75 Ohm available.

 

Any help would be appreciated.

[Prodos8]

The 82ohm will work just fine or you could put two 150ohm in parallel for 75ohm.

[Longhorn Engineer]

You can use

 

1. 75ohm

2. 2 150ohm in parallel

3. combination of resistors in series to get a total of 75ohms

4. or a 82/68 ohm

 

Just needs to be *close* to 75ohms.

 

dam prodos beat me to it!

Edited October 18, 2007 by Longhorn Engineer

[Bripilot]

You can use

 

1. 75ohm

2. 2 150ohm in parallel

3. combination of resistors in series to get a total of 75ohms

4. or a 82/68 ohm

 

Just needs to be *close* to 75ohms.

 

dam prodos beat me to it!

 

Thanks all... I figured "close enough" would be fine, but I wanted to make sure. My other option would be to cannibalize one of my other XL motherboards for a few 75Ohm resisitors, but I'd prefer not to butcher a good machine for parts. =)

[Sincity]

FRY's has them in 5 a pack (2% tolerance) for about $1. I bought a pack earlier this week.

[Bryan]

It mainly has to do with the requirements of sending high-frequency (video) signals over long cables. If the source amp has a 75ohm impedance, the cable's characteristic impedance is 75 ohms, and the termination (in the monitor) is 75 ohms, then the signal is efficiently transferred and you avoid ghosting and other high-frequency nasties (especially as the cables get longer). 10% is close enough for non-professional use.

 

BTW, 75 ohm cable doesn't measure 75 ohms on an ohmmeter, but rather it has frequency dependent characteristics that come into play at higher speeds.