ODYSSEY AV MOD

Started by VideopacBelgium, March 24, 2013, 12:25:35 AM

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gertk

Quote from: Rene_G7400 on April 04, 2013, 04:41:52 PM
Quote from: VideopacBelgium on April 04, 2013, 04:31:34 PM
On some TV, the wall, the ball and the players are black instead of white!
I can turn them to white with a resistor 27 ohms between the video out and the GND.
Is this correct?

27 Ohm is a bit low. Does it work with 82 Ohm too?


Or put a trim pot of 100 ohm in series with that 27 ohm and see what happens when adjusting it.
My guess is that there is no black level defined in the signal aka 'no back porch' (a thing which a lot of older ZX81 have also on modern TV sets) messing up the automatic black level adjustment.
>>G7000 G7200(P+S) G7400 N60 JET27 VG5000 ZX80 ZX81 ORIC-1 COMX35 Aquarius<<

Rene_G7400

My guess is that the TV has a capacitor on it's input, so no DC current can flow through the transistor without emitter resistor.

VideopacBelgium

Finally I put a trim pot of 100 ohm.
On this tv (pic) I have to adjust the trim pot at 27 ohm to get this screen.
On another TV (LCD or other) adjusted to 80 ohm.
Videopac digger

gertk

Quote from: VideopacBelgium on April 10, 2013, 11:36:40 PM
Finally I put a trim pot of 100 ohm.
On this tv (pic) I have to adjust the trim pot at 27 ohm to get this screen.
On another TV (LCD or other) adjusted to 80 ohm.

At least you used a trimpot from the same era  :D

With an emitter follower you need a resistor to ground from the emitter, maybe a series resistor of 75 ohms to the TV input might help too, some TV also react strange to DC voltages so a relatively large series capacitor (220 uF) resolves this.
>>G7000 G7200(P+S) G7400 N60 JET27 VG5000 ZX80 ZX81 ORIC-1 COMX35 Aquarius<<

VideopacBelgium

#19
Quote from: gertk on April 11, 2013, 06:57:58 PM
At least you used a trimpot from the same era  :D

Yes, I use the Q2 142N4 from the RF oscillator board as the emitter-follower transistor and a trim pot from the shop near my work, as I said in another topic, they have a lot of components from the seventies and eighties (now for sale at -50% till 30 april)
Videopac digger

Rene_G7400

The TV's I'm using have a 75 Ohm resistor on the video input to ground, therefore I didn't need an extra resistor in the Odyssey1 video output.

gertk

Quote from: Rene_G7400 on April 12, 2013, 09:33:26 AM
The TV's I'm using have a 75 Ohm resistor on the video input to ground, therefore I didn't need an extra resistor in the Odyssey1 video output.


Normally you would have a series resistor to match the (coax) cable impedance but then you also need to have your input signal at about double the required level on you load impedance.. An oscilloscope can give you some clues what is going on.. 
>>G7000 G7200(P+S) G7400 N60 JET27 VG5000 ZX80 ZX81 ORIC-1 COMX35 Aquarius<<

KylJoy

Reviving this topic!  :)

I am interested in a composite output modification (or s-video or RGB for that matter) but I want to keep the original RF, too.  How would I go about doing that, please?

Thanks in advance!

Rene_G7400

More info about AV mods can be found in the hardware section of this forum:
https://videopac.nl/forum/index.php?board=7.0

grips03

I have a O2 composite PCB design at oshpark.com.  Search for grips03 under shared projects.