Enterprise Forever
:UK => Hardware => Display => Topic started by: jackleather on 2007.March.06. 17:37:23
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Hi, i am just about to do the Composite Video Mod (http://ep.homeserver.hu/Hardware/Composite/Composite.htm).
As the transistor used in the project (BC182) is hard to get hold of now could anyone advise on a replacement e.g. 2N3904.
Has anyone done this mod? could someone give me the jist of the instructions, which is written in Hungarian, on the page above. Are the any pitfalls that i should watch out for?
thanks
Jack
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Your TV/monitor not have RGB input? (Scart connector) The RGB cable is the best for the Enterprise.
I used simplified version for color composite output:
[attachimg=1]
On the monitor connector I used the original composit output pin, and added a switch for select the monochrome/color mode. (For TEXT 80 mode the monochrome is better.)
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thanks, i will try this.
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I used simplified version for color composite output:
What is the value of the resistor you have added?
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What is the value of the resistor you have added?
110 ohm
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i was just wondering, after doing a composite video mod on a zx81...wouldn't a single transistor buffer circuit be safer? or is TR8 doing this job? could i still add a 2n2222a?
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I tried to make a colour composite modification, but it didn't seem to work in my case.
I used 2N3904 instead of BC182.
[attach=1]
But there’s only a messy picture on the screen.
[attach=2]
(this is IS-Basic start screen)
https://www.dropbox.com/s/7do91aypyvpsm1e/tc4k_load_Telegr.mp4?dl=0
Where could I have gone wrong?
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The composite video needs a 75ohm impedance cable. In my opinion from transistor output to the TV. On the other side the cable lenght at transistor must be as short as possible. Keep the transistor close to the IC output.
At 1MHz (where the composite signal bandwith has), long wire acts as inductivity.
Next to your transistor i am not sure that a big electrolit capacior is a good idea. A small 10uF tantal cap would be better.
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The composite video needs a 75ohm impedance cable.
In my opinion from transistor output to the TV. On the other side the cable lenght at transistor must be as short as possible. Keep the transistor close to the IC output.
At 1MHz (where the composite signal bandwith has), long wire acts as inductivity.
Next to your transistor i am not sure that a big electrolit capacior is a good idea. A small 10uF tantal cap would be better.
I'll try to shorten the wires.
I suspected that this might only cause slight image interference. Because standard B/W picture is very good.
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If the circuit doesn't work with 2N3904, try to use the "default" BC182 transistor instead...
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The BC182 is a general purpose transistor with the following spec:
BC182 TO92 NPN 60V 0,1A 0,35W
The circuit has a ca. 75ohm resistor on the emmiter, which is a negative coupling, thus the frequency response of tha amplidifier circuit is better.
As I remember, the circuit needs the 12V supply of the EP board, that way the transisor amplifier circuit responds faster.
The video signal has level from 0,5V to 2V with max 5MHz.
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On the screen left " from the 'base' of the TR8 transistor
OR the 13.pin of the U35 IC"
On the right "Monitor connector B1 pin"
Nowdays we are not using electrolite capacitor for the coupling the resistor to the output. A tantalium or polymer capacitor is better. The voltage of the capacitor as I mentioned, can be small, like 16V or lower.
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Another option would be to remove the resistor (marked in the attachment) and replace it to a 75ohm, connect to a capacitor in series, and then connect to the output B1 (or a 75ohm bnc connector)
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While waiting for the delivery of new elements, I found an old 10V capacitor in the electronic waste. After the replacement, the picture became more legible.
[attach=1][attach=2]
https://www.dropbox.com/scl/fi/htew675gue37cvuvvtjnd/Comp2_Telegr.mp4?rlkey=14ws6ageq84brd9x4dthqmpuv&dl=0
https://www.dropbox.com/scl/fi/up3wel89xk3n7bdydaw5v/Comp3_Telegr.mp4?rlkey=jajgdvpsuyb35rk0dcprlzf4x&dl=0
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I removed the R101 resistor and changed to a 75ohm (in my case it was easier to find an SMD resistor in my drawer) and then i connected to the RF output (see picture).
I do not need the capacitor, because yesterday I placed a capacitor in the old CRT TV, and I am going to use the enterprise only with that TV.
Of course the external connection of the RF modulator has to revoved. To avoid interference.
Result beautiful color picture.
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Chuckiegg
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Tuby128, thanks a lot!
Didn't mean to bother you so much :oops:
I got a good picture using the TV tuner card. But it seems that my modern composite-HDMI converter didn't like the color EP signal (it converts BW composite quite well).
[attach=1]
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Here's a video:
https://www.dropbox.com/scl/fo/3oh3jcxulpl3z02cgihj3/h?rlkey=chos1xibvgxbq5yqaldlqxkcu&dl=0
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I have problems with the color composite signal.
Now in winter time the 230V on the wall connector rapidly changes. Probably the electrical grid in our street is not that good.
I measured with multimeter, every time when in the wall socket the 230V drops with 7-10V, I can see some brightness change on the TV when I use Composite.
This is not the old CRT, now I am using a 2000s SonyTrinitron.
This kind of brightness change does not happen, when I use the R-G-B mode signal.
I suspect, that the 12V circuit of the Enterprise is not stable. The Monochrome Composite signal is made by the PNP-transistor, which uses 5V. That signal seems to be stable.
The color composite I made, was using the 12V to amplify the signal to 75ohm load.
Maybe it is time to change all the electrolite capacitors on the Enterprise board. On the other side the "Brick like" power supply has probably some electrolite capacitor, which also need some refresh.
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Maybe it is time to change all the electrolite capacitors on the Enterprise board.
My EP has long required a total maintenance inspection (I described my problem once before (https://enterpriseforever.com/maintenance/broken-ep/msg37901/#msg37901)).
But at the moment I am not morally ready for this procedure. I know how to solder a little, but not professionally (so I have, perhaps quite simple, questions on circuitry and element parameters)
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I think the best solution modernize the machine with EPower/RePower, and L2 module. For a stable power voltages and stable Nick clock.