Building and testing the video mixer
Contents |
The video mixer
The video mixer in the spectrum takes the inverted video signal (/Y), and the colour signal from the chroma generator section and merges them. Out of this is a composite video signal which is then fed into the Modulator.
In my case, i dropped the modulator and fed the video signal directly to a RCA jack, mainly because the UM1233 is very tricky to get hold of. This signal i can connect directly to the Composite pins of a SCART socket.
Theory
THe /Y signal (Black and white composite video signal) is inverted by TR1. This voltage is then merged in with the signal from the chroma section and buffered by TR2.
Schematic
This is the original part of the schematic.
The changes I made were as follows:
- I removed the UM1233 and connected IN to the center of the RCA plug and the shield to 0v
- I used MPS2369 transistors instead of the original ZTX313 Mainly because no-one has made ZTX313's for years.
Finished product
Testing
As this is supplied by the end point of various circuits within the speccy, its pretty easy to test with access to a working speccy 48 as +5V, 0V and /Y are outputted to the edge connector. When these are connected, you should get a black and white picture. Changing the border from 0-7 should result is various scales of greyscale.
to test the colour signal, you need to connect the chroma signal to pin 13 of the LM1889 which will involve removing the keyboard.
