Building and testing the Chroma section
Contents |
The Chroma generator
Theory
THe ULA supplied U V signals, but is not able to modulate these signals into something that can be used for a TV directly, This job is done by a LM1889 chip which outputs the modulated colour information which is then later merged with the Black and white (Y) information in the Video mixer section.
The /Y signal is used in the circuit, apparently to clamp the U and V signal to +5V when sync happens. (I could be mistaken about this).
This version is the Issue 3+ version. Earlier versions used passive components which needed manual adjustment via Preset resistors. The transistor version requires no such adjustment, which is why i chose it.
Note, there is a bug in this circuit, as the Oscilator is supposed to be in sync with the video signal. (WHich is generated with the Speccy's main 14Mhz crystal). Alas, it isnt. This is the cause of the Speccy's dot-crawl problem.
Schematic
The changes I made were as follows:
In the inital version, the circuit was supplied by an external +12V supply.
In the original speccy, this was supplied by a discrete DC-DC converter, which was by far the most unreliable part of the Speccy. If something has gone wrong, chances are its the transistor in the DC-DC converter (Which then generally took out one or more of the RAM chips). Originally, this was also used to supply the 4116 Ram chips as well. As i'm not planning to use something quite as obsolete as 4116's, this is the only place the +12v will be used.
As such, i decided to use a descrete DC-DC converter powered from the +5v supply, mounted on the board. Other than that, the circuit i used is the same as the above.
Finished product
Testing
Once assembled, testing was performed by connecting in the mixer, then taking the inputs from the edge connector. (They are all available) Hopefully, it should all work first go.
If the signal appears in black and white, this usually means the oscilator isnt working, (Mainly as very little else can go wrong)
If the colours are wrong, but otherwise steady, check to see you havent mixed U and V up.
If the colours are all messed up, check the input section, especially the input transistors.
