viernes, 25 de marzo de 2016

RCA board from old TV set

1 Description
This is a small breadboard (78*36 mm) salvaged from an old TV set (cathodic rays!) that I found in a street.

It features three RCA female sockets (the standard white, red, yellow) plus a 3.5 mm jack female socket (black).

For the internal connections of the RCA signals, it has two separated sockets with 3 and 2 pins (for white+red, and yellow jacks, resp.; pin spacing is 2.5 mm). There are no further connectors for the part of the circuit rlated to the 3.5 mm jack, although the components are there.

On the board one can read:
KABINLERDE KULLANILACAK (which seems to be Turkish for "USE IN CABIN")
A couple of pictures:

2 Simulation
The circuit, modeled in QUCS, is here.
(The voltage sources are not part of the board, of course).
A snapshot of the schematics:

3 Connectors
On RCA connectors see here.
With regard to the 3.5 mm 9 pin female connector, I have not looked for the model or checked in detail the connections, but I found of interest the information here. The socket is likely case C of the following image (source):
Are the sockets for input or output?
For the RCA, I think they are input:
- Protection: why would otherwise place Zener diodes here rather than on the previous stage?
- Filtering: as an input circuit the yellow RCA side (composite video) is an LC low-pass filter; the red and white RCA jacks (audio channels) are RC low-pass filters.
For the 3.5 mm jack, my assumption is that it is audio output (no technical argument: it's just the usual situation).

4 Results
Composite Video (yellow RCA jack)
For composite video see here and here.
(Why is this "composite video" and not "component video"? Not totally sure, but one: the colors of the connectors; two: the symmetry of circuits for red and white).
The resonance frequency of the LC circuit is 15.9 MHz.
The simulation of this part of the model is here.
As the sub-circuit has no resistor a short occurs at the resonance frequency - which is identified in the model results with a peak of more than 100 V at approx. 16 MHz.
(In reality there are resistances all around, I have measured about 0.6 ohm at the inductor.)
The introduction of the resistor and the Zener diode in the simulation has the result of reducing the voltage peak (from 160 to 16 V) and lowering the resonance frequency (from 16 to 7 MHz). Beyond that, the total capacitive impedance grows quickly and signals are filtered out.

Audio (white & red RCA jacks)
These sub-circuits are RC, acting as a low-pass filter, and that's shown in the output of the simulation:

However, the RC simple circuit whose behavior I have calculated with a spreadsheet has a much flatter pass branch:

What happens here ... ?
Tachán! Simulate with logarithmic steps and you get:
Still the numbers do not add up: QUCS says that at 1MHz there is 1 V, while I get 1 V at 5.9 MHz. Interestingly, the reason is related to the Zener diode: remove it and 1 V is shown at 5.8 MHz. I do not know... the Zener has a rupture voltage of 5.1 V on one side and is grounded on the other!

There a resistor voltage divider at the audio signal income: for what if both sides are directly grounded at their other ends?
No answer for this yet...

Why has the designer selected a cut-off frequency in the range of 1 MHz for the audio, and 7 MHz for the video?
According to this source, a TV signal requires 4 MHz of bandwidth, which at the end are transformed into 6 MHz. The audio is added centered at 5.75 MHz with a bandwidth of 0.5 MHz.
If this is so, the frequency carrier has been removed prior to this stage, and only the signal is left: 6 MHz for the video and 0.5 MHz for the audio. 

And why using an RC circuit for one and LC for the other?
No answer for this yet...

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