domingo, 3 de abril de 2016

Another RCA Board from another Old TV Set

The economic crisis is behind, it seems: it's time to change TVs and leave the old units in the streets!

1 Description
A small breadboard (100*33 mm) salvaged from another old TV set.

It features three RCA female sockets (the standard yellow, white, red) and a 3.5 mm jack female socket (black).
See another page of this blog with a similar unit here.

The board is labelled as from Phillips and has the following marks:
3139 127 27491
2 40     ARO
And on the back:
3139 123 5384.1 WK105
A Google search has produced no meaningful result for these marks.
A couple of pictures:

2 Simulation
The circuit is modeled in QUCS, and can be found here.
A snapshot of the schematics:
 4 Results
Composite Video (yellow RCA jack)There is nothing on this side, other than a diode: if it were a Zener (but I do not see any mark for that) it would protect the next stage from peak voltages; if it is a standard diode I do not understand its function...

Audio (input, red and white RCA jacks)
The audio channels are identical with a low-pass RC filter with extra components. As per the simulation, the cut-off frequency (70% of peak incoming voltage) is around 2.3 MHz, which is in line with what is mentioned in my previus RCA blog. (The result is the same for the simple RC circuit calculated on a spreadsheet).

Here also there a resistor voltage divider at the audio signal income, whose function is unclear to me.

Pin 9 acts as detector: when there is no jack the path 8-9 is closed with continuity; when the jack is introduced the circuit is open. The same happens with pins 3 and 6, but only pin 9 is connected to the rest of the board. (Because of this, when the red jack is in, the resistence between pins 8-9 is 150 ohm).

Audio (output, 3.5 mm black jack)
The jack is a standard 3.5 mm TRS.
This part of the board is also symmetrical: J1 is ground, J2 and J3 are the audio channels, J4-J8 and  J5-J7 are detectors, J6 and J9 are unused pins.
If there is no jack, none of the internal contacts shows continuity; when the jack is in, there is continuity between pins 4-8 and 5-7.
The circuit is a low-pass filter with a cut-off frequency (70% of peak incoming voltage) of less than 0.3 MHz - a bit on the low side as per my previous blog on RCA boards.

However, the innings of this part of the circuit are unclear to me: why an electrolytic capacitor is used (aren't they suppossed to be used only with positive voltages?)

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