Help with modification of radio Sony ICF-SW35

[Kostas1]

Hello!
I own a radio Sony ICF-SW35. I want to attach an FM Stereo indicator. The radio has a circuit named CXA1238S. Pin 4 is for FM Stereo LED indicator and is not connected from the factory. From datasheet, I have seen that this can be accomplished by connecting a LEd in series with 2.2 kOhm resistor to pin 7 (Vcc). My problem is the following: the receiver is started using a button power on/off which send a DIGITAL command to entire circuit. What happens if I attach a LED ? This supplementary current will create a problem?
Thank you!

 

[Harald Kapp]

Welcome to ep.

What happens if I attach a LED ? This supplementary current will create a problem?

Give it a try.
As pin 4 is supposed to perform this function, there should be no problem to be expected.

 

[davenn]

What happens if I attach a LED ? This supplementary current will create a problem?

Did you actually look at the data sheet ?

It shows you how the LED is connected. PIN4 goes to the collector of a transistor... that is .... it is a current sink, not a current source
it also shows you where the voltage for the LED is sourced from and the size of resistor to use ........






Dave

 

[Kostas1]

The problem is that CXA voltage supply is made from a voltage regulator LA5002, which sustain 60 mA. So, if I connect to PIN 7, the TOTAL current will increase? Or shall I use +Vcc from elsewhere?

 

[davenn]

The problem is that CXA voltage supply is made from a voltage regulator LA5002, which sustain 60 mA. So, if I connect to PIN 7, the TOTAL current will increase? Or shall I use +Vcc from elsewhere?

... what does the schematic show ?

I'll expand on that ..... how big a LED do you want ?
usually a 2mm LED is used ..... 15 - 20mA

 

[Harald Kapp]

Vcc seems to be 2.4 V. A red LED drops ~1.6V. That makes for Iled = (2.4 V - 1.6 V) / 2.2 kΩ = 0.4 mA (rounded). This will not load the regulator noticeably. You will need a low current/high brightness LED to see it light up at 0.4 mA only.

 

[dorke]

I would start with measuring the DC voltage on pin 7 relative to GND.
Is it +B(4.1V) or 2.4V ? (the schematics is ambiguous here )

Next, use a low current(2mA) high efficiency LED .

To get a 2ma current using the voltage measured on pin 7 and the LED parameters,
Calculate the value of the series resistor like Harald showed above.

 

[davenn]

Vcc seems to be 2.4 V.

yes

I would start with measuring the DC voltage on pin 7 relative to GND.
Is it +B(4.1V) or 2.4V ? (the schematics is ambiguous here )

no it's not 4.1 ... trace the rail back ... it doesn't come off the LA5002 regulator as you and the OP suggest

it comes off a different source, Q 27

it would be nice to see the schematic to the left of Q27


Dave

 

[Audioguru]

Why do you want a stereo indicator? Here in Canada FM radio stations are always in stereo and have been for many years.

 

[Tha fios agaibh]

May I ask a schematic question? I see jumper wire symbols labeled JWxx, but what does JCxx stand for?

Sometimes it's helpful to see the stereo indicator light to confirm your locked onto a distant station. At times you can be tuned to the edge of a frequency and the stereo flickers in and out. I like to know I'm in the middle of the tuned stereo range. If that makes sense.

 

[davenn]

Why do you want a stereo indicator? Here in Canada FM radio stations are always in stereo and have been for many years.

silly question !!

Sometimes it's helpful to see the stereo indicator light to confirm your locked onto a distant station. At times you can be tuned to the edge of a frequency and the stereo flickers in and out. I like to know I'm in the middle of the tuned stereo range.

EXACTLY

 

[Audioguru]

I asked about why do you want a stereo indicator because the radio is mono with "PLL Synthesized Digital Tuning".
Its chip has an FM stereo demodulator so maybe the headphones produce stereo. The radio is old so I could not find an owners manual.
You do not tune in a station on this radio, stations can be scanned or a preset station can be selected from memory.

 

[dorke]

Well,
Even in the case that you are exactly tuned to a FM stereo broadcast,
a receiver (don't know about this specific one)may be actually delivering Mono Audio !

That may happen if the station received is far and it's signal is weak,
the station's audio may have a lot of noise in stereo and much less in mono .

In most cases this is done by a manual bottom noted as stereo/mono.
But,can be done automatically by the receiver.

BTW,
Not all countries have "Stereo broadcast all over" like Canada...