Faulty 0/12v output?

[Yorks]

Hello peeps.
I am a complete novice when it comes to electronics, and would appreciate some advice.
I have a Fortec Star receiver (FSCI5100 Ultra). I want to use it's 0/12v output (RCA socket) to switch a CS012 LNB electronic switch.I have been unable to get the switch to operate correctly, and suspect the fault may be with the receiver output.
If I put one end of a multimeter on the center pin connection of the receiver (lid removed) and the other end to ground (F connector) I get the following readings.

0/12v supply off ( 0 volts in menu) multimeter DC voltage measurement 11.65v

0/12v supply on ( 12 volts in menu) multimeter DC voltage measurement 9.3v

Am I measuring at the right points?
Shouldn't I at least be getting a higher measurement when the supply (according to the menu) is switched on?
I have left the box unplugged from the mains overnight but it has made no difference.

Any help welcome.

Regards,

Yorks

 

[Llew]

Hi Yorks

It'll be the metal strip running down to the board marked 0/12V.

Voltage (or absence of) correct with mine.

Can only think you're measring at the incorrect terminal (or the switching circuit is malfunctioning)

Llew

 

[Yorks]

Hi Yorks

It'll be the metal strip running down to the board marked 0/12V.

Voltage (or absence of) correct with mine.

Can only think you're measring at the incorrect terminal (or the switching circuit is malfunctioning)

Llew

Hi Llew,
Yes, I am measuring across that connection to the f/connector (as a ground). I think it must be the switching circuit, as I cannot get a 0v reading when swiched off in menu.
Is there anything I can do to try to get it functioning again? (Big hammer for instance)

Regards Yorks

 

[Llew]

I'll have a check of the switching circuit, but I expect it's controlled by a line from the processor, I have a schematic of the STi chip, but don't recall anything obvious coming from there to do with 0/12v switching, may be wrong.

I'll have a look and come back to you later.

Llew

 

[Yorks]

I'll have a check of the switching circuit, but I expect it's controlled by a line from the processor, I have a schematic of the STi chip, but don't recall anything obvious coming from there to do with 0/12v switching, may be wrong.

I'll have a look and come back to you later.

Llew

Thanks. No rush m8. Appreciate the help.

Regards,

Yorks.

 

[Llew]

The switching transistor is the surface mount one slightly to the right of the socket - marked as Q13 (or may be Q19 - there's a solder-through blob right on the number in mine).

The three legs come out as - top leg = collector. Should have 12V when switched.

You want the bottom left leg = base, this is the switching line. This should be 12V when you switch to 0V.

Switching to 12V, this line goes to ~11.3v, switching the transistor on, giving 12V to the socket.

The bottom right leg = emitter, and is pegged at 12V with the switching in either state.

I would definitely suspect this transistor, but could be some other cause.

Not much room for mistakes when taking measurements, don't short out the legs!

Llew

 

[Yorks]

Thanks Llew,
I presume I need to measure between the leg and ground? Sorry, but I am a complete novice.

Regards,

Yorks.

 

[Llew]

Yes, that's correct.

 

[PaulR]

Is it possible that the output needs to draw a current to operate properly? If you put, say, a 12v 4w car sidelamp bulb across does it light up? If you measure the voltage across the bulb what is it?

Assuming that the 0/12v output can supply 4w (300 mA) of course!

 

[Llew]

No Paul, it's not an open collector circuit, it'll deliver 12V with no load (as long as it's within the output current limit, whatever that is.)

Llew

 

[Yorks]

I have checked the transistor and have obtained the following readings:

Base * 0v, reading = 11.6v
Base * 12v, reading = 9.4v
Emitter * 0v or 12v, reading = 12v

I presume that this proves what you suspected. Q13 transistor is malfunctioning.
Sorry if I sound like I know what I'm talking about.O-Ha

Regards,

Yorks

 

[Llew]

OK, may be duff, but would be a good idea to check through the associated circuitry from the source (not much to it).

This will be a PIO4 output of the processor, which terminates at the bottom leg of R83 (just below and to the right of the transistor. This is 3.25V (12V on) and 0V (12V off).

The top leg of that resistor will be 0.7V (12V on) and 0V (off).

This goes to the bottom left leg of the adjacent transistor Q14.

With 12V on, the top leg of Q14 should be 0V, 12V off, will be 12V.

This goes to the first transistor bottom left leg via R76 to do the switching.

Sorry if that seems a bit convoluted, but that's all there is to the switching circuit.

As long as the processor is switching OK, it's got to be one of those two transistors that have failed. (Unlikely to be a resistor).

Llew

 

[Yorks]

Hi Llew,

Here are the readings:

R83 bottom leg = 3.30v (12v on), 0v (12v off)

Q14 top leg = 0v (12v on), 11.7v (12v off)

Regards,

Yorks

 

[Llew]

Ok Yorks, processor doing its job, must be the first transistor (Q13 or whatever it is), as it seems to be pulling the 12V line down a little on Q14, which is switching OK.

If you have the means to replace Q13, you can obtain one from http://uk.farnell.com (code 742983 in the search box), about 6 pence or so each (I think they're in packs of 10 minimum).

Llew

 

[Yorks]

Hi Llew,

Thanks for your help.
I may order the transistor(s) and see if I can get someone to do the repair for me. By the way, it is Q13. Mine, unlike yours is clearly visible (just not working).!

Regards,

Yorks

 

[Llew]

OK Yorks, update us when you've got it working again

Llew

 

[Yorks]

Hi Llew,
Do you know of anywhere else where I could purchase this, as Farnell have a minimum purchase of £20 .I have searched on Maplins but they do not have the exact product and as their descriptions vary, I am not sure if they have an equivalent.

Regards,
Yorks.

 

[Llew]

I'll check for an equivalent if I can't find the exact replacement Yorks.

N.B. - It's just a general purpose PNP device, you could even replace it with a regular type transistor 2N3906 (of which the surface - mount type has the same 'innards').

Llew

 

[Llew]

Hi Yorks

Maplin do a suitable replacement, BC807-25 or -16 (code N19AF or N20AF).

These will fit the bill, P&P £3.

Llew

 

[Yorks]

Thanks Llew,

Much appreciated.

Yorks