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[cam.in.head]

hi .quick question regarding a car stereo .
what component on the circuit board is responsible of the turning on and off function when you press the momentary switch on the front ?
is it a chip or a type of transistor doing this ?
reason for asking i have a 90's blaupunkt in one of my cavs which turns on and off only when it feels like it
googling it or AI isnt too specific on this question either ??

[cam.in.head]

didnt there used to be a car stereo repair engineer on here once ?

[STEMO]

didnt there used to be a car stereo repair engineer on here once ?

Dave DND

[STEMO]

https://www.dndservices.co.uk/

[LC0112G]

Lots of generalisations, but...

The switch is usually connected directly to a microprocessor (uP). The uP is powered all the time, even when the radio is off. There are usually several operating modes, including a sleep mode where the power consumed is minimised. Pressing the switch generates an interrupt which 'wakes' the uP up from sleep mode, and it then turns power on (or off) to the rest of the circuitry.

The uP doesn't actually switch the power - that is done by a separate power regulator/control chip. The uP simply asks the regulator to turn on or off. Older designs will be a simple 'on/off' wire from the uP to the regulator. Newer designs may have a digital comunicaiton bus between the two.

So, if it were me, I'd make sure the on/off switch is working correctly, and the on/off signal is getting to the uP. Not much you can do about the uP itself, but if you can find an online circuit diagram for your radio you might be able to find the connection from the uP to the power regulator. The regulator will be quite a large device usually bolted to the metal chassis of the radio. They dissipate a lot of power so get hot. You can sometimes read the part number off them and locate replacements. 

Having said all that, if the radio is intermittent then chances are the uP and Regulator are Ok - silicon stuff is pretty reliable and unlikely to misbehave if correctly connected. It'll likely be a switch, crystal, resistor or capacitor playing up.

[cam.in.head]

thankyou very much for taking the time to write that lengthy reply .much appreciated .
so
its not the momentary power  switch for sure as its a removeable facia and the facia works fine on another unit .
i also have another unit that switches on and off fine but has other issues so was hoping i could swap the relevant parts(s) from that into my required unit .this is good cosmetically and has good cassette deck and cd changer  operation on it too (easier than swapping all these onto the other unit for sure !)

[Marks DTM Calib]

The era you describe is unlikely to be 100% mcu controlled, from personal experience, there is usually a transistor circuit that is used as de-bounce, I have seen these early SMT transistors go leaky and cause all sorts of random issues with operation (repaired many of the Vx high end stereos with this issue!).

[cam.in.head]

il have a play tonight .il open up the facia panel and see if the on off button goes to a specific pin and then follow it from there . like i say i have a unit that switches on off fine so the faulty part and its replacement are already in front of me ! .i just need to locate it !!!

[Marks DTM Calib]

Have you got any pics of which model it is ?

[LC0112G]

I have the circuits for the radio (SQR88?) and CD player (CDP09?) used in the LC - Both Blaupunkt blue dot. What model number radio are you having problems with? I might be able to get the circuits and have a look.

Mark is correct about the de-bounce circuit - but is depends on the actual fault. Are you saying when you press the on/off button it sometimes works, sometimes doesn't? Or are you saying that the radio randomly switches on some time (seconds/minuites?) after you have pressed the on button?