Omega Owners Forum
Chat Area => General Discussion Area => Topic started by: cam.in.head on 07 December 2025, 16:50:49
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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 ??
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didnt there used to be a car stereo repair engineer on here once ?
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didnt there used to be a car stereo repair engineer on here once ?
Dave DND
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https://www.dndservices.co.uk/
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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.
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thankyou very much for taking the time to write that lengthy reply .much appreciated .
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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 !)
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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!).
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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 !!!
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Have you got any pics of which model it is ?
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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?
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sometimes the unit will switch on and work fine .sometimes it wont no matter how many times you press the button or how long you hold it .removeable facia (and another spare facia all work fine on other units )
it can work fine for a few days even .then one day its non responsive .rear iso connector is fine and powers and grounds are too.( bench testing same issue)
il get the model number tonight.there were quite a few basically identical units ,some with cd changer socket on the back and others without .all named after us places .
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this is all very much appreciated .
these units i had an issue with a few years back where the cassette sound was distorted and i swapped virtually every component i could on the board to eliminate it so it would be brilliant to gave a scematic for them with a clud as to what does what as i dont intend to change thses units .i have them in 3 different cars and likd them and they are not too flashy looking for an older car .they are in my two mk1 cavs and my 85 carlton .(2 using the boot mounted 10 disc changer)
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so this is how annoying it gets .wouldnt power up yesterday in garage.now set it all up for a bench test and it works .whats the difference ? warmer in house maybee ???
anyway ive opened up an facia and i think the first microorocessor is in there .it has a ST6240/c8 in it so maybee the power button triggers this and then the signal is fed into the main unit from gere .cannot find anything substantial on heatsinks apart from the two audio amps( TDA7375) so i will start googling chip numbers.
this unit is a blaupunkt san diego dj and the other (same base units ) are milano rcr127 and avignon rcm ??
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so if my understanding of your description is correct .the front facia button sends a signal to a microprosessor then this sends a signal to the main board regulator to turn on .
so i have confirmed standby power is present at the facia socket (5v) and im pretty sure this uP is in thd defacheable facia panel .so i know 100% this uP is ok .when the unit next fails to turn on il heck that standby power is there and if it isnt thats the best fix .if it is then i know its something internal .it could fault tommorow or next month ! .
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Is it one of these? https://elektrotanya.com/blaupunkt_san_diego_dj31_sch.pdf/download.html
If it is you can download the circuit diagrams from the link on that page.
It looks like the processor on the faceplate (Page 24) is only used to interface the various front panel keys. Communication with the main board is over an SPI (Serial Peripheral Interface) using 4 pins - MISO, MOSI, SCK and CE. However the on/off function has it's own pin - Pin 8.
It looks like on the main board pin 8 comes in on page 19, but goes straight off again to the power supply page. I think that's page 8, and comes in on the bottom left corner. Couple of transistors there (V2054 and V2051), and what looks like a thermistor (R2051). That's where I'd start poking about.
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hi .no jts not that model .apparently and confusing the san diego dj name was used on different models.i did struggle to find a schematic when i was trying to sort a previous issue of distortion on cassette only .i can find a pic and email it to you if that would help ?
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The circuits seem to cover lots of models - all basically the same, but followed by another city or state name. Cleveland, Dresden, Boston, Madison, Freiburg, Las Vegas, Canberra, San Diego, Essen, Alicante, Lausanne, Konstanz, San Remo, Santa Cruz, Dusseldorf, Kansas, Bologna, Hawaii, Maryland, Munchen or Modena.
Is there a numeric part number on the radio? I think the San Diego DJ RCR 127 should be "8 646 713 310"
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on the side it says
7 646 764 310
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ok so slight update on this !
the unit has worked fine in the house every time and could continue for some time (or go faulty tommorow !)
so .as i have a few of these units i went back to trying to solve one of their other common issues which is distortion on the cassette playback .( i have 3 that do this)
this started years ago and i never solved it then (just bought a replacement off ebay)
the radio section is fine and the cd control section is all fine and crystal clear . the cassette is distorted .my replacement unit has gone the same too !
i have spent every evening with a perfect deck and a faulty deck swapping components but to no success .
so far i have swapped 1st stage preamp chips ,second stage transistors and virtually every electrolytic including all the coupling ones in the sound path between stages .i have even swapped the tda4748 main ic (that was a bugger) between units andcthe distortion still stays on same unit .
tried tapping the left and right channels off into my domestic hifi amp and the signal is crystal clear .
so its crystal clear where it goes into the tda ic but the tda distirts it but the ic is ok ?
ttied powering the cassette motor from a 9v battery too !
messaged dave dnd fof advice but not interested !
so any other pointers OR are there any radio repair engiineers who i could actually visit and go through it with me ?
help !!
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The cassette motors generally have a sub board in them to regulate the speed, so check the motor (or swap it) and also check the mechanism for dry grease, tight spindles etc
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tried that .the complete decks were the first thing i swapped !
its as if the coupling capacitors are faulty but swapping them unit to unit doesnt swap the fault !
its very frustrating !
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one more update (for anyone whos interested!)
ive just found and bought 2more units off ebay .one works fine .other has slight cassette distortion !
also i tried a previously perfect new old stock unit that i fitted into my carlton a few years back and lo and behold the cassette is very bad on that one now as well !
so even if its being affected by the cold now im 100% sure they never used to be so theres something very weird going on especially to affect so many identical models all with an identical issue. i did message dave dnd at the begginning of this thread as im sure he would have worked on many of these sets in the day but he wasnt too helpfull ! fair enough hes running a business and doesnt seem to be on this forum anymore .
my next move is to either buy one of the small preamp boards of try to idolate ond from another unused different old radio cassette !
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Can't see it being related to motor speed. That would normally result in either slow/fast playback or more likely a wow-ing type variation in the audio output as the motor speed hunts up and down.
My first guess would have been it's related to tape pickup head alignment. It doesn't take much misalignment on the tape/head to cause a muffled type sound. However, if I understand correctly you're saying you've picked off the signal prior to the main amplifier and played it through a different amp, and it sounds ok? If that's correct, then probably not alignment.
Have you got a signal generator and an oscilloscope? If so you can usually inject a signal somewhere close to the tape pickup heads and then follow it through the circuit using the scope to see where the distortion is introduced. I'd go with a 1KHz signal at low amplitude (perhaps a few mV) to start with and crank up the amplitude once you're happy. Distortion on pure sine waves is usually pretty easy to spot by eye (on a 'scope), and even easier to hear.
There are some fairly complex frequency compensation circuits on cassettes due to the different charicteristics of various metals used in the tapes, plus other compression stuff (Dolby B/C for example). Don't rule out faults in these compensaiton circuits.
Haven't got time to look deeper today - need to go to Street/Clarks Outlet centre tonight. Shops close at 7pm and parking is only free after 6pm so a narrow window to get in and out without paying. Not looking forward to it. :(
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hi .no its definately not a deck issue ive swapped decks ,that was the very first thing i did years ago and now as well .i dont have a signal generator or oscilloscope but yes if i seperate the sound circuit after the head preamps and the main input ic (at the coupling caps) then i get a good sound out AND a good sound if i feed into the main ic so this should be impossible .one way should be faulty but it isnt .im back on it now and looking at dome smd caps see if changing them makes a difference !
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Ok, survived Clarks village - just.
Thinking about it on the drive home, it seems unlikely that you would get distortion on both left and right channels due to a single fault if the fault is in a capacitor or resistor. I mean all the circuits I've looked at the signal path is basically separate for L and R, so a single capacitor in the signal path going duff might distort one channel or t'other but not both. The chances of both L&R capacitors failing at the same time is tiny, and the chances of the same thing happening on multiple radios? Nahh, Occam's razor says it's something else.
You mention a TDA4748. Are you sure of that part number - or could it be a TDA7348? TDA7348 appears to be an audio processor & switch, switching the input between radio and tape inputs, and is used in lots of Blaupunkt stuff. TDA4748 is an obsolete switch mode power supply chip, and I can't see it on any of the circuit diagrams I've found.
The other possibility is power supply. When the tape deck motor is running the unit will probably be drawing 1A-2A more that when it's running on radio or CD. I'm wondering if this extra power is enough to cause the supply to drop and/or get noisy, enough to upset the supplies to the analogue sections of the tape and speaker drive circuitry. That would be enough to cause distortion on the 'tape' inputs, but allow the radio and CD to operate normally. It would also affect both L & R of the tape equally. It might also explain why if you pick off the tape signals and feed them into an external amp there is no distortion - the speaker drivers will also take a lot of power - perhaps there is only enough power to rune either the amps or the motors, and not both?
To test that theory I'd locate the supply pin to the tape deck - probably +12V - and power it externally from a bench supply. Need to be careful though - you don't want to blow up the cassette motors.
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hi .just checked yes sorry its a 7348 ic !. im on a similar way of thinking and did power the drive motor seperate from a 9v battery ! still distorted!
this is why its driving me mad.if the input to the 7348 works fine (i swapped this chip from other unit) and the output before it is good when fed into another amplifier then the only common link there is the coupling caps but swapped them too . its theoretically working perfect on paper but not in reality .
ai is totally useless on questions like this too as it tells me i could have a faulty tape,dirty or misaligned head ! all basic stuff which are eliminated by swapping deck complete .
please dont let this get into your head too and drive you mad as well ! its just something that shouldnt be happening and is common to a few units now so once i find the offending part it should mend them all !
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I think it's going to be a struggle to diagnose without an oscilloscope.
The TDA7348 appears to have 3 identical stereo inputs. Pins 8 & 12 for the radio, 9 & 13 for the tape, and 10 & 14 for 'Aux' which I'm guessing is the CD in this instance.
The tape head pre-amp looks like it's a dual op amp - either an RC4558 or LM833. However, there is a mute circuit between the output of the preamp and the tape input of the TDA7348 which is controlled by the microprocessor. If the mute is being turned on and off rapidly it could sound like distortion and would affect both L & R channels.
So basically I'd want to 'scope the output of the preamp, the input to the TDA7348, and the mute circuitry. It should be obvious from these where the distortion is occurring.
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thankyou for that .il try work out the mute circuit and disable it .see what happens
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after further testing i have concluded that "some " rogue component in the second stage of the preamp (just a transistor in second stage)is causing the issue .even thou everything seems to test similar in values to a good unit .im going to try some freeze spray when i get some but in the meantime on one faulty unit i have fitted the standalone preamp board from a philips 752 this morning which has eliminated the problem completely .thus proving my theory that there something there which is either a design flaw or an age related one .either way this does solve it and make the xassette crystal clear again .needed slightly higher coupling caps (4.7 instead of 2.2uf)
other idea which im contemplating is modding to a usb stick input as well as or instead of the cassette .might try this on one unit to see how it sounds .
the realy annoying thing here (althou its a learning curve) is the amount of parts /ic's etc that ive swapped over now and years ago when i knew less about what i was doing ! and didnt need swapping at all !? hey ho !
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after further testing i have concluded that "some " rogue component in the second stage of the preamp (just a transistor in second stage)is causing the issue .even thou everything seems to test similar in values to a good unit .im going to try some freeze spray when i get some but in the meantime on one faulty unit i have fitted the standalone preamp board from a philips 752 this morning which has eliminated the problem completely .thus proving my theory that there something there which is either a design flaw or an age related one .either way this does solve it and make the xassette crystal clear again .needed slightly higher coupling caps (4.7 instead of 2.2uf)
other idea which im contemplating is modding to a usb stick input as well as or instead of the cassette .might try this on one unit to see how it sounds .
the realy annoying thing here (althou its a learning curve) is the amount of parts /ic's etc that ive swapped over now and years ago when i knew less about what i was doing ! and didnt need swapping at all !? hey ho !
Hallelujah !!!! ;D
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its certainly more fun than christmas telly !
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None of the circuits seem to show a 'second stage of preamp' as such. The rcr87_essen circuits show the tape preamp (an RC4558) feeding a couple of transistors (V1120 and V1130 both BC549C's), and from there into the TDA7348.
This is also where the tape mute circuit comes in - transistors V1310 (BC808), V1311(DTC1432KA), V1312(BC848C) and V1313(BC848C). This is where it gets sort of interesting. The Mute circuit appears to be fed from a couple of microswitches mounted on the cassette mechanism, via connector P1350/N1950. It looks like pin 2 of this connector should go to +5V when the cassette is inserted. Pin 6 of the connector should be open circuit when not-muted, and shorted to ground by a second microswitch when muted.
It's not obvious from the circuits what controls the microswitches, but I'd guess it's loading and unloading the cassette tape. I'd be much more suspicious of these microswitches than any semiconductor device. Given the 'single fault affecting both L&R channels' issue, it's most likely the feed to the base of V1310 is dicky - from pin 6 of P1350/N1950.
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hi and thankyou for all this .
i mistook the other transistors as another amp stage !
yes the 4558 is the preamp ic and it all goes eventually into the 7348 .on one unit i have fed the head into a seperate preamp board robbed from a philips 752 and wired it straight down to the 7348 coupling caps.all works well so thats one unit sorted and proves im in the right area .
as for the other units i will now check the muting switches for any resistance leakage (or could i eliminate them completely ?
and yeah that 1350 plug contains the 12v motor supply and also two microswitches .one looks like it tells the motor to turn on and the other looks like it tells the system wether you are in play or fast forward/rewind and not play so yes i agree i this is the mute command ?
will have a play with this wire .thankyou again .will report back
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this also sort of ties in with the fact that even on a good unit if you get your finger close to that gren wire from the mute microswitch you can make it sound distorted but never saw the connection .!
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BUT it also isnt a problem with the microswitch as such because ive swapped complete plug and play decks between units so more a case of an issue with the muting circuit itself .funnily enough of all the smt components that i swapped none of those transistors (or nearby resistors/caps )were touched!
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i think were onto something here !
after the connector on the board for the mute switch (the green wire switched to gnd) there is an inline resistor and then its straight to that first transistor .i have removed the resistor effectively taking the mute switch trigger out of the equasion .this appears to have cleaned the signal up considerably . so maybee that transistor had some leakage and was partly switching . i beleive it might /shouldhave a resistor somewhere to keep it pulled off (or not?) but it seems like weve got it .no noticeable difference to how the rest of the unit works . was affected more in damp or cold weather so may have been partly switching that transistor on ? will try the one in the car on a damp cold day and then remove the resistor in situ and see how it sounds .
if this is the fix then you have solved me a problem that has been frustrating me for years and i spent a hell of a long time replacing stuff /swapping stuff i didnt need to ! learning curve or what !!
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If I'm looking at the correct circuits - the resistor is R1310, a 1Kohm, connected between the base of V1310 (a PNP BC808) and pin 6 of connector P1530/N1530. When pin 6 of the connector is shorted to ground (by a microswitch on the tape deck) the transistor will conduct causing the mute circuits to activiate. The microswitch should be open circuit in normal tape operation. There is no resistor shown to pull the base high or keep the transistor "off", so the base is indeed floating during normal operation.
Lifting R1310, or disconnecting the wire into pin 6 of P1350, will have the same effect - and will stop the mute circuit from working. However, if that works it's not a leaky transistor - it's a problem with the wiring to the connector or the microswitch. However, it might be worth wiring a high value resistor (10K or more) between pins 3 and 6 on the connector. Pin 3 should be a stable 5V supply, and 10K should be more than enough to ensure the transistor (V1310) stays off when it should be off.
PS - I'm dying with man-flu at the moment, so might not be able to add any more unless I make a miraculous recovery.
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this conversation went way way over my head pages ago! ;D ;D ;D
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beleive me this has been screwing my head up for ages too . i started with this a few years back as one of my stereos went distorted .i bought another off ebay and the problem SEEMED sorted but i was curious what was actually wrong with the original unit .didnt realy find the faulty component then so gave up temporarily .THEN the same issue started happening to my other "same " stereos so i thought its a obviously a component that goes faulty with age .makes sense because they used to work and wouldnt be designed to not work right in winter !!
so now (if the problem is found ) il find out over the next few weeks, then although i still dont know which component is actually failing this workaround may just solve the issue anyway and doesnt seem to affect its functioning .radio plays fine ,cd changer and cassette and the display shows what it should and the selector switch does what it should .il try the suggested pull up resistor as the next stage if any issues present themselves .
a great big thankyou to anyone who helped especially mr LCO112G whom i am extremely gratefull to .
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ITS SORTED !
even cutting the track right at the transistor base and removing that resistor didnt solve it .that transistor is so sensitive ! you only needed to get your finger near it and it started to conduct! so i added a 10k resistor across its base and emitter .that keeps it at 4.9v and stops it picking up any interference or reistance to earth . i just confirmed it by putting the unit outside for a few hours .its quite cold and frosty here at the moment .just brought it in and in no time it was wet with condensation BUT CASSETTE SOUND IS STILL OK !!!
so il be doinn this mod to the others .thankyou immensly for your help .i wouldnt have thought of doing this .its either a design fault ,component of who knows what !!
the other issue of not always turning on is not as important now as ive now got useable spare units !
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Glad your perseverance and determination paid off. :y
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yes i usually carry on until i find the problem .just didnt expect this to take a few years !. i couldnt have done it thou without mr lco112g whom ive probably srcewed up his head over this !
however ive learned a lot from him and im now going to have another go at thd original thread issue .(not turing on some of the time !)
i bought two units this month and added the pull up resistor to both of them as they didnt sound perfect in the cold bug now do .one of them however only turns on 9 times out of ten so im ready to bore you all on this issue now !!!!!!!
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The best circuit that I've found that most resembles your description is for the blaupunkt_rcr87_essen_rcr127. You can download that from here : https://elektrotanya.com/blaupunkt_rcr87_essen_rcr127.pdf/download.html
It may not be identical in every respect to your radio - but the tape and audio sections at least sound to be the same.
Once you've got the circuit it's usually fairly easy to diagnose audio faults, although it's much easier if you can follow the audio path through using an oscilloscope. More modern radios tend to be digital all the way through, so they are much more of challenge, especially if virtually all the circuity has been integrated into a single chip.
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thankyou
will print them off at work tommorow and have a play !
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nothing obviously faulty standing out so i swapped the main 40 pin up chip.a heat gun with a shroud managed this ok .fault still remained so added a "reset' button to wake up the processor when it faults .this works every time it fails to power on so i can live with this .
put the reset button on the facia to short the control pin to gnd .works great and looks factory too !
so all problems solved realy .thanks to all for helping . cheers.