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Old 9th Mar 2009, 12:22 pm   #1
'LIVEWIRE?'
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Question Alpine 7148L Car Stereo digital Tuning Fault

Basically, the LCD Display is totally blank, the tuning 'free runs', by which I means the radio can be heard searching for stions but doesn't stop, and the controls, other than the mecnical waveband switch, have no effect. The +5v supply to the uP is present, as are both the permanent and accessory +12v supplies, but the 4.5MHz Crystal Controlled Oscillator doesn't appear to be working, and the o/ps from the uP to the display are all at zero. Having worked in the car audio trade for over 23 years(1971-95) I am familiar with how digitally tuned radios work, and am beginning to suspect that the uP is faulty, in which case this early 80s stereo is scrap, but I don't have, and cannot find asource for, a service manual, or even a circuit diagram, for this model, so I may be missing something that can be fixed. Anyone know what it might be??
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Old 9th Mar 2009, 12:48 pm   #2
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Default Re: Alpine 7148L Car Stereo digital Tuning Fault

The only additional thing I'd check is that none of the buttons are stuck down or leaky

Nick.
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Old 9th Mar 2009, 4:21 pm   #3
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Default Re: Alpine 7148L Car Stereo digital Tuning Fault

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Originally Posted by 'LIVEWIRE?' View Post
but the 4.5MHz Crystal Controlled Oscillator doesn't appear to be working, and the o/ps from the uP to the display are all at zero.
What about the crystal itself? Sometimes they go dry jointed or the crystal may have failed. Should be able to find something from RS or maybe even Maplin. They are usually quite cheap.

Vibration and bumping from our wonderful road surfaces may have seen it off!


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Old 9th Mar 2009, 5:18 pm   #4
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Default Re: Alpine 7148L Car Stereo digital Tuning Fault

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The +5v supply to the uP is present, as are both the permanent and accessory +12v supplies, but the 4.5MHz Crystal Controlled Oscillator doesn't appear to be working, and the o/ps from the uP to the display are all at zero.
If there's no oscillation, then that is most probably your problem; don't expect anything else to work.

Just probing around a crystal oscillator often gets it working, but it's a bit impractical to have to get an oscilloscope out every time you want to listen to the radio A crystal or ceramic resonator is basically a delay element, wired between the output and input of a Schmitt-trigger NOT gate. So, one lead of the crystal is driven high; and after awhile, the other lead goes high -- now the output of the NOT gate changes to low, and after awhile the other lead of the crystal goes low; and the output of the NOT gate goes high again. You should see a nice square wave (the drive signal) on one leg of the crystal and a rough sine wave (the delayed signal) on the other end. Re-soldering the joints on the crystal and/or any nearby capacitors (usually a few pF on one or both leads of the crystal) might fix it. Otherwise, replace the crystal.
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Old 9th Mar 2009, 5:57 pm   #5
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Default Re: Alpine 7148L Car Stereo digital Tuning Fault

It's best to use a x10 probe as the capacitance of a x1 probe often stops them working even if they're OK.
Is it really a crystal (metal can) or a ceramic resonator?
What is the processor?
Alan
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Old 9th Mar 2009, 6:21 pm   #6
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Default Re: Alpine 7148L Car Stereo digital Tuning Fault

You've probably already checked for this one, but I remember a while back spending an awful long time trying to find a fault on a car stereo only to eventually twig that the illumination lamps were blown in the removeable "security" front and they were required to pass current into the radio to enable it to work properly. The radio thought the security panel was missing and so refused to work!

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Old 9th Mar 2009, 8:33 pm   #7
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Default Re: Alpine 7148L Car Stereo digital Tuning Fault

Biggles, This Radio is about 25 years old, and it doesn't have a removable security panel-it's a 'spindle fit' set. Come to think of it, the pilot lights don't work, so I'll replace them & check again. Alan, it is actually a low profile 4.5 MHz Crystal, and I am wondering whether to replace it and/or the two 22pf ceramic caps, having already resoldered the joints of these components to no effect. The Processor is a large DIL IC made by or for Alpine, and programmed for the series of stereos in which it was used. The radio is in my workshop abt 3/4 mile from here, and I didn't make a note of it's type no., but, sure as eggs are eggs, it will be obsolete, and unobtainable.
Correct me if I'm wrong, AJS, but don't you mean a NOR gate-I can't think what a NOT gate is. I remember being taught about AND, OR, NOR, NAND gates, though. Unfortunately, my 'scope is out of action, and I'm too involved with other things to find the time to fix it. Catch 22 I know! Thanks for reminding me about the theory, though.
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Old 9th Mar 2009, 8:37 pm   #8
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Default Re: Alpine 7148L Car Stereo digital Tuning Fault

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I can't think what a NOT gate is.
http://en.wikipedia.org/wiki/NOT_gate
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Old 9th Mar 2009, 11:27 pm   #9
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Default Re: Alpine 7148L Car Stereo digital Tuning Fault

I stand corrected!
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Old 14th Mar 2009, 11:48 pm   #10
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Default Re: Alpine 7148L Car Stereo digital Tuning Fault

Having rechecked around the 4.5MHz Crystal, I decided to replace the two 22pf capacitors associated with it, plus a decoupling capacitor on an adjacent line. Hey Presto! The radio now works fine on all 3 wavebands, although there are intermittent problems with the 'Tact' switches used for station selection, etc. Changing them is gonna be a fiddly job, but one that must be done for the sake of reliability. I should, however, have checked those 22pfs sooner. Only being repairing radios for 37 years, that's all!!!
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