Board layout for 69 Chevelle radio

12/30/2010 10:44:45 AMRogers flipdial

I have the schematic for this radio - a model 91APB1 - but no board layout - its for my buddy's all original 69 Chevelle - it stopped working and needs a recap and realign - having the board layout would speed up the recapping.
I asked him if he wanted to add a jack so he could hook up an mp3 player and he said yes.
The way I would do things is to have a mono cable coming from the mp3 to the car radio and just have the jack switch the audio - I've read that some people install a micro switch so that it switches over when you turn the dial all the way to one end.
For mono audio - are people just using one channel from the mp3 - or are they converting the mp3 files to mono - (I know I can do that with Audacity - but it might be pretty time consuming)
Steve

12/30/2010 11:52:28 AMWarren

For a mono input to the radio, it is common to join the left and right channel from the MP3 player together at the stereo input jack. Use shielded ground wire out of the radio to the input jack. A make and brake jack can be used, to cut out the radio and just use the amplifier section for the player. Similar to how a headphone jack would work to cut out the speaker when headphone plugged in.

12/30/2010 8:36:17 PMEdd

Sir Rogers Flipdial. . . . .

My Samuels of that unit is currently out at the “mule barn” . . . . . BUT . . . not to worry . . . as I can ease the locating caps on PCB’s task by approximately 75% for you . . . . PLUS , helping for any other Pee Cee Bee’s in the future.

Have the board where you can make alternate views of foil and component side.

Place a high intensity red laser pointer on the FOIL LAND SIDE, close to where you THINK a capacitors lead wire is coming down from.
And THEN you view from the component side of the PCB.

Move the pointer just to the side of the land such that it will be illuminating THROUGH the board just to the EDGE of the capacitor.

When viewed from the component side, that little glow thru the board should then show you where your pointer is physically located. Then, move the pointer inwards again, towards your perceived guesstimation of the wires location.

Move your eye ball viewing back to the foil side and your pointer should then be resting right atop the land that the wire terminated into.

Mark the land with a blue Sharpie check / dash mark. . . . . . locate that caps OTHER associated wire and its land, mark it, and then circle the two with the Sharpie.

Move on to your marking up of all of the other caps wires in the unit.

And to think . . . . that way y y y back in 1985 . . . I built this apparatus up by using a 5KV gas helium laser driver, feeding into a surplus surgeons fiber optic endoscope.

Now a red $7-8 premium Laser pointer will fill the bill and be small enough to “kiss” the PC board directly.

(Those little pressed tin can cylinder $3 “toy” Lasers won’t quite fill the power output bill . . . take a PCB with you for testing before buying . . . less you have a Laser already. )

(BTW . . .some old blue or green tinted PCB’s really dampen the red spectral pass thru level.)

The next most helpful bit on cap ID will be the use of a dental mirror in reading the cap values printed on their sides, unless you just flat have to pull them out to be able to read values.

I avoid that with some strips of ½ by 3 in clear plastic strips with some shiny chrome foil stuck to the bottom ½ by ½ end so that you have a micro mirror that will let you manipulate in and read values printings as close as 1/8 of an inch to a capacitor.

Thassit . . . .

73's de Edd

:For a mono input to the radio, it is common to join the left and right channel from the MP3 player together at the stereo input jack. Use shielded ground wire out of the radio to the input jack. A make and brake jack can be used, to cut out the radio and just use the amplifier section for the player. Similar to how a headphone jack would work to cut out the speaker when headphone plugged in.
:

12/31/2010 11:05:22 AMRogers flipdial

That is a Fantastic method - I feel like an idiot for not using that before - I've never owned one of those laser pointers so it never occurred to me.
I'm perfectly capable of tracing out the circuit - but I have very limited time to spend on repairs these days - so I try to get the board layout because I know it will save me a lot of time - and as I get older I get lazier I guess, and since I have about 70 radios waiting for repair I don't like to spend extra time on any of them.
25 years ago - straight out of college I started fixing 2-way radios - At that time the radios still used leaded components mixed with chip components - many of them had multilayer pcb's - the manuals would be a small book - if you had a 3 layer pcb - that was in turn plugged into another 3 layer pcb -and you wanted to measure something - you had to unfold all the board layout diagrams and follow the traces through to find out where you actually get to a point where you place your test lead - multi-layer pcbs gave a lot of trouble back then - especially in a mobile environment - and especially here in our area - where inside vehicle temperatures could range from 100 degrees in the summer to 30 below in the winter (I've actually seen minus 40 several times) - the boards expand and contract and the plated through holes break their connections to the other layers - then the whole mess becomes intermittent - sometimes you could locate the faulty trace and install a jumper wire - that was when the radios were worth fixing - now they're so cheap - nobody wants to pay to have anyone troubleshoot anything - and now the radios have been condensed to one or two pcb's - someday I'm going to organize a series of pictures that show the progression of miniaturization of the stuff I work on.
The laser pointer would have saved me a lot of time and eye strain over the years.

12/31/2010 11:24:46 AMRogers flipdial

Thanks - I would caution anyone about joining the left and right channels - many devices use a BTL output - there could be a dc voltage present - I would imagine most MP3 players would have an IC driving the speaker outputs and might have some sort of short circuit protection - but still I wouldn't do it.
My thinking - correct me if I'm wrong - is that since I'm talking about an MP3 player - most of what will be played will be in stereo - if I only use one channel - it won't sound very good - I either need to MIX the two channels - or change the music to MONO recordings - I realize its a vintage AM car radio - the audio quality isn't going to be very good anyway - but I might as well make it as good as I can.

12/31/2010 12:22:42 PMWarren

Agree with most of what you are saying. These little portable mp3 players have just a few mw of output. There is no protection circuit. By connecting the two channels together the mp3 player don't care. Have done it this way with an mp3 player for AM broadcast unit. Try this on a single speaker with both channels tied together. See if this works for you.