Any info on adding an aux. input to vintage radio?

[CandyAppleRed 10]

Numbers and bottom photo. To orient yourself, the bottom photo shows the On/Off, Volume and Balance Pots at the top.

 

 

 

[CandyAppleRed 10]

I will sleep much better knowing that this radio passes the radiation test!

[CandyAppleRed 10]

I did find the speaker wire color code if anyone else is interested:

 

Black, White tracer = Ground for all speakers

Pink, Green tracer = Front Left

Pink, Blue tracer = Front Right

White, Green tracer = Rear Right

Orange, Green tracer = Rear left

[danno 128]

Your radio is from a 1975 Ford, with the D5 in the part number. I will need to check my schematics and layout tonight to see if it shows something that will allow me to tell you what to do next. I have an oscilloscope, and with that it would be easy for me to find the place to cut, even without a schematic. Do you have a digital voltmeter that will read AC volts under 1 volt? If you have one, that could also give me a way to find out more.

[CandyAppleRed 10]

Yeah, I knew it's a 75. I have two digital multi-meters. One simply has a AC volt position, the other has a 600VAC position and a 200VAC position. I don't really know if either will register a reading below 1VAC. I would think they would, but don't know for sure. I'm surprised there's even AC current in there anywhere.

[CandyAppleRed 10]

During my research I seem to remember reading that an aux. input could be just wired to the volume pot with a switch to turn off the AM/FM input and replace it with the input from the aux. input. Am I wrong in assuming that the aux. input can just be correctly wired to the volume pot? There's actually a stack of 3 pots and the on/off switch. I assume one pot is volume, one is balance, I don't have a clue what the 3rd one if for. I would think that with an ohm meter a person could figure out which pot is the volume one. Not sure how to figure out which of the 3 contacts on the pot is the "in", the "out", and the "slider" though.

[danno 128]

Yes, there are 3 potentiometers on the shaft. One for left volume, one for right volume, and one for balance. If you wire your new input to the volume, you will not have balance control. But you possibly can wire it to the balance knob. Put you ohmeter on it to see which one is the balance. The way the circuit works, there is an output from the decoder amplifier, which goes from this to the balance knob. This same connection then goes on to the tone and volume knobs. You would need to cut the connection between the balance knob and the decoder amplifier and connect your MP3 input to this.

 

So you would be close and almost correct in your statement about connecting to the volume control. I hope some of this makes sense.

 

I also need to find a schematic and layout for your radio. I do not have one for your exact model. I have a real good guess of what to modifiy on the one you have and would have no problem figureing it out if I had it with me, but to tell you is difficult. Do you have the schematic and layout? Sam's Technical manuals makes a who set of these, you would need the one for your radio.

[danno 128]

The picture no longer is visible. What do I need to do to see it? It worked yesterday, I don't know what happened.

 

What is the part number for your radio again?

[danno 128]

The problem I mentioned in my previous reply might not have been clear. In your previous reply, you put a photo of the radio with the part number and bottom of the circuit card. Apparantly these photos are not maintained on the thread, so you need to post them again. I need to know the part number of your radio.

[CandyAppleRed 10]

Sorry, just got home. All the photos come up for me. Try again, it may have been a server glitch. I don't have any schematics for the radio. I'll check back a bit later and repost the pics if necessary but I think they're working.

[danno 128]

I can see the photos I included, but not the ones you put in. All I see is a red X. I sent a different message about the red X, and I was told that gets put in place of attachements after some time has passed. You can probably still see them because you put them there.

 

Anyway, repost them and I will save them to my computer next time.

Danno

[CandyAppleRed 10]

Here's the photos. I really appreciate all of your efforts. I didn't expect this to turn into a week long effort on your part. I thought it might be as simple as a few paragraphs from you, and a few soldering iron burnt fingers on my part. I know that you do this, at least on occasion, for money and I don't feel right about taking up this much of your time pro bono. I think a more appropriate route for me would be to just install a small amplifier. I can then hook up the radio and an aux. input to that. An additional benefit would be some more wattage. I doubt this thing puts out more than 7 or 8 watts on it's own. In any case, here are the pix, though I think it best to just go the amp route. I would have taken the radio apart this much anyway to clean and lube it so no harm done there. Even though it's a new old stock radio there seems to be a lot of cardboard box dust in it.

 

 

 

[danno 128]

Once again I cannot see the photos. I see them in other threads, so I don't think it is me. Are you attaching the photo or just a link to it?

 

Your idea of an seperate amp will certainly work. You can usually go directly from the headphone output of your MP3 to the low level input ( with the RCA plugs) of your amplifier. Volume control is a pain, though, to do it on a mp3. It is usually a 2 hands deal, risky when driving.

 

You are right in that it is not a simple deal to add the mod to these radios. For me, with the radio in front of me, it is simple. If you still want to try it, we can continue with the description...

Danno

[CandyAppleRed 10]

I would certainly like to get this done. I just don't want it to become a thorn in your side. I also have a tiny concern with experimenting with a NOS radio. But I'm willing if it's not too much bother for you.

 

I was linking to the photo. I'm not sure why that wouldn't work. It works on all the other sites I've done it on. I wonder if anyone else is having trouble seeing the pix.

 

I'll try attaching the file and see what happens.

[69RavenConv 286]

I can see all the pics. This is an interesting thread, I've been following. Are you sure you're logged in Danno? That's the only reason I can think of that would prevent you from seeing the photos....

Edited April 19, 2011 by 69RavenConv

[danno 128]

I have been logging in every day, but today all the photos are viewable. Yesterday and all last week they were not. I swear I have not changed anything in my computer. Today they are viewable, yesterday they were not.

 

I will look to see if I can get a schematic and layout for the circuit card. Then we can proceed to the next step and you can decide if you want to tear into this when I get the schematic.

 

More later...

Danno

[CandyAppleRed 10]

Awesome. Thanks.

[danno 128]

I am going to pick up a schematic for your radio tonight. It is in SAM's photo-facts AR-188, and it turns out the Minneapolis Library has a copy of it. The library is a free source for these photo-facts, a nice thing to have available here. I will do some photocopies and I will know a lot from this.

[danno 128]

Ok, I got a schematic and layout, they are attached. The modification can be done with a few of the components right behind the volume control.

 

The schematic that I attached represents the wires and circuits of the radio. You can see my modifications to the circuits. There are 2 places where the signal goes from the decoder amp to the balance control that need to be cut. This is where you disconnect the signal from the AM/FM radio coming out of the decoder amp and insert your wires from the MP3.

 

The easiest way to do it is cut R313 and R314, they are both right behind the volume control on the circuit card. They should have blue, grey, and red stripes on them. This is where the MP3 signal goes. You will need new resistors for R313 and R314, you will need to get them at a Radio shack store. These are 6800 ohms, you will need 2 of them, the wattage of the resistors does not make any difference, get them as small as possible ( probably 1/4 watt).

 

So clip off the brown portion of R313 and R314 so you have something to solder the wires to. The wires will go to your switch or relay can connect to these leads.

 

Is this making sense so far? Are you totally confused? I have left off some details, we can go into those later.

[CandyAppleRed 10]

This is going to be fun! I'm not confused. We're clipping the signal after resistors 313 & 314 and injecting the mp3 channels into the capacitors at connections 110 & 121, then using a double pole, single through switch to go between the radio and mp3 player, piece of cake...

 

It's going to take me a while though to get my bearings. I just did a quick look-see at the radio and found the capacitor between 121 & 122 no problem. Tracing to the resistor between connection 121 & 108 is going to take a bit more time. As you know, there's printed circuits on both sides of the board. The ones on the bottom are pretty easy. The ones on top are a bit harder to follow because of the forest of ceramic disc capacitors and other junk in the way of seeing the circuit. I'm sure I can "git-er-done" though. I'll have to pick up a switch and some wire. I'm thinking 8 gauge should be good. :P

 

I'm going to go take a better look at it now. I'll poke back in here if I run into any questions.

 

Thanks!

[danno 128]

Good, it sounds like you are following me on this. Very few people could.

 

I realized one thing after sending the email. The way I have it, if the balance is all the way to one side, it shorts out the MP3 output on that side. Most radios I have done this on have the balance after the volume, so it is not a problem. So for this, I would add 100 ohm resistors on teh MP3 outputs. I added them in the schematic I attached.

 

Let me know of your progress...

[danno 128]

By the way, those resistors you are looking for are right next to those capacitors. Those potentiometers are the volume and tone controls, and at the very back of it, right next to those capacitors, is the on/off switch for the radio. I am wondering if the resistors you are looking for are under that switch. You could make the connection at those capacitors if it is easier.

[CandyAppleRed 10]

Okay, made note of the resistor addition to the mp3 in's. I just got back from Radio Shack. They didn't have any 6.8K resistors so I picked up 4.7K and 10K figuring close was good enough, but wasn't sure whether to go higher or lower. I will check around and see if I can find 6.8K at another store. About a dozen years ago I tossed out a box of about 200 shinny new resistors and a bunch of other items I thought I'd never use anymore. Figures....

 

I'll take a closer look at the radio after supper. Dial 911 if you see smoke!! :P

[danno 128]

If you will be making another trip to Rasio shack, it would probably be fine to use 6200 or 7500 ohm resistors in place of the 6800 ohm values. The 5000 ohm might allow distortion at high volumes and the 10000 ohm might not allow the volume to go very high. You can try the 5000 ohm part and see how it works. If it does not distort too bad when you are on an FM station, then you can probably get by using it.

 

But to be safe, see if you can find the 6800 ohm ( or close) resistor. Also pick up the 100 ohm part at the same time.

[danno 128]

I am curious if anyone else is considering undertaking this task? 69gmachine, are you going to try? I know it can be done, but every radio is just a bit different.