Hi all,

I have this 1990 OEM AM/FM/Cassette Delco radio and have the Radio Repair Schematic for it too. I will be removing the Radio antenna from my car and would like to make AM and FM their own AUX Audio inputs. Meaning, when FM is selected, AUX input 1 is playing. When AM is selected, AUX input 2 is playing. Leaving the Tape deck intact to play my tapes.

Its been 10 plus years since I looked and read ANALOG schematics. So I am having difficulties figuring out where I can place the AUDIO inputs to make this happen. There is a clear connection point 'AUD L OUT' & 'AUD R OUT' at plug 'J1' which I have tested and will work, but has only ONE AUX Audio Input. It over-rides ALL Audio ANALOG, AM and FM both which doesn't allow for selectable input.

U1 , U2 , and U3 are for the FM section. U4 is for the AM. I do see that pin 19 at the U3 is labeled 'AUDIO INPUT'. I did cut the circuit after 'C50' that leads to U3/pin 19 and soldered a 3.5mm Audio jack at U3/pin 19 to test if it would play through the Radio Amplifier. But I heard nothing.

I ask if someone can look at schematic below and guide me to a solution that will allow for (2) external inputs that are selectable when AM and FM.

The following Schematic for the radio section to the ACTUAL Delco radio that I have.

Thank you.

TONY

 

The circuit shown is incomplete in that iit shows none of the audio amplifier section and none of the switching arrangement or the tone controls. Nor any details about the cassette deck interface.

 

The circuit shown is incomplete in that iit shows none of the audio amplifier section and none of the switching arrangement or the tone controls. Nor any details about the cassette deck interface.

Hi MisterBill,

I have added the LOGIC BOARD schematic and PCB Layout documentation I have for the Delco radio Below.

Thank you for your time trying to guide me to a solution.

TONY

 

Here is the AUDIO BOARD schematic and PCB Layout documentation I have for the Delco radio Below.

 

I do NOT have the CASSETTE schematic and PCB Layout documentation.

 

I don't see the auxiliary inputs anywhere on those schematics.
Must be on the cassette board unfortunately.

 

OK, and I have been thru the circuits a few times and it certainly appears that the AUX input, or inputs, no hint if there is one or a pair, are associated with the cassette module. That would make sense. In that case, those inputs might do the change over from cassette to aux simply by inserting the plug, and having "cassette" selected, if that is an option.

I seriously suggest not hacking into the radio circuitry, both because it is quite complex, and because it is totally unforgiving of any error in connection that might cause a momentary short circuit. Also, those radios are rather miserable to work on.
So please take a look at the "AUX" input and let us know, if you can tell, if it has only three wires, in which case the seection is from elsewhere, or if it has at least five wires/connections, in which case the act of plugging in will make the change.

 

The question is, how are the auxiliary inputs selected normally?

 

Hi all, . . . . . . . .
TONY

Your solution lies in the switch that selects AM/FM. What that solution is will be depend on the Audio part of the unit.
Assuming that the selector switch also handles the power feed the the AM and FM part of the Radio, I would use an Analog switch to select the Aux 1 or Aux 2 to feed the Audio part.
The Am ans FM tuner outputs will have to be disconnected.
As others have mentioned, the most vital schematic is missing.

 

The question is, how are the auxiliary inputs selected normally?

I was taking a guess that it may be selected by inserting the plug, which might simply operate a direct switching, or else by some other scheme.
The diagrams get confusing because the "U" seems to be applied to both IC devices and also module assemblies. And it seems that the AM and FM receivers are not so very simple. I see an ugly disaster with that approach. The system is rather seriously complex.

 

I don't see the auxiliary inputs anywhere on those schematics.
Must be on the cassette board unfortunately.

He us trying to replace the FM and AM selections with two new aux inputs, as I understand it. There are no existing aux inputs.

The audio source is switched by the microcontroller via the chip called “mute control”. But it only switches between the cassette and the radio. Both AM and FM feed into the same input of that chip, so that complicates it. You could add 1 aux input by replacing the radio inputs to that chip, but if you want two, you would need to select between them by some other means.

 

It could also work by adding a connection to the "Cassette Input" location. That could be either just in parallel, or an alternative arrangement. That point is probably more accessible because of the cassette being a separate assembly.

 

He us trying to replace the FM and AM selections with two new aux inputs, as I understand it. There are no existing aux inputs.

The audio source is switched by the microcontroller via the chip called “mute control”. But it only switches between the cassette and the radio. Both AM and FM feed into the same input of that chip, so that complicates it. You could add 1 aux input by replacing the radio inputs to that chip, but if you want two, you would need to select between them by some other means.

You understand Correctly. Me wanting 2 AUX inputs is tricky to say the least. Because there are (2) audio sources already (AM and FM), I was hoping that it would somewhat straight forward. But of course, it is NOT because both AM and FM go to one single Audio input.

I was taking a guess that it may be selected by inserting the plug, which might simply operate a direct switching, or else by some other scheme.
The diagrams get confusing because the "U" seems to be applied to both IC devices and also module assemblies. And it seems that the AM and FM receivers are not so very simple. I see an ugly disaster with that approach. The system is rather seriously complex.

I agree ... after taking a hard look at things. I did more testing today on the Receiver board. Looking at J1 8pin plug ... Pin 4 and 5 alternate 5V when selecting between AM and FM. So may guess is, since both are sharing the SAME Audio output. The power to the each receiver (AM or FM) simply turns OFF to allow the other to play. Meaning, when FM is selected. The FM circuit has 5V to work and AM has NO power and stays OFF. Then the opposite for AM to work.

When using the cassette. The RECIEVER audio is bypassed/turned-OFF and only the cassette can be heard. But the last playing Receiver portion, AM or FM, continues to plays while the tape is running. Just the Radio Audio is not heard.

I haven't fully figured out Pin 6 purpose. I know it says MUTE. But what (and When) is it MUTING.

TONY

 

When using the AM and FM buttons to activate the aux inputs do you want the display Off as well?

 

It appears that the TS does not want to add another switch. That would certainly be the more confusing to anybody else. How does the AM receive mode drive both channels with mono? With the FM stereo two channels being separate.
Or is it the novelty of having the AM/FM selector do something else??

 

This is how I would do it using a MAX4614 analog switching IC.
When FM is selected Aux #1 is activated and AM selects Aux#2
On the Receiver board the 5 volt outputs from VR1 should connect to the MAX4614 only in order to shut down the AM and FM circuits.

 

When using the AM and FM buttons to activate the aux inputs do you want the display Off as well?

Having the display is fine.

It appears that the TS does not want to add another switch. That would certainly be the more confusing to anybody else. How does the AM receive mode drive both channels with mono? With the FM stereo two channels being separate.
Or is it the novelty of having the AM/FM selector do something else??

Pressing the Tuner knob inward selects AM and FM option. I wish to use no other switches. I like the clean factory look.

 

This is how I would do it using a MAX4614 analog switching IC.
When FM is selected Aux #1 is activated and AM selects Aux#2
On the Receiver board the 5 volt outputs from VR1 should connect to the MAX4614 only in order to shut down the AM and FM circuits.

I never thought of using the VR1 Pins 3 & 5, but that makes sense. I did measure the output and it IS 7.8V, not 5V.
Shouldn't it be like this, for the MAX4614 output to go to the AUDIO INPUT?

I tried to find the Voltage regulator VR1 "16012040" datasheet, but that part number pulls nothing in on Google.

 

I never thought of using the VR1 Pins 3 & 5, but that makes sense. I did measure the output and it IS 7.8V, not 5V.
Shouldn't it be like this, for the MAX4614 output to go to the AUDIO INPUT? View attachment 332009

I tried to find the Voltage regulator VR1 "16012040" datasheet, but that part number pulls nothing in on Google.

No leave the audio lines as shown.

 

I did measure the output and it IS 7.8V, not 5V.

Good to know as it was hard to tell from the schematic.
Adding R1-R4 will drop the voltage to a safe level with this mod.