Hello,
I have an old radio with cassette tape player and I am trying to convert it to aux. I noticed there is already a similar thread opened at:

https://forum.allaboutcircuits.com/threads/connecting-aux-to-tape-head.147054/

but I don't want to steal it. I already tried connecting aux directly to the tape head, the sound was terrible. Then I found the dolby chip (as said in the thread). There are two of them so I assumed they are for sperete channels. Since those chips were made for noise reduction via tapes, I connected the aux to the outputs of those chips (pin 7 on both chips to the aux signal channel and pin 9 on one of the chips to the aux ground). When I try to play a song, I need to keep volume on the phone at about 80%, if I go over that the sound becomes distorted. Otherwise sound is quite good with barley any noise, but the volume is too low. It's not completly quiet as it was when I tried to skip the whole tape related circut and connected aux directly to the amplifer (atleast I think that's what it is), but it is about half of the volume as it was via cassette. So here I come to my question, is there a way I could make that sound any louder? I am assuming that this dolby chip also amplifies the sound besides noise reduction. I also tried to feed the aux to the A and B inputs (pin 5 and 2) on the chip since there are some amplifiers on the scheme in data sheet, but the volume was even lower.
I attached the data sheet of dolby chips and some pictures (the pins are soldered to the aux on the other side).

Thanks for help in advance!

 

Welcome to AAC!

For starter, tell us the make and model of the radio. Maybe there is a circuit diagram out there in the wide world web.

Taking the signal from the tape head is not the ideal place to get a usable signal. You want to tap off the signal at the audio amplifier stage.

Show us some overall view of the radio and we may be able to identify the audio output.
Show us where the loudspeaker connections are on the board. The audio amp may be nearby.

 

When I try to play a song, I need to keep volume on the phone at about 80%,

What output from the phone are you using to connect to the tape player?
At the "amp connection" is that 3 wire plug going to the volume control?
You might try connecting to pin 5 of the NE649 through a 1uf capacitor and see if that helps.

 

Oh, I didn't think of this. Which way are you going?
What is the input source?
Where is the sound being reproduced?

 

You can usually tap an audio signal from across the volume control, not the wiper, but from the gnd end and the "hot". Feed it out via a 100nF cap.
That is assuming it has an analog control, and not a digital one.

 

Welcome to AAC!

For starter, tell us the make and model of the radio. Maybe there is a circuit diagram out there in the wide world web.

Show us some overall view of the radio and we may be able to identify the audio output.
Show us where the loudspeaker connections are on the board. The audio amp may be nearby.

Oh I forgot about that. The radio is Benytone MPC-1S. I already looked for documentation but couldn't find anything.
Here is what the main unit looks like:

Those screws at the top are for radio antennas (FM/AM), I assume the whole pcb on that "level" is radio related. On the second pcb there is a so called "phono" input on the left and on the right there are speaker outputs (L/R for 4 Ohm and 16 Ohm speakers with a switch). Since the volume control, equalizer and heat sink are there, I assume there is amplifier nearby.

What output from the phone are you using to connect to the tape player?
At the "amp connection" is that 3 wire plug going to the volume control?
You might try connecting to pin 5 of the NE649 through a 1uf capacitor and see if that helps.

I am using 3.5 mm jack.
That 3 wire plug is going to the first pcb on top, since volume control knob is connected to the second pcb, I don't think it's going there.
I will try that, might take me a while to find/order a capacitor.

Oh, I didn't think of this. Which way are you going?
What is the input source?
Where is the sound being reproduced?

I am going from phone jack output to the radio, currently signal wires are still soldered to 7th pin of the dolby chips.
Don't know the answer to that sadly.

You can usually tap an audio signal from across the volume control, not the wiper, but from the gnd end and the "hot". Feed it out via a 100nF cap.
That is assuming it has an analog control, and not a digital one.

I would rather feed the signal to the tape related circut, since radio has a switch for other inputs, which still work. Tape motor was stuck anyway so there wasn't much to lose. But anyway I could still add another switch, if that would work.

I can't come close to the part where volume wiper connections are going without taking everything apart, but if that's the only way then I can try.

The volume control wiper connections in case I can connect it here:

 

https://www.user-manuals.com/service-manual-BENYTONE-MPC1S.html
Is what I found.

 

I am still not clear on what you are trying to do.

I see an integrated system consisting of three components, a tape deck, a radio receiver and an audio amplifier.

You are attempting to extract a signal from where and sending it to where?

Or you want to use an external signal source and input it to the amplifier?

 

https://www.user-manuals.com/service-manual-BENYTONE-MPC1S.html
Is what I found.

Let's say I buy this, I probably won't be able to share it here due to copyright?

 

Let's say I buy this, I probably won't be able to share it here due to copyright?

Probably correct.
But you can share a part of the circuit I think. So, do a screen grab of the audio input section.

 

I am still not clear on what you are trying to do.

I see an integrated system consisting of three components, a tape deck, a radio receiver and an audio amplifier.

You are attempting to extract a signal from where and sending it to where?

Or you want to use an external signal source and input it to the amplifier?

I want to use external signal source... jack connection from my phone to the radio, preferably somewhere to the tape deck system or directly to the amplifer.

 

What are the RCA connectors on the back?
Is one an input already?

 

What are the RCA connectors on the back?
Is one an input already?

Yes, the ones on the left are L/R inputs for "phono". They have low frequency boosted and playing via aux from phone to them sounds distorted. It sounds similar as if I connect aux to the tape head.

 

Use the FUNCTION select knob to select which input you would like to select.

Choose any one of the inputs. Disconnect the existing source and feed your AUX input instead.

PH (phonograph) input has RIAA equalization. This would be the most convenient input for your AUX input if you bypass the RIAA circuitry.

 

One method is to feed your signal in at the volume control, using the outer two connections,or at the function switch as said above.

 

There is a ribbon cable connecting function select knob with the switch on pcb. Where do I have to solder the aux? I haven't seen a switch like that before.
I got the manual:
View attachment MPC1S_SM_BENYTONE_EN_Stran_07.jpg
Also the scheme of the amplifier area if it's of any help:
View attachment MPC1S_SM_BENYTONE_EN_Stran_06.jpg

 

The second attachment shows a mic input,
I would try that. May need to reduce the signal from the phone using a couple of resistors.

 

I suspect that the mic input is for recording to cassette only.

 

Most likely MrChips.
Urbi do you have a schematic in that manual?

 

Afaik mic is for recording only... I tried all different modes and couldn't get sound from mic input.

There is a diagram of everything combined, but it is barely visible:
View attachment MPC1S_SM_BENYTONE_EN_Stran_16.jpg

And a cassette circuit:
View attachment MPC1S_SM_BENYTONE_EN_Stran_08.jpg