Как найти линейный вход на магнитофоне

This is a write up of how I attached a line in to an old radio cassette recorder.
I had an old Philips Radio Cassette Recorder (model number DR193). The radio was for MW and SW frequencies. The cassette player and the radio no longer worked. The single speaker worked – this I knew only by the noise it produced when I worked the radio needle.
There was no line in, just a line out.
So, I opened up the radio to see if I could somehow cut a wire here and there and attach a line in. In the end I managed to add a 3.5mm pin to the radio and now it can play any device with a 3.5mm jack.

I must mention here that I do not know the reason it works and I just managed to make it work by chance.

Before starting on I must also mention that I got the idea for this project by reading the excellent Instructable by TheRealAnubis (https://www.instructables.com/id/Adding-a-line-in-to-a-boombox-with-a-tape-player/)

Apart from not having any knowledge of how to go about the objective, I did not have any proper tools either! Hopefully, if you decide to follow this, you’d be better equipped.
The stuff I used was
1. A simple screwdriver
2. A Philips head screwdriver
3. A pair of forceps
4. An old gel pen refill
5. A used safety razor blade
6. Electrical tape

This may seem a bit unusual list but let me explain

The screwdrivers are obvious. The Philips head screwdriver was used to open up the body and the regular one to open a single screw holding the board.
Forceps and pen refill – This helped a lot with handling the thin wires. The inside of the radio was a bit crowded and using fingers all the time was a bit hard. These two helped a lot.
Razor blade – To peel back the casing of the wires. I could have burned that but these provided a lot more precision. If you decide to use one then hold the wire from below and make the blade go upwards – away from your fingers (or forceps)
Electrical tape – I did not have any soldering equipment so I used the tape.

Upon opening up the radio I found this. I have labelled the parts which I initially thought I should concentrate on to get the task done.

#1 This is a small board. It has a connection to line out. It is connected to the speakers and to the radio board (#3) by wires #2 & #5 respectively.
#2 These red-black wires connect the speaker to board #1
#3 The green radio board. It has a knob to operate the needle.
It is connected to board #1 by a pair of wires (#5),
It is connected to power source by a pair of wires (#4)
It is connected to the cassette player board (the orange buff coloured one) by 2 sets of wires of 3 and 4 wires respectively.
#4 These yellow wires connect the power source to the radio board (#3)
#5 These red-blue wires connect board #1 to board #3

I concentrated my attention on wire #5. These were the only ones that connected either of the board to board #1. Board #1 caught my eye because it had a line out and was the only one to be attached to the speakers too.
So I thought that if any sound was to be played by either the radio or the cassette player it had to go through board #1.
So it I cut the wires leading away from board #1 and make my attachment to them then I could succeed.
Obviously, I failed to reason that with only board #3 getting any power and me cutting the connection between board #3 to board #1 I was heading for sweet failure!

I have failed to explain/mention what I intended to add to the radio. I planned to add a wire with a 3.5mm male pin so that I can attach any device with a female 3.5mm pin to the radio’s speakers.

For this purpose I cut in half a 3.5mm male male connector wire. After cutting in half, I pealed back the casing to expose (the truth?, Nah!) the naked (man!?! Nah) metal wires.
I used the safety razor blade to peal off the plastic casing. There was an external casing (black) and an inner casing (yellow)

So back to the story. I had explained how I thought that I needed to intercept wire #5 – the red-blue wire that connected board #1 to board #3.
So I cut wire #5, pealed it back to expose the metal wires and connected it to the metal wires of the 3.5mm connector wire.
Just as I had pealed back the 3.5mm connector wire, I used the safety razor blade to peal back the casing of wire #5

To test if sound was being produced I held the two sets of wires together (attached). Very faint sound was produced so I knew I was going in the right direction.

I did not have any soldering equipment so I connected the two sets of wires by simply twisting them together.
When I played the iPod I was in for a surprise! The sound was produced even if there was no power source. Apparently, the speakers must be drawing power from the iPod itself. The sound was not faint or distorted either but it was not very strong either. It was enough to fill a quite / small room with light music but was not much for a larger room or a louder place. Also, since this was not what I had intended to do I decided to go back.

I untwisted the wires and connected the original wire (#5) together. I tested it again and there was no sound when there was no power but when I connected the power there was the old, and familiar, noise of static once again.

After noting that I had made a mistake by cutting wire #5, and after reattaching it, I looked for the other wires that I should have cut instead. I observed that there were 2 sets of wires connecting board #3 with the orange-buff coloured cassette player board. One set had 3 wires and the other had 4 wires.
I thought that I should intercept either one of these sets of wires. The reason behind this was that the speakers were connected to only board #1 and board #1 was connected to board #3 and not to the cassette player board. So, for the cassette player to work the sound must travel from the cassette player board to board #3, then to board #1 and then to speakers of line out. Hence, I needed to target the two sets of wires connecting board #3 to the cassette player board.

But I did not know which set of wires carried the sound.

I looked closely and found that the place where the set with 3 wires was connected to the board, the boards were marked with plus and minus. There was no marking where the third wire connected the board and I assumed that it must be the earthing wire.

If you look at the photo you would see the reattached wires #5 underneath which I have placed a white paper (just to act as a background for the photo). You can also see the two sets of wires coming from underneath the board #3 and connecting with the cassette player board (on the upper right side)

Although the set of wires with 3 wires were labelled I did not know if these were that carried the sound or not and I had no way of knowing as I did not have the expertise.
Finally, I decided to go with them mostly because 1. They were labelled and 2. They were fewer in number so if it turned out that I had made a mistake I would have to re-attach lesser number of wires.

So I cut the wires and peeled them using the razor blade.

In the photo you can see the cut and peeled wires red and black.

After peeling back these wires I connected my 3.5mm wire to them. I made the connection but no sound was produced. I made connections red-red and yellow-black but no sound was made so I connected red-black and yellow-red and no sound was made.
I was pretty sure that I had made another mistake.
But accidentally I touched a wire from the 3.5mm connector to the radio board (#3) while the other wire was still in contact with the board’s wire and there was some noise!
I had connected red-black and yellow-board and there was noise.
I swapped the connection and this time I connected yellow-black and red-board and there was noise.

So I secured the connection red-black and started to move the yellow wire of the 3.5mm connector on the pin to locate a good connecting point.

I secured the wire connection by making a simple loop over the connection and then using an electric tape to just secure it there. The tape also acted as a shield to isolate the wires from touching any other connection points.

I also covered the other connection with the electrical tape.

In the photo both the connections which have been secured by the tape can be seen.

That’s it. I was done. The back cover was closed, all the screws put again.
So the radio does not work now, it never did, and the cassette player had died a long time ago. But now I have a working speaker in a retro housing. The only thing that I had to sacrifice was the handle.

In the photos I have connected an iPod Shuffle to it and also an old Nokia phone. The phone has a radio and a 2.5mm connector so I used a 2.5mm male – 3.5mm female connector for the phone.