So, it's biamped? That's unusual. Not surprising that the tweeter includes a blocking capacitor in case of wiring mistakes. What is more amazing is that it doesn't have any sort of line input (or output). DVD and MP3 players and Minidisc and computers and set-top boxes were things one could reasonably expect to connect to a sound system back in the misty past of 2004. Maybe even a phono preamp...

Anyway, through the magic of Google, I found the service manual:
https://elektrotanya.com/panasonic_sa-pm21e.pdf/download.html
There should be some place to interrupt the signal from the FM tuner and insert a selector switch and jack for line input. Like C202 and C402 (located on the Main PCB, to the lower left (west-southwest) of IC800), or the traces for pins 5 and 7 (of CN805) of the "tuner extent pcb". (I chose the tuner because the brain might mute the CD or tape when they're not playing.)

On the other hand... maybe you have a car FM modulator lying around, or can pick one up cheap? Then just wire it up to a 12V wall wart, which will be easier and safer than hacking the innards of this thing.

 

Interesting to see you figured out how to open it. Just out of curiosity, would like to see a picture of the tweeter cone - front end of the speaker. I'm leaning to think it's a domed tweeter. A piezo wouldn't have a magnet.

 

If the tape player is seldom, or never, used, then a simple trick will be to use a switching jack for the line input, so that unplugging the line input re-connects the tape deck. Plus you avoid the cost of the switch, and finding a place to mount it.

 

Just out of curiosity, would like to see a picture of the tweeter cone - front end of the speaker. I'm leaning to think it's a domed tweeter. A piezo wouldn't have a magnet.

Photos in Google show it:

 

Sorry I have not posted in a few days. I do long shifts some days and by the end, I am not conversational. Also something else has popped onto my plate, I need to sort, but I will give full answers shortly. Once again, thank you to everyone for your time and contributions, thus far I have been hugely educated.

 

The tape channel has a preamplifier connected to the tape head, and then tape equalization, and after that the signal goes to the selector switch.So at that point a line input could be substituted, presuming that the switch is an actual mechanical switch and not done in the electronics. So is there a physical selector switch??
After looking at the photos and seeing the labels on the connectors another idea comes to mind. It looks like the cable from the tape deck connects to the radio circuit board. If that is true, then at that cable a line input could be made, depending on how much of the preamplifier circuit is on the tape deck. Is it a play-only deck, or does it have recording ability as well? And can we get one of those good photos showing the text next to the connectors from the tape deck. Adding a line in connection may just have become simple. Possibly.

Hi again,

No physical selector switch. If you press any button on any function, it jumps to that function.

I was wondering how it did this. Each function must send a 'listen to me now' signal to the 'brain'. If something is already playing that stops. It must send a 'quiet please' to the previously operating function. The tape deck click-clacks to stop when you move focus to cd with cd-open. Talking about the tape deck, it records.

Inside, the line from the tape deck connects in 2 places. To the board with the radio input. Here is the first:

And the second:



Because of the control signalling I just described, I think wiring something to substitute the tape deck, is not in any way 'drop in'. I am probably looking at using a car FM transmitters.

So these can be cheap, and run off a wall wart. Much simpler to set up, no messing around inside it. Now the sound will not be the cd quality from the server, lost over the fm transmitter, but I am not sure she will notice this. This unit is in the kitchen and has always got poor fm reception - that doesn't matter here. What might matter: in a car the transmitter is it the chassis faraday cage, does this block noise, which will not be blocked in a home kitchen? Does the transmitter and receiver need to be placed in an old metal biscuit tin together? (earthed?)

 

Photos in Google show it:

Cone.

 

The really great thing about FM is the capture effect, where the stronger signal eliminates the weaker signal. So if the signal from the FM adapter is adequate it should totally block out the noise.
An interesting alternative is an external input cassette, which places what looks like a cassette tape head against the tape head in the deck and drives it with an external signal. Those things are so very simple it might be worth investigating.

Now an intersting thing is that I own a stereo system with a similar form factor, but a different brand and quite a few years older that also has quite poor reception without an adequate external antenna. And that is both AM and FM.

 

There are various costs and qualities for an FM transmitter. Cheap simple ones are made for an AM radio muffled sound or telephone voices (no highs and no lows). Better transmitters are good for music.

The audio signals feeding the FM transmitter must use shielded audio cables since ordinary wires will be antennas that pickup mains hum and other interference. Audio wiring added inside the radio might also need to use shielded audio cables.

 

The really great thing about FM is the capture effect, where the stronger signal eliminates the weaker signal. So if the signal from the FM adapter is adequate it should totally block out the noise.
An interesting alternative is an external input cassette, which places what looks like a cassette tape head against the tape head in the deck and drives it with an external signal. Those things are so very simple it might be worth investigating.

Now an intersting thing is that I own a stereo system with a similar form factor, but a different brand and quite a few years older that also has quite poor reception without an adequate external antenna. And that is both AM and FM.

Back in the 90s, when we got our first CD player, a 'Discman' my brother wanted to listen to it in the car. There was no line in, and he read about these tape devices and purchased one. He was 13.

We went on holiday - set off in the car - and put the tape device in the tape deck. We pressed play on the CD player and indeed CD sourced sound came through the car. It was just not good enough to realistically listen to it for any length of time. Sometimes it had lots of pop and crackle, other times it had a cyclic pitch shift. Maybe we were using what a 13 year old can purchase with pocket money in 1994. Are these any good in 2021, or it that so niche I am really looking to purchase a modern antique?

I wandered if FM had some sort of inherent noise squelch. Radio hiss vanishes when you hit the station frequency. Now I know.

I'll google some options for both.

 

If you can find an old car FM transmitter that fits in where a cassette tape fits then it has nothing to produce pop and crackle and no motor to produce pitch shift.
The wiring or power supply of the transmitter might pickup alternator whine or clicks when an electrical item in the car is turned on or off.

Some FM radios have squelch that cuts off a weak station or blocks in between stations hiss.

 

I did have one of those adapters that went into an 8-track slot, it worked OK. And certainly some of the cassette adapters could have some issues, and some cassette mechanisms will not allow such an adapter to even live. So an FM package should be the good choice here. AND in a home environment alternator, ignition, nor fuel pump noise should be an issue.