Adding aux input to Sony PMC-202 stereo system

[mg3np1]

This header connects the tape deck head for playback and erase, could someone tell me its pitch?
Im trying to create an AUX input by disconnecting this and soldering a 3.5mm male to a cable i want to make

this is 3 pins per channel it would appear (L/R) and 2 erase could i just twist the (L/R) together to solder to a 3.5mm stereo male.

hope this makes sense

 

[(*steve*)]

1) 0.1 inch (2.54 mm) is a common pitch and that seems pretty close to it.

2) the signal levels are so different between a head and an aux signal that it's probably not practical. I would be looking along the signal path after the preamp and before the main amp (the volume control is often placed there) where the signal is probably line level or similar.

 

[mg3np1]

Thanks for your reply

I am a complete novice, but eager to learn

Having looked at the service manual it appears that the tape head, cable plugs directly into the main board of the hifi, I was hoping the preamp stuff was done on the main board with the volume circuits running from a separate board also connected to the main board

I am not sure if I have understood this correctly

 

[KrisBlueNZ]

If you want to add an auxiliary input, the sensible place to connect it is at the top end of the volume control. (Or at the input to the tone control section, if that comes before the volume control.) If you have the service manual, you should be able to figure out where to intercept the signal. Post a link to the service manual if you would like any more specific advice.

 

[mg3np1]

Id appreciate any advice here is copy of the service manual, I am looking at the RHS of pg 18

https://www.dropbox.com/s/bx3szq6ux4lc4y9/sony_pmc-202.pdf?dl=0

Thanks in advance

 

[KrisBlueNZ]

OK, the best place to insert your line input is between the input selector, IC309, and the preamp, IC304. Specifically, between R141 and C126 (left channel) and between R241 and C226 (right channel).



Here is my suggested modification. I've assumed you want to use a stereo jack socket with switching built-in, so when you plug something into the socket, the normal source is disconnected and overridden by whatever you plug into the socket. The alternative is to use a switch to select between the normal input source and the jack socket.



All changes are shown in blue. There are two track cuts, and six new components: two 1 µF capacitors (show as electrolytics but you could also use film capacitors) and four 100 kΩ resistors, which are needed to prevent loud pops when the external signal is plugged in and unplugged. And of course there's the stereo switched jack socket.

 

[mg3np1]

Upon reflection I dont think this project is suitable for a complete novice - is it I have never made track cuts before to a pcb and theres no header in those position is there?

Im guessing I could mount the circuit on breadboard?

If Im over-complicating this please let me know

Thanks for your time
mg3np1

 

[mg3np1]

Just a quick add - isn't the signal from a laptop soundcard already at line level? Basically I wanting to play my computers sound through this system in stereo

 

[(*steve*)]

Just a quick add - isn't the signal from a laptop soundcard already at line level? Basically I wanting to play my computers sound through this system in stereo

Yes, that's why Kris suggests you inject it there.

 

[mg3np1]

So I have to physically make changes to the tracks on the pcb
?



tbh this is way beyond my skill level - I was hoping to use jumper wires to connect to the socket for the tape head shown in the first post

oh well have to make do with my mix mic input (mono)

thanks tho

 

[(*steve*)]

It's not that hard. Once you locate the correct position, you use a sharp knife to cut through the track (typically in 2 parallel cuts, then remove a small piece of coper between) then solder wires to each side of the cut.

If you can show us a picture of the PCB from both sides, we may be able to show you where to make the cuts. One important thing is to get the jack socket though.

 

[KrisBlueNZ]

As Steve says, cutting tracks is not a big deal, but you can avoid that if you lift one end of R141 and R241. Then you can connect your four jack socket signal connections to the lifted ends of those resistors, and the holes you lifted them out from.

You can attach the resistors directly to the terminals of the jack socket. You don't really need a circuit board.

As Steve says, the tricky part is finding a suitable socket. Digi-Key have several. Have a look at the data sheets to see what you need. You might be able to find one locally.

http://www.digikey.com/product-detail/en/SJ1-3515/CP1-3515-ND/738681
http://www.digikey.com/product-detail/en/SJ1-3535/CP1-3535-ND/738693
http://www.digikey.com/product-detail/en/SJ1-3545/CP1-3545-ND/738702
http://www.digikey.com/product-detail/en/35RAPC4BV4/SC1461-ND/528670

(I've assumed you want a 3.5 mm socket for compactness, not a 1/4" socket.)

 

[mg3np1]

Hi,
Thanks for the replies
I am a disabled person which means my fine motor control is rubbish, so for me this is out of the question

But Im confused as to why my original idea of wiring up a 3.5mm jack (or two rca females) to the header for the cassette tape head/reader block. this 6 pin cable, 8 if include the erase head appears to go straight the pins on the tape head, I assume the the 6 wires have been twisted into pairs, there doesnt appear to be any signal processing going on, so I thought if you interfaced the 6 pin header with a 3.5mm plug or 2 rca jacks you would be able to connect a computer as a signal source with all the pre-amping/main amplification done by the system?

What am i missing?

Thanks

Nilesh

 

[KrisBlueNZ]

Oh. Could you desolder and lift one end of R141 and R241? It sounds like the whole project may be difficult. The tags on the socket are pretty small.

The signal at the tape head is at a very low level, and quite low impedance. You would need to attenuate the computer signal using resistors connected as close as possible to the connector. You would get a lot more hiss because of the high gain of the tape head preamp. Finally, the frequency response of the tape head preamp is not flat - it implements NAB equalisation, to "undo" the curve applied during recording. Google NAB equalisation for more information.

I'd expect that if you could modify or hack into that connector, you would also be able to lift those resistors and wire up a jack socket...

 

[mg3np1]

Yes, the whole process seems difficult tbh I hope you dont feel I have wasted your time (sorry)

Its the process of taking the unit apart that is the main issue (and putting back together) - Im sure I could get a friend to help I guess

I understand how to hook up the resistors, but how would you hook up the caps?

Are there any preamp boxes that I can buy to flatten the signal? I could certainly hook a few resistors to the computer:

http://cpc.farnell.com/_/128-234uk/cd-aux-switch/dp/AV23847?in_merch=Products From This Range

http://cpc.farnell.com/unbranded/spr-6/pre-amplifer/dp/AV19963?in_merch=Products From This Range

Im clutching at straws now lol

Sorry dont mean to be annoying at all...

 

[mg3np1]

http://cpc.farnell.com/jsp/search/productdetail.jsp?sku=AV2099861&N=0

 

[KrisBlueNZ]

That's fine. You're not annoying!

I think connecting into the cassette circuit is a bad idea. And a microphone preamp won't help.

Yes, see if you can get a friend to help.

The caps could just be mounted to the socket, with the other end connected to the wires going to the board. Or when you lift R141 and R241, if you lift the ends that go to IC309, you can put the positive wire of each cap into the hole where the resistor lead used to go. (You can connect the aux input circuit before the resistors, instead of after them like I showed in the diagram.)

 

[mg3np1]

Good Evening Kris,

Your last posts confuses me I need remove R141 and R241 or attach the caps to thend of them going away from IC309, to that end can I just twist the wires from the caps onto the legs of the resistors and tape them up or do I need to physical solder them?

Time to find a willing friend I feel..

Would it be possible to do all this situ as I can access the main board from the back you see ..... could i use crocodile clips for example

I trying to avoiding taking this thing apart hehe

 

[KrisBlueNZ]

OK this is going to be messy to describe step-by-step but here goes. Yes you can do it all on the underside.

Get a 1 µF capacitor and cut both its leads to about 8 mm long and tin the ends. Trace the track from IC309 pin 13 to R141. At the R141 end, melt the solder and add some new solder, and solder the positive lead of the 1 µF capacitor to that pad. Solder a wire to the other (negative) lead of the capacitor.

Find the other end of R141 and disconnect the resistor's wire from the pad. If you can't get to the component side and pull the resistor end out of the board, you can suck the solder off the pad using a solder sucker, then push the wire lead through the board using a sewing pin or similar. Then solder a wire to that pad.

Bend the 1 µF capacitor so it sits flat against the board and its wires will not touch anything nearby, then attach it to the board using hot melt glue (or at a pinch, a good sticky double-sided foam pad) so it is firmly fixed in place.

You now have two wires from the left channel, to connect to the aux input socket. The one from the capacitor needs to go to the left channel "NC switch" pin of the socket (the second-to-top connection point on the socket in the diagram in post #6) and the one that goes to the pad on the board needs to go to the left channel contact pin of the socket (the top connection point in the diagram).

Repeat the process tracing the track from IC309 pin 3 to R241, using a second 1 µF capacitor and two more wires. The one from the capacitor needs to go to the third socket connection point in the diagram, and the one from the pad needs to go to the fourth socket connection point in the diagram.

Then you just need the ground connection from the ground terminal of the socket to pin 6, 7 or 8 of IC309. And you can probably mount the four resistors directly on the socket.

Ideally you should use screened cable for the four signal wires, but as long as they're not too long, and don't go near the mains transformer, I think you'll be fine using standard insulated hookup wire.

 

[mg3np1]

Hi,

Thanks for the walk through much appreciated,

Does it have to be a 5 contact 3.5mm stereo jack I can only find ones with 4 contacts locally, and this one:

http://cpc.farnell.com/1/1/553-jack-socket-stereo-switched-t-214j.html

This is very much a learning project for me more than anything, can anybody tell me what the purpose of the caps - what do they do to audio signal? Deliver it in one burst to the PreAmp?

will these caps do for above circuit?

http://cpc.farnell.com/rubycon/50ml.../dp/CA07525?in_merch=Products From This Range

Cheers