[OZ4]

I been trying to fix the original stereo decoder in this Sony STR7065. And finally came to the conclusion that the IC was bad.

Now since the IC is not obtainable. I ordered what is suppose to be a MPX decoder for a car radio. I ordered three different ones and got lucky that at least one of them is actually what it was advertised as.

Here is the original circuit.    

And here is the block diagram. There was no 38K signal at pin 18. Both the 19k signals were present at the two filter and trap. So at that point I gave up on the original IC.      


So the next option is this. It works like charm.     
The 8volt regulator is at the top and the two transistors at the bottom right are to switch on the stereo lamp.(not LED)  The pot at the right is for tuning the VCO.
   
The other chip on the little board is a noise blanker that I don't feel like trying to build the filter network for it.

After about a whole day of messing with it. It now looks like this.    

Now if I only knew how to use the software to make circuit boards. I will most likely make it by hand.

 I will add more here when I get back to it.

[OZ4]

Here's something new
This is the IF seen thru my SDR radio. This is KLOS 95.5 of the backside of the transmitter.
 Isn't true that the 10.7 IF is suppose to be centered?
   

[OZ4]

Here's the transmitter right off the  antenna. you can see the green area says it is wide band area. I think broadcast FM is + and - 75khz . I think it looks like junk. Something wrong I don't think is should look like that. I will try some others. Sorry that pic looks pretty bad.
   

[OZ4]

Here's the MPX circuit all made up. I even used Kicad software to arrange the layout. It helped. 
   

Here's the I.F. thru the SDR radio with different software. When I tune the receiver I can see the limits of the I.F. filters.
It looks much better now. The new software works better. The MPX board didn't affect it. 
   


And here it is all a glow.
   
It's a slighty doctored pic. It's hard to get a good pic  the auto iris keeps bring up camera gain.

[Ron Ramirez]

Interesting. What led you to choose the AN7418S?

How hard was it to work with given it appears to be surface mount?

I am interested because I've built a couple solid-state MPX decoders based on the LM4500 IC, which itself is now obsolete and hard to find. So if there is a good chip which can be used to build a board based on an available IC, I am definitely interested - assuming it works well with good separation. I see it has a connection for an optional separation control, which is a plus for some of those early Fisher tube-type FM tuners.

It appears that you added some transistors - to amplify the signal going into the MPX decoder chip? Do you have a diagram you can share? Thanks.

[OZ4]

Oh sure let me see if i can scrounge something up.

[OZ4]

Here's the nitty gritty right here.
   

For the P.S the 17.5v from the receiver is dropped first by a resistor then thru a 78L08v reg. 

The other two transistors are to isolate the 17.5v stereo lamp circuit from blowing out the led driver in the 8v chip.
 First is a pnp which is turn on by the led drive which then turns on a npn to pull down the actual lamp ground for the stereo ind.

Only other thing of note is the cap for the vco. It calls for a polystyrene one. I think it's supposed to be more stable than most other types.

The reason I chose this IC chip was ebay for about 13 bucks. They were part of a module board for car stereo.  the boxes say panasonic on them.  
I am sure there are obsolete by now. There are still some chips by them selves on ebay.
And I do have a magical link to a pdf data sheet I found. It's to somewhere in japan.   It took me almost an hour to find it.
And yes it works good. It's always nice when something you build works.

[OZ4]

here's the diagram I built it from. This IC is only the MPX decoder. The other one I built for the Fisher has both a noise canceling circuit and a MPX decoder. And it was much more difficult to build because it was so small with no carrier to convert it to .1 inch pin pitch.

[EdHolland]

Neat project, and I like the use of the SDR to examine the IF

[Ron Ramirez]

Very interesting. Thank you.

[radiorich]

Hello OZ4,
Wow nice job and like Ron I too have used that LM4500 and also would like to build a new one to use with a old tube Harmon Kardon Receiver .
Well Ron I think Chris might have done the leg work for use finding this IC .

Anyway OZ4 you did outstanding job and what great old Sony .
to use the software to design a circuit board nowadays is a lot simpler then when I was designing boards back in the 80s and 90s .

Sincerely Richard

[OZ4]

I didn't need to adjust any levels. It is happy just the way it came. The only thing is there is no deemphasis on the output. The sony has it already from the original circuit. 

Your LM4500 IC looks to be the easiest to work with as it's a old school dip package. There are still some on ebay. As far as function it looks a lot like the AN7416. 

Now I am waiting for new driver transistor for the output. The bias keeps drifting up. It already blew out the finals on one channel. I found one of the drivers had a gain of 86 and the bad channel one had gain of 2 and kept getting really hot.

[radiorich]

Hello OZ4,
Wow anyway I have a Amplifier that I need to rebuild again because it lost a channel it is a 1970s Luxman that is 50watts RMs per channel .

[toptip]

In 1965 the French company “Continental Edison” made a fancy radio for the Citroën DS Pallas called the “Radioën HiFi” — it is “Hi-Fi” because for the first time it had FM. In any case it is mono and quite rare. I recently got one.

In the 80s, Sanyo made a bunch of clock radios and others called, “Stereocast.” The FM on these were mono. But you could get a small adaptor that plugged in and produced stereo (but strangely, only through a headphone output). I have one of these. 

This weekend I tried to connect it to the Radioën. I just picked up the signal from the ratio detector. It worked perfectly. I may have to do some signal matching. I also had to be careful because the Radioën is 12V, negative ground, and the Sanyo is, again strangely, 6V, positive ground. The Sanyo has two jacks — a 3.5mm and a 2.5mm. The signal and positive ground are on the 3.5mm and the -6V comes in from the tip of the 2.5mm (the “body” of the 2.5mm is not connected). Inside there is a Sanyo A3311 MPX chip.

Now I have to figure out how I can operate this +12 / -6V combo in a car. Any ideas?

[RodB]

You might be able to find an isolated DC-DC dual output converter converting 12 volts to + & - 6 volts at one amp.