SONY STR-6046 lazy FM stereo

[spaghetti]

Hi, I have 2 of these vintage receivers, they both show the same issue with FM stereo, there are very few stations that are received in stereo. I am trying to figure out if I can do something to improve the situation.

I recapped the IF and MPX sections of one receiver, but things are unchanged. Most of the cap values were off by 50%. Of course the stronger the signal, the more likely FM stereo will engage. But I noticed a strange behavior: I expected stereo to kick in when the signal is at the highest, but many times it kicks in earlier and goes out just before the signal reaches its peak.

There's not much to adjust on the MPX board, I uploaded the tuner section schematics here: https://i.imgur.com/rcdrCYBr.jpg and the adjusting points here: https://imgur.com/KfYUhei
so my guess the issue is in the FE or the discriminator.

Any suggestion of what could be the issue? What parts should be aligned? I have an oscilloscope and a signal generator that covers also FM frequencies with a 1Khz internal oscillator.

Thanks!

 

[39cross]

Seems like a complete alignment is in order.

 

[spaghetti]

Seems like a complete alignment is in order.

Yes, I guess so. Any specific suggestions for possible actions I can take?

 

[fredtroy]

What are you using for an antennae?

You wrote, "signal generator that covers also FM frequencies with a 1Khz internal oscillator", is that an FM modulated 1Khz? Does it produce markers?

My guess is that you will spend more on test equipment than the receiver is worth, but there isn't anything necessarily wrong with that. If you don't want to get into that, and you can improve your antennae, try that.

 

[spaghetti]

I'm using a FM dipole antenna.

Regarding the generator, it produces an FM modulated signal, but it's not the best. It's been hacked by a guy I know, the generator was supposed to produce only AM modulation. The signal is a bit distorted, still it works, I used it in the past to improve other bad receivers, but they had different issues and circuits.
I don't know about "markers", so my guess is it cannot produce them.

 

[fredtroy]

If these receivers are for you to listen to, try a better antennae. If these are to repair and sell, start looking for an FM alignment generator. Like FM, these have fallen out of favor and consequently you can find alignment generators inexpensively if you are patient. If you acquire one, start with IF (IFT101) then the discriminator (IFT201) and that will probably all you will need to do.

 

[spaghetti]

I'd like to keep one of them in my collection and sell the other in order to fund new expensive hifi purchases. These SONY receivers are nothing special but they surely have the looks with the shiny knobs and the real veneer wooden cabinet.

Last night I spent one hour tinkering on one receiver, and I feel there are small improvement. Now stereo kicks in very close to the max signal amplitude, and I think I am getting also more stations in stereo.

What I did was basically tune IFT101 for max output signal on the scope and slightly adjust IFT201 for centered stereo on max signal.

Today I'll go and buy some 0.2uF caps and follow the IFT101 alignment as described in the SM, it looks complicated but I think I have the equipment for the "signal generator alignment" procedure.

 

[fredtroy]

That's excellent, congrats.

 

[rcs16]

Most tuners need a means to measure THD so you can adjust the detector and sometimes IF filters for lowest value. Cheapest way is a good sound card and s/w to display the fft spectrum.
The other is a comp stereo gen to adj for the stereo stuff like channel sep, sub carrier rejection, I used a HP 8904A for that function.
need a freq counter to adj the VCO if using one of those chips that need adjustment.