Hitachi AM-FM SR-704 receiver.. no stereo reception but strong signal..

[jpdesroc]

Hi all,
I'm a tech that has been working on audio stuff for years
but just starting in the radio freq AM/FM alignment.
So far I've been successfully able to realign some AM radios nicely
but now I have an issue with an vintage Hitachi AM-FM SR-704 receiver.
The FM section receives strong signals from FM radio station
(both strenght and centering meters) but their is no stereo reception..
only mono is received. The STEREO pilot light is not burned but won't lit.
I'm suspecting a weak 19khz pilot signal coming up in the demultiplexer IC.
My question is: can I troubleshoot the FM receiver front end ONLY using
a 10.7mhz carrier signal modulated at 19khz to pinpoint the pilot levels ?
I don't have any stereo RF generator on hand.. only a generator
that goes up to 20mhz and can be modulated AM or FM (mono).
Thanks for your help.
Jean-Pierre Desrochers
Canada

[Nuvistor]

Well you could use an off air stereo signal that will have the 19Khz at the correct level to trace the problem.
If mono is working the circuit upto and including the detector will be ok. You should be able to see the 19Khz tone on a scope after the detector.

[jpdesroc]

Here is a partial section of the overall schematic:
http://www.arcenson.com/public/Hitachi%20SR-704.jpg

I'm not home now so I'll have a look tonite.
I'm trying to figure out where to check the incoming
19khz pilot..
Pin 2 of IC103 HA1196 ??

[jpdesroc]

Look at the original post,..
I put a link to a better image..

[Nuvistor]

The 19Khz signal will probably be contained within the IC, unless you have a block diagram of the internal of the IC it will be difficult to find. Presuming the pilot tone complete with the DSB L-R is fed to the MPX IC, no immediate reason to suppose it’s not.

I would check the voltages on the IC HA1196, and check the setup of the controls R237 and R238.

If nothing is towards, check the components around the IC, after that perhaps the IC is faulty.
I have not fault found on these IC’s before so it’s very general info. Hopefully someone will have more experience.

[jpdesroc]

The inners of both IF and demux ICs are shown here:
http://www.arcenson.com/public/IF_&_demux_ICs.jpg

[Nuvistor]

Has far as I can tell the 19Khz signal is contained within the IC, there is a possiblity it is on pins 14 and 15 but I cannot be sure.

The important adjustment is the 76khz VCO, that is divided to 38khz for the carrier insertion then divided by 2 to 19Khz. This 19Khz signal is phase compared with the incoming pilot tone on pin 13, the output of the phase comparator corrects the 76Khz VCO. I would expect some audio mixed with the pilot tone on pin 13.

If the 76khz VCO is far off frequency or is not oscillating there will be no stereo.

My advice for what is worth is to check those points in post #5.

[mark2collection]

If my Leak 2000 receiver was anything to go by ...

In the Hitachi, check:-

C301 (Most likely a duffer)
C302 (Possibly a duffer)

Have you tried removing the original C302, then connect one end of a new capacitor to the input pin, other end of the new cap to your signal generator, set to 19Khz? This 'should' make the beacon illuminate.

Optionally, just replace C302 if a capacitor meter isn't to hand & it's a bit of a job to get to the cap ...

Mark

[stevehertz]

See my answer to this thread: https://www.vintage-radio.net/forum/...d.php?t=143165

BTW, this is a vintage audio thread, not radio.

[jpdesroc]

Originally Posted by Nuvistor

Has far as I can tell the 19Khz signal is contained within the IC, there is a possiblity it is on pins 14 and 15 but I cannot be sure.

The important adjustment is the 76khz VCO, that is divided to 38khz for the carrier insertion then divided by 2 to 19Khz. This 19Khz signal is phase compared with the incoming pilot tone on pin 13, the output of the phase comparator corrects the 76Khz VCO. I would expect some audio mixed with the pilot tone on pin 13.

If the 76khz VCO is far off frequency or is not oscillating there will be no stereo.

My advice for what is worth is to check those points in post #5.

You were right Nuvistor. The problem was that the 76khz VCO was way off !! it was at around 220khz !! What is weird is that the VCO components C310 (360pf) including R311 (24k) and the freq adj pot R237 (10k.. cleaned).. were all OK.. Hmmm so I put a new polystyrene cap a little higher (500pf) in place and BINGO ! The 76khz was back with a nice beacon light coming in on stereo stations. The receiver has been running for several hours without any driftings problems .. Thank you all for your generous help. JP

[Nuvistor]

I suppose it’s possible that a fault within the IC is causing the change of frequency, if however it’s now stable and a long term cure that’s what matters.
Glad you gave it working.

[stevehertz]

Originally Posted by Nuvistor

I suppose it’s possible that a fault within the IC is causing the change of frequency, if however it’s now stable and a long term cure that’s what matters.
Glad you gave it working.

I was going say, you shouldn't have to increase a stock value by some 50% in such a critical position to make it work. To me it points to an as yet unidentified faulty component (or problem) whereby changing the value of that cap just happens to balance things out and create a working condition. Nonetheless great news that it is now working.