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

[arcenson]

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

 

[Nigel Goodwin]

I've NEVER seen such a fault, and I've been repairing audio/TV professionally for over 45 years.

Do you have a service manual (or at least a schematic) for the receiver?, if not download the datasheet for the decoder IC (assuming it uses one). There 'may' be an adjustment on the IC for the 38KHz PLL, and if that's too far out of alignment the PLL won't lock.

 

[Les Jones]

Are you sure that the ststion you are tuned to is transmitting in stereo ?

Les.

 

[arcenson]

Yes the stations I'm receiving are ALL stereo broadcasting.

Here is a partial section of the overall schematic:
**broken link removed**

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

 

[Nigel Goodwin]

I'm trying to figure out where to check the incoming
19khz pilot..

Why would you want to?, what kind of fault could 'magically' make the pilot tone disappear?.

As I said previously, it's FAR more likely to be PLL misalignment which appears to be R237 on the circuit, with TP5 as the test point. Preset resistors commonly make intermittent contact after years of use, probably a quick turn or two either way and placing it back where it was could well cure the problem.

 

[arcenson]

..Why would you want to?, what kind of fault could 'magically' make the pilot tone disappear?.
A bad cap or ceramic filter that would chop all the freq from 15khz and above..
No more pilot to read from.

..probably a quick turn or two either way and placing it back where it was could well cure the problem.
I'll check these trimmers..
Thanks.

 

[Les Jones]

I found a data sheet for the HA1196 on the web. It does not look like you can get to any signals on the phase locked loop. One thing you could check is that the voltage on pin 12 changes when the mode switch is changed between mono and stereo.

Les.

 

[unclejed613]

i used to do alignments a lot. you need to do the alignment using a sweep generator. it sounds like you have the IF peaked too much, and so doesn't have 38khz of bandwidth which is required for the subcarrier. you probably don't even have 19khz of bandwidth, and so you get no pilot tone. you need a signal generator that can output an FM signal at 10.7Mhz, with +/-30khz of deviation. actually, more like +/- 40khz...