QUESTION: FM Stereo Sensitivity

Your notion of "halfway" is sort of optimistic.

Odds are the stereo detector stage needs alignment or else it has some other issue with it. Take it to a tech. The audio output of the FM detector goes through a coupling cap into a second discriminator stage. All kinds of things can go wrong with it.

No, the stereo demodulation circuit is bad.

If you have a 300 ohm source, you use the 300 ohm input. If you have a 75 ohm source, you use the 75 ohm input. Usually there is a balun inside the case which transforms one to the other, and the balun is a little lossy, so one of the inputs will have slightly better sensitivity. Which is basically academic since you will need the correct one for your source anyway.

--scott

--
"C'est un Nagra.  C'est suisse, et tres, tres precis."

To add to Scott's reply:- The 300 ohm input will be balanced twin feeder. This is the cable with two parallel wires, spaced about 12mm (1/2") apart. The 75 ohm input is unbalanced and uses coaxial (screened) cable. In the UK, we prefer to use the 75 ohm input as screened cable protects the antenna signal from interference, especially from mains wiring and/or metal objects which could change the impedance of the balanced unscreened cable.

Having said that, Scott is right in that if you have a 75 ohm antenna, use coax, if you have a 300 ohm antenna use balanced twin.

S.

--
http://audiopages.googlepages.com

The SA designation is still used for their receiver line, and has been since Matsushita formed the Technics brand. How old is it?

Depends very strongly on where you are, geographically speaking...and where the receiver is physically located.

Those work acceptably in urban settings. If it's hooked up right, and there are reasonably strong stations within 15-30 miles, you should get stereo.

Depends. Obviously if the gear is old, it could be defective.

It's not resistance, but impedance...probably beyond a concise explanation. Suffice to say: hook twinlead (what you have) to the 300 ohm connection and coax (like cable TV lead) otherwise.

jak

Scott:

I am fully aware of the Technics' questionable "hi-fi" status. I simply bought this as a nostalgia trip. Believe me, the Technics SA receiver I had as a kid was AUDIOPHILE stuff compared to what I had been listening to before I got it.

For the record, the receiver I just got is an SA-400, made from 1978 to 1979.

Stereo notwithstanding, can you get decent sensitivity at both ends of the FM band?

I used to work on a lot of these types of receivers. Our shop's standard "tune-up" for these included FM sensitivity adjustments (i.e. RF alignment) at several points along the FM band, and IF and stereo demux alignment. For the RF alignment, you could probably approximate the optimal tuning if you can find relatively weak stations at both ends of the band.

For the IF and stereo demux, though, we used a combination of a dedicated signal generator and a 'scope. Not sure if or how you could accomplish that w/o some specialized test equipment.

I was always a little surprised at how far off these old technology receivers could be, even when they weren't that old.

If you're getting decent signal meter deflection, I agree with the other poster(s) - you probably need your IF/Stereo demux aligned.

Good Luck!

EeAaDdGgBbEe (a 12-string)

You probably have this info, but: 45wpc at 0.04 THD and 0.04 IMD rated from 20-20khz bandwidth with a 90 dB s/n ratio (phono). 10.8 fm mono IHF sensitivity with a 1.2 dB fm capture ratio...cost $360 in 1979 dollars (over $1000 today, by the consumer price index).

By some measures, I'd day that qualified as "HiFi".

jak

see if you can locate a schematic or serivce manual and look for a muting or stereo threshold adjustment...

these receivers are designed to switch to mono when the signal is weak and sometimes there is an adjustment for the threshold...

or the tuner could be out of alignment or broke such that it thinks the signal is weak when its not weak...

Mark

Need to align the stereo section of the tuner which requires some equipment that generates specific signals.

75 ohm is coax type connection, 300 ohm is twin lead. The often have a common input terminal.

VR301 is the VCO adjustment. It's located near the tuning gang. This would be the easiest thing to adjust that might correct the problem. First take note of where it's set so that if the adjustment doesn't work, you can return it to its prior state. Then turn it slightly one way or the other and see if your stereo signal light comes on for more stations. If not, go the other direction. Without a frequency counter, you'll need to trial and error this adjustment.

Good luck.

--
David Farber
David Farber's Service Center
L.A., CA

75 is commonly unbalanced, or uses ground reference. I do have some 72 ohm balanced lead next to me, and there is no shield. You can buy balanced shielded however. I used to use some 300 ohm shielded lead for a TV antenna way back. One of my tuners I started to use, which I bought of Ebay, is slightly off tune, and you have to hit mono to get good reception. It can also affect the stereo and muting thresholds.

greg

and

error

It's highly likely this is the problem. This setting can drift to the point where the decoder can fail to switch to stereo. I've seen the same issue in other stereo receivers.

Oh, yes. In fact, the tuner pulls in most stations quite clearly even when there is NO antenna connected at all.

With an antenna, the signal strength meter and FM fine-tuning meter both respond VERY strongly, throughout the entire FM band.

The signal strength meter typically registers between 3.5 and 4.5, on a scale of 1 to 5, and the fine-tuning meter always allows me to precisely "center" the tuning onto a station.

LOL!!! Good to see a fellow guitar player who understands my "name"!!!

David:

Thanks so much for the information. I work on analog stuff (tape decks, turntables), but I'm not too well-versed in tuner circuitry. This will be a big help.

FWIW, I do have a frequency counter, but it's not in hand just yet. (Just ordered it.)

It's the VCO adjustment...

Mark Z.

The problem with these in an urban area is they do little to sort multipath problems - which are all too common with high buildings etc around. And multipath can cause very audible distortion.

--
*Virtual reality is its own reward*

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

True, but irrelevant to the OP's issue.

jak

OK, I accessed VR301 - plainly labeled "VCO" by the way - and turned it clockwise a very tiny amount.

That did the trick!

I now have FM Stereo indication on all stations!

Thanks so much to David, William, Mark, and everyone else who offered information/suggestions.

My garage sale find is now 100% functional!

would

take

and

the

error

I looked at the service manual again and the alternative method of alignment says to adjust VR301 until the stereo light comes on. When you get your counter, hook it up to TP301 via 100k ohm resistor and adjust VR301 for

19KHz. It might help if you move the tuning dial so there is no station being received during the test.

--
David Farber
David Farber's Service Center
L.A., CA

But .. is the output stereo? The indicator could be bad ... it is on my old (and very excellent) Sony tuner.

This is supposedly a pro group. Record the output and look at the waveforms in your favorite editor. Are they completely different? Are there places where they are 90 degrees out of phase? Then it is working OK in stereo on that station. Or just listen!

Doug McDonald