[EADGBE]

(Please forgive me if I say anything dumb in this post...I am a home
enthusiast who is experienced at working on amplifiers, tape decks,
and turntables, but I haven't really worked on tuners at all.....)

As you all may recall, my Technics SA-400 receiver's "FM Stereo"
indicator would not light up, even when receiving any of the strong FM
stations in my area.

A few folks kindly told me how to adjust the VCO circuit. I used my
frequency counter and hooked it up via a 100k resistor to the
appropriate test point and was able to adjust the VCO perfectly to
19kHz.

For a few weeks, the receiver worked great. I was able to get "FM
Stereo" indicator to light AND could plainly hear the stereo image of
my favorite FM station.

But now, the same problem has happened again. The "FM Stereo"
indicator does not light and I do NOT hear stereo.

I was able to verify that the "FM Stereo" light DOES work.
Interestingly, the last time the "FM Stereo" light worked, I was
dialing in a station, the light came on for half a second, then went
out.

It's as if the receiver is TRYING to give me FM stereo, but for some
reason cannot.

QUESTION: What procedure(s) do I need to do to align the FM tuner so
this problem won't happen again? I have a frequency counter, a signal
generator (20Hz to 20kHz), and a Nakamichi T-100 audio analyzer. My
scope, unfortunately, is non-functional at the moment.

Many thanks in advance........

[Randy Yates]

EADGBE writes:

(Please forgive me if I say anything dumb in this post...I am a home
enthusiast who is experienced at working on amplifiers, tape decks,
and turntables, but I haven't really worked on tuners at all.....)

As you all may recall, my Technics SA-400 receiver's "FM Stereo"
indicator would not light up, even when receiving any of the strong FM
stations in my area.

A few folks kindly told me how to adjust the VCO circuit. I used my
frequency counter and hooked it up via a 100k resistor to the
appropriate test point and was able to adjust the VCO perfectly to
19kHz.

For a few weeks, the receiver worked great. I was able to get "FM
Stereo" indicator to light AND could plainly hear the stereo image of
my favorite FM station.

But now, the same problem has happened again. The "FM Stereo"
indicator does not light and I do NOT hear stereo.

I was able to verify that the "FM Stereo" light DOES work.
Interestingly, the last time the "FM Stereo" light worked, I was
dialing in a station, the light came on for half a second, then went
out.

It's as if the receiver is TRYING to give me FM stereo, but for some
reason cannot.

QUESTION: What procedure(s) do I need to do to align the FM tuner so
this problem won't happen again? I have a frequency counter, a signal
generator (20Hz to 20kHz), and a Nakamichi T-100 audio analyzer. My
scope, unfortunately, is non-functional at the moment.

Many thanks in advance........

If you would have worked the number of hours you've spent on this
newsgroup and otherwise with this problem, you could have bought a new
receiver with the money.
--
% Randy Yates % "Midnight, on the water...
%% Fuquay-Varina, NC % I saw... the ocean's daughter."
%%% 919-577-9882 % 'Can't Get It Out Of My Head'
%%%% % *El Dorado*, Electric Light Orchestra
http://www.digitalsignallabs.com

[Richard Crowley]

"EADGBE" wrote ...
But now, the same problem has happened again. The "FM Stereo"
indicator does not light and I do NOT hear stereo.

But you failed to mention whether whether your 19KHz calibration
is still holding?

QUESTION: What procedure(s) do I need to do to align the FM tuner so
this problem won't happen again?

It may not be merely an alignment issue. Your receiver may
actually be broken and in need of repair.

[EADGBE]

On Dec 1, 10:40 am, Randy Yates wrote:

If you would have worked the number of hours you've spent on this
newsgroup and otherwise with this problem, you could have bought a new
receiver with the money.

Thanks very much. That comment was neither accurate nor helpful.

I've hardly spent "hours" talking / reading about this problem.

I have a total of eight dollars invested in the receiver.

It looks and sounds excellent...apart from the FM stereo problem,
which can probably be cured fairly easily. I am no stranger to
component-level repairs and have worked on MANY a circuit.

I'm merely looking for a little advice about a circuit with which I
have
had no prior experience. I've already learned some valuable info,
and
am simply asking for a bit more info....

.....is that REALLY too much to ask?

[Chronic Philharmonic]

"EADGBE" wrote in message
...

[snip]

A few folks kindly told me how to adjust the VCO circuit. I used my
frequency counter and hooked it up via a 100k resistor to the
appropriate test point and was able to adjust the VCO perfectly to
19kHz.

For a few weeks, the receiver worked great. I was able to get "FM
Stereo" indicator to light AND could plainly hear the stereo image of
my favorite FM station.

But now, the same problem has happened again. The "FM Stereo"
indicator does not light and I do NOT hear stereo.

It appears as though the free-running PLL frequency is drifting. This might
indicate that the PLL IC might be failing, the power supply regulation is
not consistent, or component values are drifting. The trimmer that you
adjusted could be dirty (you might try sweeping through its entire rotation
range a couple dozen times, and then re calibrating the PLL).

I was able to verify that the "FM Stereo" light DOES work.
Interestingly, the last time the "FM Stereo" light worked, I was
dialing in a station, the light came on for half a second, then went
out.

It's as if the receiver is TRYING to give me FM stereo, but for some
reason cannot.

Because the PLL free running frequency drifted too far off the pilot
frequency for the PLL to lock to the stereo pilot.

QUESTION: What procedure(s) do I need to do to align the FM tuner so
this problem won't happen again? I have a frequency counter, a signal
generator (20Hz to 20kHz), and a Nakamichi T-100 audio analyzer. My
scope, unfortunately, is non-functional at the moment.

If the receiver has good sensitivity and noise and distortion are
acceptable, then the PLL is the only thing that comes into play with this
kind of problem. Don't start rocking the FM alignment. Without an RF
generator with a low distortion FM modulator, all you can do is mess it up.

You don't really even need a frequency counter for setting the PLL though.
Just set the adjustment halfway between the points where the stereo light
goes off when tuned to a stereo broadcast. The PLL locks to the station's
stereo pilot, so the free running frequency only needs to be close enough
for the PLL to capture lock.