Old McIntosh FM tuner poorstereo separation
Andy Cuffe
detects stereo stations, but the channel separation is very poor. It's
almost impossible to tell if it's in mono or stereo mode by just
listening to it. Does anyone have any experience with these? Does it
sound like a failure, or an adjustment?
--
Andy Cuffe
balt...@psu.edu
Jerry G.
these units if the tubes start going weak the selectivity starts to
decrease. After retubing, a check of the IF alignment is in good
order. I would not suggest checking the IF alignment with old tubes
in the unit.
I used to service tube tuners when I started out... The main problems
were with the tubes going out of specs. A tube checker may say that
the emission is acceptable, but I found the best test is to try the
tubes.
--
Jerry Greenberg
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"Andy Cuffe" <balt...@psu.edu> wrote in message
news:38FD2A...@psu.edu...
Paul Pu239
someone there will help.
Jerry G. wrote in message ...
:The first thing I would do is retube the IF and front end stages. In
:
TIM SCHWARTZ
>
> I am working on an MR-71 tube FM stereo tuner. It receives fine and
> detects stereo stations, but the channel separation is very poor. It's
> almost impossible to tell if it's in mono or stereo mode by just
> listening to it. Does anyone have any experience with these? Does it
> sound like a failure, or an adjustment?
> --
> Andy Cuffe
> balt...@psu.edu
Andy,
I have to disagree with one of the other posts. The LAST thing I would
do is retube. I might test the tubes, making sure to put them back in
the same socket they came out of. (Of course, replace any dead tubes.)
I would first look in the multiplex section for a stereo problem.
Check that the stero lamp is working and that it is the correct lamp
(1847 or 47).
Do you have an FM alignment generator and oscilloscope available? Is
the basic sensitivity and front end alignment OK? These tuners by now
often have defective IF and detector transformers, which are
unfortunately no longer available. Have you made sure the power
supplies are all OK?
I realize that there are an awful lot of possibilities here, but as
these units get older, more thingsneed to be looked over.
Regards,
Tim Schwartz
Bristol Electronics (an authorized McIntosh servicer in NJ)
Peter Maus
> >
> > I am working on an MR-71 tube FM stereo tuner. It receives fine and
> > detects stereo stations, but the channel separation is very poor. It's
> > almost impossible to tell if it's in mono or stereo mode by just
> > listening to it. Does anyone have any experience with these? Does it
> > sound like a failure, or an adjustment?
> > --
Check the tubes. Replace the bad ones. Likely it may not need an
alignment if the mono reception is distortion free.
However, there is, in the stereo decoding circuitry, a trimpot
that controls the amount of L-R injection. This is what sets the
amount of stereo separation. A small amount of oxide at the tip of
the wiper on this trimpot, (and after 40 years there's going to be
SOME) will alter the amount of L-R injection. Locate that trimpot,
and run a small piece of vellum, or drafting mylar under the contact
point on the wiper. Construction paper will also work. Note and mark
the original position of the wiper before you begin.
An alternate procedure would be to note the original position and
swing the wiper through it's range a couple of times. And put a
drop of CAILUBE MCL on the resistive element.
Check also coupling caps in the decoder circuit and verify that
the L-R injection voltage is correct.
This is a common complaint on old stereo tuners. In most cases
that cross my bench, the problem is no more complicated than this.
Donald Borowski
demultiplexer section. I have a Fisher 500C receiver that came to me with all
original tubes (including the 7581A outputs that were still good due to a power
supply problem!). It had a problem with stereo reproduction that turned out to be a
weak tube in the demultiplexer. The weak tube didn't allow the the demultiplexer to
lock onto the subcarrier pilot, thus the stereo problem.
Donald Borowski
Agilent Technologies, Spokane PGU
TIM SCHWARTZ wrote:
> Andy Cuffe wrote:
> >
> > I am working on an MR-71 tube FM stereo tuner. It receives fine and
> > detects stereo stations, but the channel separation is very poor. It's
> > almost impossible to tell if it's in mono or stereo mode by just
> > listening to it. Does anyone have any experience with these? Does it
> > sound like a failure, or an adjustment?
> > --
Andy Cuffe
>
> Andy Cuffe wrote:
> >
> > I am working on an MR-71 tube FM stereo tuner. It receives fine
and
> > detects stereo stations, but the channel separation is very poor.
It's
> > almost impossible to tell if it's in mono or stereo mode by just
> > listening to it. Does anyone have any experience with these? Does
it
> > sound like a failure, or an adjustment?
> > --
> > Andy Cuffe
> > balt...@psu.edu
>
> Andy,
> I have to disagree with one of the other posts. The LAST thing I would
> do is retube. I might test the tubes, making sure to put them back in
> the same socket they came out of. (Of course, replace any dead tubes.)
I already did that. All tested good for emission and most are the
original tubes. I did try swapping out some of the tubes temporarily.
Replacing the MPX amp tube significantly improved it's ability to
recognize stereo stations, but didn't change the sound at all.
> I would first look in the multiplex section for a stereo problem.
> Check that the stero lamp is working and that it is the correct lamp
> (1847 or 47).
The lamp works fine and it correctly IDs stereo stations.
> Do you have an FM alignment generator and oscilloscope available? Is
> the basic sensitivity and front end alignment OK? These tuners by now
> often have defective IF and detector transformers, which are
> unfortunately no longer available. Have you made sure the power
> supplies are all OK?
Overall reception is good. It pulls in all the local stations with a
standard FM ant. It's not a sensitive as a modern receiver, but seems
fine. I don't have a schematic, so I can't verify the correct
voltages. I did check all the electrolytics and looked for obvious
problems. It's in very good condition because it was mounted in a rack
and hardly touched.
> I realize that there are an awful lot of possibilities here, but as
> these units get older, more thingsneed to be looked over.
>
> Regards,
> Tim Schwartz
> Bristol Electronics (an authorized McIntosh servicer in NJ)
--
Andy Cuffe
balt...@psu.edu
Andy Cuffe
>
> > Andy Cuffe wrote:
> > >
> > > I am working on an MR-71 tube FM stereo tuner. It receives fine and
> > > detects stereo stations, but the channel separation is very poor. It's
> > > almost impossible to tell if it's in mono or stereo mode by just
> > > listening to it. Does anyone have any experience with these? Does it
> > > sound like a failure, or an adjustment?
> > > --
>
> alignment if the mono reception is distortion free.
>
> However, there is, in the stereo decoding circuitry, a trimpot
> that controls the amount of L-R injection. This is what sets the
> amount of stereo separation. A small amount of oxide at the tip of
> the wiper on this trimpot, (and after 40 years there's going to be
> SOME) will alter the amount of L-R injection. Locate that trimpot,
> and run a small piece of vellum, or drafting mylar under the contact
> point on the wiper. Construction paper will also work. Note and mark
> the original position of the wiper before you begin.
> An alternate procedure would be to note the original position and
> swing the wiper through it's range a couple of times. And put a
> drop of CAILUBE MCL on the resistive element.
> Check also coupling caps in the decoder circuit and verify that
> the L-R injection voltage is correct.
>
> This is a common complaint on old stereo tuners. In most cases
> that cross my bench, the problem is no more complicated than this.
Mono reception is fine. Where is this pot? The only pot I can see in
the mpx section is the stereo light adjust. Are you talking about this
specific model, or old FM tuners in general?
--
Andy Cuffe
balt...@psu.edu
Timeman
stations with adequate strength, but there is no channel separation. I
would, therefore, leave the RF and IF sections alone.
You can try to tweak the separation pot in the multiplex circuit, if
there is one. Failing that, tweak the 38 kc transformer and see if the
situation improves. If no improvement is noted, return this adjustment,
as well as any others you may have touched to exactly where you found
them.
The multiplex circuits may require a complete realignment. This can be
an onerous task unless you have a service manual, the appropriate
equipment and, last but not least, expertise.
Regards,
Steve Cwynar
Andy Cuffe wrote:
>
> I am working on an MR-71 tube FM stereo tuner. It receives fine and
> detects stereo stations, but the channel separation is very poor. It's
> almost impossible to tell if it's in mono or stereo mode by just
> listening to it. Does anyone have any experience with these? Does it
> sound like a failure, or an adjustment?
> --
> Andy Cuffe
> balt...@psu.edu
Asimov
Old McIntosh FM tuner poorstereo separation (18 Apr 00 23:39:01)
AC> I am working on an MR-71 tube FM stereo tuner. It receives fine and
AC> detects stereo stations, but the channel separation is very poor.
AC> It's almost impossible to tell if it's in mono or stereo mode by just
AC> listening to it. Does anyone have any experience with these? Does it
AC> sound like a failure, or an adjustment?
Did you check the stereo demodulator's last tube just before the audio.
This is typically a pentode oscillator which switches a 2 diode matrix
at 38KHz. If the tube is weak the incomming 19KHz may not be enough to
shock it into oscillation. Perhaps an open coupling transformer winding
too. Thus you might detect the stereo pilot but the diode matrix doesn't
separate well. (this shoulda been a bra commercial!)
... She said it was either her or the Ham Radio... OVER...
--
| Return Address: asi...@mnetREMOVE-THIS.axess.com
| Standard disclaimer: The views of this user are strictly her/his own.
| Signature auto-added at gateway.
Andy Cuffe
>
> I would not rule out tubes, but I would concentrate on the ones in the stereo
> demultiplexer section. I have a Fisher 500C receiver that came to me with all
> original tubes (including the 7581A outputs that were still good due to a power
> supply problem!). It had a problem with stereo reproduction that turned out to be a
> weak tube in the demultiplexer. The weak tube didn't allow the the demultiplexer to
> lock onto the subcarrier pilot, thus the stereo problem.
>
> Donald Borowski
> Agilent Technologies, Spokane PGU
>
> TIM SCHWARTZ wrote:
>
I've already tried several subs for both tubes in the MPX section.
Replacing one did improve its ability to detect stereo stations, but had
no effect on the sound.
--
Andy Cuffe
balt...@psu.edu
Peter Maus
Andy Cuffe wrote:
>
> Peter Maus wrote:
> >
> > > Andy Cuffe wrote:
> > > >
> > > > I am working on an MR-71 tube FM stereo tuner. It receives fine and
> > > > detects stereo stations, but the channel separation is very poor. It's
> > > > almost impossible to tell if it's in mono or stereo mode by just
> > > > listening to it. Does anyone have any experience with these? Does it
> > > > sound like a failure, or an adjustment?
> > > > --
> >
> > Check the tubes. Replace the bad ones. Likely it may not need an
> > alignment if the mono reception is distortion free.
> >
> > However, there is, in the stereo decoding circuitry, a trimpot
> > that controls the amount of L-R injection. This is what sets the
> > amount of stereo separation. A small amount of oxide at the tip of
> > the wiper on this trimpot, (and after 40 years there's going to be
> > SOME) will alter the amount of L-R injection. Locate that trimpot,
> > and run a small piece of vellum, or drafting mylar under the contact
> > point on the wiper. Construction paper will also work. Note and mark
> > the original position of the wiper before you begin.
> > An alternate procedure would be to note the original position and
> > swing the wiper through it's range a couple of times. And put a
> > drop of CAILUBE MCL on the resistive element.
> > Check also coupling caps in the decoder circuit and verify that
> > the L-R injection voltage is correct.
> >
> > This is a common complaint on old stereo tuners. In most cases
> > that cross my bench, the problem is no more complicated than this.
>
> Mono reception is fine. Where is this pot? The only pot I can see in
> the mpx section is the stereo light adjust. Are you talking about this
> specific model, or old FM tuners in general?
> --
> Andy Cuffe
> balt...@psu.edu
It's in the MPX section. After the subcarrier is detected, and
demodulated, the injection is made just ahead of the output, to
create the matrix for stereo. Yours may not have it. Some updated
models didn't. Many tuners of period had this trimmer. Macs of this
vintage, including my own MR-71 had it.
Did you say that you have SOME stereo separation? And did you
turn the trimmer that adjusts the light?
As was also posted here earlier, verify that the light is the
correct number. The light is not just an indicator, it is an
integral part of the circuit. If it is not the correct one, stereo
performance will be compromised.
Andy Cuffe
>
> I am working on an MR-71 tube FM stereo tuner. It receives fine and
> detects stereo stations, but the channel separation is very poor. It's
> almost impossible to tell if it's in mono or stereo mode by just
> listening to it. Does anyone have any experience with these? Does it
> sound like a failure, or an adjustment?
> --
> balt...@psu.edu
Someone sent me the following precedure which worked perfectly. I used
a scope with a diff input instead of going by ear. It now sounds as
good as any other FM tuner I've heard.
1. With the tuner connected to a stereo amplifier, tune in a known
stereo
station.
2. Turn the volume down, then temporarily disconnect the right speaker
from
your amplifier, then take the GND lead from the left speaker and connect
it to
the SPKR lead or the right amplifier. This puts the left speaker between
the
two hot leads of the amplifier so that the ONLY sound that will come out
of it
will be L-R.
3. Turn up the volume a little, than adjust the 19Khz coil slightly one
way or
the other for maximum sound. Note which way you turned the coil. It
should take
no more than 1/4 turn. If there is a SEPARATION control, it should be
fully
CCW.
4. Reconnect speakers as usual. If you should discover that L and R are
reversed, you need to repeat the procedure but adjust the 19Khz coil the
opposite direction.
--
Andy Cuffe
balt...@psu.edu
Brian McAllister
stereo Tuners is a small (usuall 1 to 4 Mfd.) Electrolytic in the Mpx.
circuit. Can cause all kinds of strange problems when open.
Brian McAllister
Durham NC
Vintage Radio and Audio Web Pages
http://mcallister.simplenet.com
Andrew R Mitz
--
Thank you for posting your progress on this newsgroup. Many of us
have benefited from it.
-- Andy
