Maybe I'm thinking like an electronic cow, but the description of the problem feels like a logic malfunction. To me. Sorry…
In order to keep consumer electronics “user friendly”, quite early in the development of FM radio receiver design automatic muting was added, gated off a “signal strength” assessor circuit, easily derived from the first IF mixer output. That way, as one would “turn the knob” (back when FM receivers had knobs that actually connected to variable tuning capacitors), the sound output would mostly be silence punctuated by near-to-well-tuned broadcast signal output.
Things became substantially more complicated with STEREO FM broadcasting; and with “channel capture” FM demodulator signal locking. But in the end, the output(s) of the IF mixer(s), the stereo discriminator and channel sideband separator, and those pesky front-panel buttons ("Mono / Stereo") were routed to a logic subcircuit that'd decide how to mute the channel, and when to let it pass thru. Signal “quality” of course.
So, given that, it feels like the logic subcircuit is either malfunctioning, or that the stereo 'signal strength' circuit isn't outputting '1' when it determines there is enough signal to warrant it. Either would result in the behavior indicated.
It is substantially LESS likely that you'd just happen to have a signal strength on all available channels, across the whole reception band that'd be low enough not to get at least a squeak out of one station or another. Seriously. Even in the “bad old days” when one had squirrelly twin conductor ribbon antennas that would inevitably come undone and fall off the antenna binding posts, even then, you'd still get a channel or three.
Just recalling,
GoatGuy
In order to keep consumer electronics “user friendly”, quite early in the development of FM radio receiver design automatic muting was added, gated off a “signal strength” assessor circuit, easily derived from the first IF mixer output. That way, as one would “turn the knob” (back when FM receivers had knobs that actually connected to variable tuning capacitors), the sound output would mostly be silence punctuated by near-to-well-tuned broadcast signal output.
Things became substantially more complicated with STEREO FM broadcasting; and with “channel capture” FM demodulator signal locking. But in the end, the output(s) of the IF mixer(s), the stereo discriminator and channel sideband separator, and those pesky front-panel buttons ("Mono / Stereo") were routed to a logic subcircuit that'd decide how to mute the channel, and when to let it pass thru. Signal “quality” of course.
So, given that, it feels like the logic subcircuit is either malfunctioning, or that the stereo 'signal strength' circuit isn't outputting '1' when it determines there is enough signal to warrant it. Either would result in the behavior indicated.
It is substantially LESS likely that you'd just happen to have a signal strength on all available channels, across the whole reception band that'd be low enough not to get at least a squeak out of one station or another. Seriously. Even in the “bad old days” when one had squirrelly twin conductor ribbon antennas that would inevitably come undone and fall off the antenna binding posts, even then, you'd still get a channel or three.
Just recalling,
GoatGuy
Your AT-27 is out of alignment. The quadrature detector coil T102 has drifted over the years and is no longer centered.
To fix this, you need a non-magnetic flat blade adjustment tool with approximately 1.5mm x 0.5mm tip. A bamboo chopstick that has one end shaped will work.
Tune to a strong FM station and slowly adjust T102 (see attached pic with the green arrow) on either direction until the station is no longer muted. You don't need to turn the ferrite core that far, only a few degrees. Note the old position with a marker so you can return it to the old setting if this doesn't work.
There is another adjustment for the FM "TUNED" sensitivity SR101 10k. Tweak this trimmer resistor if the above step didn't work. Same procedure, mark the old setting with a permanent marker first so you can revert to the old setting.
Good luck!
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I tried your suggestion (both options), but that did NOT improve the situation.
Here's what I noticed though:
With rabbit ear antenna, there ARE stations where the tuner locks (TUNED is lit) and STEREO reception is possible (albeit not very good quality).
HOWEVER, I noticed that the frequency that the tuner locks to is OFF relative to official frequency by several kHz.
BTW, after tweaking the pots as suggested and returning them back to their original position, I can no longer play ANY station in STEREO and scan would NOT tune to any station automatically.
Go figure
PS Anyone got a service manual of the AT-27 and is willing to share? Thx
Here's what I noticed though:
With rabbit ear antenna, there ARE stations where the tuner locks (TUNED is lit) and STEREO reception is possible (albeit not very good quality).
HOWEVER, I noticed that the frequency that the tuner locks to is OFF relative to official frequency by several kHz.
BTW, after tweaking the pots as suggested and returning them back to their original position, I can no longer play ANY station in STEREO and scan would NOT tune to any station automatically.
Go figure
PS Anyone got a service manual of the AT-27 and is willing to share? Thx
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Hi qwer1304,
I do a lot of tuner alignments. DF96 is right, your tuner is off even worse than it was. I think it's time for a technician to work on it. You should never attempt to align a tuner without the instruments for the job. Sorry for that advice, but it is clear to me that you aren't going to get anywhere.
Where are you located? Maybe there is a good technician close to you. It isn't uncommon for tuners to drift out of alignment over 20 years or more of use.
-Chris
I do a lot of tuner alignments. DF96 is right, your tuner is off even worse than it was. I think it's time for a technician to work on it. You should never attempt to align a tuner without the instruments for the job. Sorry for that advice, but it is clear to me that you aren't going to get anywhere.
Where are you located? Maybe there is a good technician close to you. It isn't uncommon for tuners to drift out of alignment over 20 years or more of use.
-Chris
I tried your suggestion (both options), but that did NOT improve the situation.
Here's what I noticed though:
With rabbit ear antenna, there ARE stations where the tuner locks (TUNED is lit) and STEREO reception is possible (albeit not very good quality).
HOWEVER, I noticed that the frequency that the tuner locks to is OFF relative to official frequency by several kHz.
BTW, after tweaking the pots as suggested and returning them back to their original position, I can no longer play ANY station in STEREO and scan would NOT tune to any station automatically.
Go figure
PS Anyone got a service manual of the AT-27 and is willing to share? Thx
I did mention that T102 requires "few" degrees of adjustment. It is quite sensitive but don't despair. Here are schematics of tuners that use the same discriminator IC LA1265 as your AT-27L. These schematics were taken from the following: Sony FH-215, Onkyo T-4130/T-4140, Sanyo DC D30 and Kenwood T-7X/T-7LX.
You need to figure out where the "NULL" testpoints are on your AT-27 by tracing the connection between pin 9 and 20. The internal picture of the AT-27L from the internet lacks detail. There are at lest 2 testpoints around IC101. You need a digital voltmeter for this and a very strong FM station.
I deal with FM tuners on a regular basis. I reverse-engineer, modify, re-design and align them. Here is an example of the FM tuners I have worked on..
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Hi ej25awd,
Nice instrument you have in the AP. My tuner modifications are always light and concentrate on sound quality in the IF, detector and MPX sections. Of course also the audio sections when it is to solve problems. Some of this can be a lot of work.
One tuner job that struck me was the one modified with the huge capacitors. I see them come in like this as well. Things like that have to come out first. It's great to see another person doing excellent work as you seem to do.
-Chris
Nice instrument you have in the AP. My tuner modifications are always light and concentrate on sound quality in the IF, detector and MPX sections. Of course also the audio sections when it is to solve problems. Some of this can be a lot of work.
One tuner job that struck me was the one modified with the huge capacitors. I see them come in like this as well. Things like that have to come out first. It's great to see another person doing excellent work as you seem to do.
-Chris
BTW, I found partial schematics of the AT-27L on the net.
I marked on the attached sheet the relevant parts (T102, SR101, pin 9 and pin 20). Also uploaded all the schematics I've found. There's a testpoint TP7 connected to pin 9, and there's a resistor R121 between pin 9 and pin 20 as with, e.g. Onkyo T-4130/T-4140, but there's no testpoint at pin 20 (analog of TP-2 in Onkyo T-4130/T-4140).
So, where do I measure the NULL voltage?
Thx,
David
I marked on the attached sheet the relevant parts (T102, SR101, pin 9 and pin 20). Also uploaded all the schematics I've found. There's a testpoint TP7 connected to pin 9, and there's a resistor R121 between pin 9 and pin 20 as with, e.g. Onkyo T-4130/T-4140, but there's no testpoint at pin 20 (analog of TP-2 in Onkyo T-4130/T-4140).
So, where do I measure the NULL voltage?
Thx,
David
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Chris,
The older AP System One and Two are perfect for FM tuner work. I have both and they have all that I need for FM-related work. You can't evaluate IBOC filter designs and FM stereo tuner IBOC immunity with newer APs that has a DAC only generators. Their frequency doesn't go high enough. Analog AP generators go up to 204kHz which is just 1kHz shy of the ideal frequency range for testing the 5th harmonic of the sub-carrier.
Tuner work can be quite involved. Some are more complex and require a lot of time, effort and understanding. Simple tuners can have complex problem like frequency drift that requires a lot of patience to characterize and fix. We can discuss more of this in private if you want as it is getting OT.
- Jov
Success!
Opened the AT-27L today:
1.There's a testpoint TP8 not in the schematics on the other side of R121, so measuring was simple.
2.The discriminator was WAY off (about 4V). After tuning T102 it's imperceptibly (with my DVM) close to 0V and stereo detection is back.
3.I played a little bit with SR102, but there was no major change.
Thx
PS Some stations are still off by ~100KHz. Any way to recalibrate this?
Hi qwer1304,
That's very good news. That amount of offset in the detector would throw everything off. Now that you have that adjustment tamed you can very carefully try the others you have tried to see if everything else comes into alignment. Very slow motion with the tool and a little care might help. Try working with a weak station as these will respond more strongly to a small adjustment.
-Chris
That's very good news. That amount of offset in the detector would throw everything off. Now that you have that adjustment tamed you can very carefully try the others you have tried to see if everything else comes into alignment. Very slow motion with the tool and a little care might help. Try working with a weak station as these will respond more strongly to a small adjustment.
-Chris
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