Technics ST-S707 tuner signal cutting out - diyAudio
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Old 27th January 2016, 06:08 PM   #1
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Default Technics ST-S707 tuner signal cutting out

I'm having some issues with a Technics ST-S707 tuner that I recently picked up.
I'm hoping someone with more expertise than myself (nil) with tuners can help me out with this. The tuner pulls in a signal pretty well but as the audio level goes up, the signal seems to cut out.
It is NOT my receiver/amplifier because it is happening at the tuner. In fact I don't even have to have the tuner hooked up to anything to observe it.
I can observe it on the tuner display with the Quartz lock, FM IF and signal center display actually pulsing off and on in concert with the audio level being received.

What I mean is, if, say the DJ shouts suddenly, it cuts out momentarily with the associated pulsing off/on of the previously mentioned display parameters. It's like the signal level is overloading something causing it to cut out - but the signal strength, while strong, is certainly not THAT strong.

Any ideas where to start looking. I checked all the electrolytic capacitors (changed a few suspect ones), but that had no effect on the issue.
Does it just need a tuner alignment or are there some sick parts to go after?

TIA
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Old 27th January 2016, 07:39 PM   #2
gpapag is online now gpapag  Greece
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Does it happen with the AM too?

George
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Old 27th January 2016, 08:03 PM   #3
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Didn't try it but I'll check tonight
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Old 28th January 2016, 01:17 AM   #4
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Well, it doesn't happen on AM although the signal was so bad it took a while to determine it is an FM only issue. I did take another look at the signal strength and the signal strength seems to be dropping enough that it comes out of "lock" but it definitely happens in conjunction with the "level" as I mentioned previously.
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Old 28th January 2016, 06:43 AM   #5
gpapag is online now gpapag  Greece
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Quote:
Originally Posted by BlownRx7 View Post
Well, it doesn't happen on AM although the signal was so bad it took a while to determine it is an FM only issue
.
This is encouraging.
Now as this tuner is well equipped with capabilities, use the reduction method to do the initial FM troubleshooting without poking inside.
Ensure the FM antenna is properly connected.
Dial on a strong signal.

25 kHz/100 kHz tuning selector to 25kHz.
Auto/ Manual selector to Manual.
Muting to OFF
Mono/ Stereo to Mono
RF narrow
IF narrow

If the problem is due to some switch settings, the above combination is the least interfering.
If with these settings you have no signal cut-out problem, change switches position –one at a time-starting with the switches at the bottom of the list.
Keep notes on a paper of the results for each switch position.
If you want to, report back.

George
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Old 28th January 2016, 04:45 PM   #6
DF96 is offline DF96  England
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Most FM tuners have an off-tune mute facility. This usually uses signal strength, but some also use the degree of off-tuning. Obviously they need to remove the audio before using the discriminator output to detect off-tuning, and there will be some sort of low pass filter to do this. My guess is that an electrolytic has dried up or gone open circuit, so the muting circuit is now seeing the audio instead of just a DC level.

Try to identify the FM IF and discriminator chip. Find a datasheet for it. You will probably find that one of the pins has a cap attached to ground to do the low pass. Look for a similar cap in your tuner circuit.
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Old 29th January 2016, 03:16 AM   #7
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Thanks George and DF96 for some excellent advice. I'm feeling a little stupid for not reading the manual before messing around and asking questions. At any rate, the tuner has a variable muting function that can be set to 20, 30, 40 or 50db. It defaults to 30db. It can be turned off but it automatically comes back on each time a new station is tuned and must be manually turned off again, if desired.

So I can get rid of the annoying muting but it still goes in and out of stereo mode in concert with the audio level peaks. So, is it possible I still have a bad cap or does it just need an alignment?
Here is an internal photo and the subject MUTE OFF/SCAN LEVEL button
Attached Images
File Type: jpg ST-S707 switches.jpg (35.7 KB, 42 views)
File Type: jpg ST-S707 internals.jpg (41.6 KB, 40 views)
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Old 29th January 2016, 11:05 AM   #8
DF96 is offline DF96  England
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See my post 6. If you are able to follow my advice, do so. If not, find someone else who can. Do not attempt re-alignment unless you really know what you are doing and have the necessary test equipment. As I said, the most likely cause is a cap, so you just have to find it and replace it. Do not be tempted to replace every cap, or every electrolytic; that is a good method for stopping it working altogether.
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Old 30th January 2016, 07:16 PM   #9
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DF96
Got it, thanks. I definitely was NOT going to try re-alignment. I do not have the expertise or the equipment.
How do I go about identifying the FM IF and discriminator chips(s)? I don't have a schematic ...
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Old 31st January 2016, 12:05 AM   #10
gpapag is online now gpapag  Greece
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Page 2, IC102

George
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File Type: pdf technics_st-s707_schematic.pdf (306.4 KB, 12 views)
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