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2270: signal strength to zero and no stereo decoding

Traverso

Traverso

Active Member
Hello,

I try to fix the following issue with a 2270.

When operating the FM section, the signal is zero for the signal strength meter and stereo beacon never light s up.
However:
- the audio from FM radio is working,
- the stereo beacon is ok for other inputs than FM such as Tape 1 or Tape2,
- the signal strength is working for AM,
Also, I cannot figure out where is elaborated the signal strength meter indication.

In addition, when I switch on the muting no more audio can be heard whatever the position of the corresponding adjusting potentiometer.

Thanks a lot for the help


Alain
 
Last edited:
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The signal strength meter is driven by the IF signal at H206, rectified by H229 and H230, to J207, then amplified by the circuit including H553.
The stereo beacon (and muting) is enabled in part by the same signal from H206.
Since you have no stereo beacon AND no signal strength meter, I suspect the problem is related to a defect somewhere in that area.

There is a good description in the service manuals for the 2245 and 2270, but it's kind of hard to follow.
The 2230 is slightly different.

IF signal strength is monitored by H306. If IF signal strength is above muting level, H306 is ON, H307 is OFF.
If there is excessive noise, then H552 is ON which is wire-anded to J305, keeping H306 OFF.
If the discriminator is outputting DC due to the tuning being off station, either H560 or H561 is ON which are wire anded to H307 as well.

If the station is properly tuned, not noisy and the signal strength is high enough,
H306 is ON, SS OK
H307 is OFF, SS OK AND
H552 is OFF, Noise is OK AND
H560 is OFF Tuning centered AND
H561 is OFF Tuning centered
If SS, Noise and center tuning OK then:
H308 is ON allowing audio signal to pass to the center tap of the 38kHz transformer.
H302, 19kHz amplifier is enabled by H303.
If the 19kHz pilot is present from the broadcast station, then it is doubled to 38kHz by H304 and if of sufficient amplitude to decode stereo, H305 turns on causing the stereo lamp to illuminate.
The left and right matrixed audio signals are recovered by H317-H320, de-emphasis is applied, then they are amplified, filtered of high frequency noise and sent along through the selector switch to the preamp.

Tom
 
Thanks to all of you and the details by Tom.

The problem was on the IF board P 200. When testing the H 206 final transistor, Ve=Vb=Vc=0 Volt
The transistor was not fed. The culprit was a broken resistor R257, 100 Ohms. One can see it on the corresponding attached pic.

After it was changed, everything was back to normal.

Happy with this

Thanks again


AlainIMG_1665.jpg
 
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