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Old 06-29-2022, 06:58 AM   #1
Nevillet
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Default Sony CFS-B11 Radio cassette boom box

Hi all,

I am trying to diagnose this small radio/cassette player where the audio cuts out after a few seconds - then comes back, then repeats.

I've checked the voltage and current coming from the PS board and it seems fine.

I've also powered it from a bench PS and the fault remains.

All of the electrolytics on the audio board are normal with no high ESR's or bulging tops.

Any ideas?

I've attached the service manual for the unit.

Many thanks.

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File Type: pdf sony_cfs-b11.pdf (12.07 MB, 80 views)
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Old 06-29-2022, 07:14 AM   #2
lotas
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Default Re: Sony CFS-B11 Radio cassette boom box

Check the headphone jack, there is a contact lock for the speakers.
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Old 06-29-2022, 08:20 AM   #3
Nevillet
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Default Re: Sony CFS-B11 Radio cassette boom box

It doesn't seem to be the headphone jack. I've connected directly to the outputs of the TA8229K (P3+P5) and it still happens
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Old 06-29-2022, 08:30 AM   #4
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Default Re: Sony CFS-B11 Radio cassette boom box

I've connected to the input pins (P11 + P12) same problem it cuts out.
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Old 06-29-2022, 09:24 AM   #5
lotas
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Default Re: Sony CFS-B11 Radio cassette boom box

Does the sound disappear on all functions "FM", "AM", "TAPE"?
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Old 06-29-2022, 09:29 AM   #6
Nevillet
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Default Re: Sony CFS-B11 Radio cassette boom box

yes it does on FM/AM/SW (the tape deck is non functional due to lack of belts).
I've checked the supply voltage to the tuner IC (CXA1238S) (and when working the supply is 4.2V then it disapears to 0.2V when there is no audio o/p
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Old 06-29-2022, 09:48 AM   #7
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Default Re: Sony CFS-B11 Radio cassette boom box

check the input to the amp i.c. - it could be cutting out from an internal fault.
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Old 06-29-2022, 09:49 AM   #8
lotas
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Default Re: Sony CFS-B11 Radio cassette boom box

If there is a bolon with liquid for cleaning contacts, then you can puff to the contact groups of the switches and click them back and forth (contact may disappear in them).
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Old 06-29-2022, 09:58 AM   #9
Nevillet
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Default Re: Sony CFS-B11 Radio cassette boom box

I think it might be the amplifier that is intermittantly shorting which drags the supply down.
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Old 06-29-2022, 09:59 AM   #10
Nevillet
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Default Re: Sony CFS-B11 Radio cassette boom box

The switches have been disasembled and cleaned with super 10
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Old 06-29-2022, 12:43 PM   #11
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Default Re: Sony CFS-B11 Radio cassette boom box

Quote:
Originally Posted by Nevillet View Post
yes it does on FM/AM/SW (the tape deck is non functional due to lack of belts).
I've checked the supply voltage to the tuner IC (CXA1238S) (and when working the supply is 4.2V then it disapears to 0.2V when there is no audio o/p
The supply to IC301 should be 7.6 volts on pin 1, Pin 18 is a 4.8v output that is switched by the ic, This output controls the base of Q303 which supplies 4.1 volts to the tuner board, that's why that voltage is changing.
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Old 06-29-2022, 12:51 PM   #12
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Default Re: Sony CFS-B11 Radio cassette boom box

Quote:
Originally Posted by Nevillet View Post
The switches have been disasembled and cleaned with super 10
Did you have this problem before or after you disassembled the switches?
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