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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 8th Jul 2019, 11:05 am   #1
SteveCG
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Default TDA 1200B AM/FM radio chip

My question is this: does this chip have an intrinsic long term defect?

I have now encountered this chip twice, both in Roberts radios:
the FM3 and the Rambler II.

In both cases it is the AM section that has problems - for the FM3 low gain on both LW and MW, for the Rambler II no Local Oscillator or sign of life in the RF part either. For both of them the voltages on the pins are in agreement with the ST (a chip maker) application notes.

I've checked the switch contacts, looked for dry joints, etc.

I was wondering whether others have had similar experiences with this chip?

As an aside the only difference between the FM3 and the Rambler II seems to be that the printed circuit board is fully populated in the FM3 - with the tone button position on the Rambler II being the VHF button on the FM3. Both had a plastic frame loudspeaker and both used the TBA820M audio chip.
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Old 10th Jul 2019, 11:32 am   #2
SteveCG
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Default Re: TDA 1200B AM/FM radio chip

A postscript thought:

Both the Roberts radios have a socket for an external power supply (9 volts). This, I note, has a -ve centre pin. This is rather unusual as most externally powered devices have a +ve centre pin. I just wonder whether an incorrectly polarized psu has been used and has damaged/taken out the TDA1220B chip?
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Old 10th Jul 2019, 3:00 pm   #3
julie_m
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Default Re: TDA 1200B AM/FM radio chip

In the last century, negative centre was the norm for DC power inputs. Positive centre is a recent thing.
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Old 11th Jul 2019, 12:43 pm   #4
SteveCG
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Default Re: TDA 1200B AM/FM radio chip

Julie_m,

That there were -ve centre pin expecting equipments 25 odd years ago I can indeed remember. That it was the norm back then is something I was not aware of. Thank you for the info.
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Old 11th Jul 2019, 1:43 pm   #5
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Default Re: TDA 1200B AM/FM radio chip

It has always baffled me why manufacturers don't spend a few pence on a diode on the power input socket to protect against reversed polarity in supply... I just can't see that that tiny extra cost couldn't be accommodated... It's just about reasonable to expect Mr Average to check correct voltage from a wallwart - polarity is probably just a step too far for most people.

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Old 11th Jul 2019, 7:06 pm   #6
AC/HL
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Default Re: TDA 1200B AM/FM radio chip

I suspect that most people use them on batteries, or if they want a mains supply they buy the recommended one from the original shop. Those who buy aftermarket either get it right first time or keep quiet about it. The worst are those reversible ones with the tiny embossed markings.
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