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Tda 1514a help please
Tda 1514a help please
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Old 17th November 2020, 06:16 PM   #1
Gmarkw2 is offline Gmarkw2
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Default Tda 1514a help please

Hi hope someone can help my understanding here. I am repairing a Cambridge Audio A1 mk3 fitted with two tda1514a chip amps. The two rail fuses were blown so I powered up through a DIM bulb tester. It indicated a problem (bright 100w bulb). I removed the two chips and found one had significant low ohms between supply pins and others. The other channel did not have shorts. I put the suspected good chip back in circuit and powered up. All ok this time with current draw and fuses but I checked the dc offset voltage across the single channel speaker output. Bearing in mind only one chip in circuit which I didnít think should matter, I am getting 2.3vdc at speaker terminal. I havenít tried speakers yet but this doesnít appear correct. Am I missing anything in my understanding or do you think the second chip has an issue also.
Many thanks
Mark
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Old 18th November 2020, 11:07 AM   #2
Mooly is offline Mooly  United Kingdom
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Tda 1514a help please
Taken at face value your 2.3 volts shows a problem as there should always be minimal DC voltage present across the speaker output.

I'm assuming this is a DC coupled circuit running on split rails rather than a single rail design with output coupling cap in which case some voltage would be expected with no load present.
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Old 18th November 2020, 11:36 AM   #3
Gmarkw2 is offline Gmarkw2
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Hi and thanks for responding. Yes split rail as in the attached. My gut said that there was something wrong with the second chip despite not causing fuse blow itself. I have not connected speakers as precaution whilst everything checks out. One chip gone the second giving the 2+ Vdc at speaker terminals. Just wondered if there was anything peculiar about tda1514a which would show this. Very simple and tda1514a spec sheet states low output offset!!!
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Old 18th November 2020, 11:44 AM   #4
Mooly is offline Mooly  United Kingdom
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Tda 1514a help please
That's a pretty standard circuit. Any DC offset should be very low and certainly under -/+ 50 mv DC.

What is odd is that failure of one chip is a common and expected scenario but one that should have zero impact on the other channel. As long as the supplies are correct the chip should produce a very low offset.

It might be worth just connecting a dummy load to this channel and seeing if the offset remains. Even a 100 ohm resistor would suffice.

On remote possibility is the chip is unstable with no load but if that were so then it would be bad design.
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Old 18th November 2020, 11:49 AM   #5
Regenpak is offline Regenpak  Netherlands
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Check C41. And the voltages on pin 1 and 9. Those should be zero. The TDA1514 is just a powerful opamp.
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Old 18th November 2020, 12:15 PM   #6
Gmarkw2 is offline Gmarkw2
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Thanks I will try that. I have 2 chips on order but just trying to further my knowledge and understanding. I will try the 100R load. and retest/report back.
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Old 18th November 2020, 12:47 PM   #7
Gmarkw2 is offline Gmarkw2
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OK thanks guys. C42 100uF checks out ok (101.2uf and Vloss 0.7% and esr 0.27Ohm)
Pin 1 Ov to gnd Pin 9 2.6v to gnd. Voltage across 1 and 9 2.6v
When 100R load placed across speaker output, 2.6vdc remains !
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Old 18th November 2020, 01:27 PM   #8
Mooly is offline Mooly  United Kingdom
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Tda 1514a help please
It would be worth just measuring the DC voltage on pin 1 which is the input. This should be zero.
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Old 18th November 2020, 03:20 PM   #9
Gmarkw2 is offline Gmarkw2
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Yes pin 1 to gnd is 0v
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Old 18th November 2020, 05:18 PM   #10
Mooly is offline Mooly  United Kingdom
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Tda 1514a help please
That only leaves the chip then really... or its oscillating and unstable and so skewing the reading a DVM shows, or...

If the unit has a hidden history then things like spillage or contamination from say a leaky cap could conceivably cause issues. It would have to be around the chip though.

All a bit of a coincidence really, to have a definite fault like a zapped chip and to then also have a weird issue on the other channel.
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