Tda 1514a help please

[Gmarkw2]

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 didn’t think should matter, I am getting 2.3vdc at speaker terminal. I haven’t tried speakers yet but this doesn’t appear correct. Am I missing anything in my understanding or do you think the second chip has an issue also.
Many thanks
Mark

[Mooly]

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.

[Gmarkw2]

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!!!

[Mooly]

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.

[Regenpak]

Check C41. And the voltages on pin 1 and 9. Those should be zero. The TDA1514 is just a powerful opamp.

[Gmarkw2]

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.

[Gmarkw2]

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 !

[Mooly]

It would be worth just measuring the DC voltage on pin 1 which is the input. This should be zero.

[Gmarkw2]

Yes pin 1 to gnd is 0v

[Mooly]

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.