DC offset on TDA7293 3-way parallel configuration

[vicdiaz]

Hi,

While troubleshooting my Linn 2250 amplifier I found than one of the channels has a DC offset of 1 volt. Each channel has 3 TDA7293 chipamps in parallel configuration.

My question is, is there a way to determine in situ which TDA7293 is faulty without replacing all 3 chipamps?

 

[FauxFrench]

With such a high output offset it should be the TDA7293 acting as "master" that is faulty. Check the voltage on pin 4 with reference to pin 8. Further explanation in the datasheet.
It may alternatively be a DC or a capacitor leaking at the TDA7293 input.

Good luck,

 

[vicdiaz]

It may alternatively be a DC or a capacitor leaking at the TDA7293 input.

Good luck,

Hi FauxFrench, when you say at the TDA7293 input, which input, IN+ (Pin 3)?

 

[FauxFrench]

Hi FauxFrench, when you say at the TDA7293 input, which input, IN+ (Pin 3)?

It can be at any of the two inputs (pin 2 or pin 3). For such amplifiers, there should be no significant input bias current flowing to or from any of the two inputs. If for instance a capacitor leaks and leaves a small current flow towards an input pin, this will result in an offset at the output when the TDA7293 tries to compensate for that current.
You mentioned that you have an output offset of 1V (clearly too much). My feeling is that the chance that an input offset voltage error (causing the output offset voltage) on a TDA7293 chip changes so much without leaving the chip fully defect is small. Does that channel play or is it "dead"? If it plays but with an offset of 1V, it is likely to be a fault in the input to the TDA7293.
Have you got a rather high impedance headphone or an oscilloscope?
Would you happen to have a schematic of that Linn amplifier?
Without loads (speakers) at the outputs, will anything get hot or in other way overloaded?

 

[sq225917]

Download the data sheet for the chip from st. David E posted the amp schematic on pink fish.

Itll be r1 or an input cap, c1 on the schematic.

 

[vicdiaz]

@FauxFrench,

I have the schematic, but while troubleshooting touched something else and power supply blew up a resistor. Now need to figure out the resistor value and type since it a weird long SMD resistor.

An externally hosted image should be here but it was not working when we last tested it.

 

[vicdiaz]

Pic for above post:

An externally hosted image should be here but it was not working when we last tested it.

 

[vicdiaz]

Schematic does not include the switching power supply diagram...

 

[FauxFrench]

@FauxFrench,

I have the schematic, but while troubleshooting touched something else and power supply blew up a resistor. Now need to figure out the resistor value and type since it a weird long SMD resistor.

Hi Vic,

The resistor value is "R047" meaning 47mOhm! It seems to be a power type accepting at least 2W (my guess).

Doing some investigation without knowing the exact circuit for the TDA7293 may be possible assuming that the application circuit from the datasheet is reasonably close.
Repairing a higher power SMPS circuit, using an IR2110S and an SG3524, without knowing the exact design is almost impossible. Try if you can find that schematics.

 

[vicdiaz]

Hi Vic,
Repairing a higher power SMPS circuit, using an IR2110S and an SG3524, without knowing the exact design is almost impossible. Try if you can find that schematics.

Will try to find one.