DC out and popping during on/off

[nonprof]

Hello Sir!

Good day! Need your usual help...I have class D amplifier which looks like SLA amp(sinus).

The problem of this amp is having a DC out of 1volt and loud popping sound every time I turn on/off this amp.

On class D what things brings out a small DC voltage on speaker? This amp that time is still working although you can measure 1 volt DC at the speaker.

I also notice the output inductor is very hot, you can't touch it for a period of second.

After an hour of fault isolation the IRF640s died.

thank you very much!

 

[Perry Babin]

I've seen similar amps have 4v of DC on the outputs with no load. With a load, the DC dropped to acceptable levels.

In a treo amp (similar) a defective Q21 was causing a loud turn-off pop.

 

[nonprof]

Thanks but, with load the DC still exist around 1V. This amp produces whining sound (with input or no input). I repaired the driver board before. I replaced the NOR gate with 74ACT00(TTL), not sure if it is requiring CMOS type. It also uses delicate LM6172 IC.

 

[Perry Babin]

TTL ICs aren't generally rated for more than 5v. Is the supply voltage across the IC more than the rated supply voltage for the IC?

Which driver board is this?

 

[nonprof]

supply voltage is 5V of 74ACT00(NOR),I don't have have the original IC, it was repaired before. This IC drives the Opto and driver transistors. ..here's the photo.

 

[Perry Babin]

Do you know if that's the correct part number IC? Attached are the photos of the same board from a massive amp. The markings were a bit difficult to read but doesn't look like hc00.

 

[nonprof]

the installed IC is ACT00(TTL). should replace it. Thank you again Perry!

 

[Perry Babin]

I've only seen one so I asked someone else and he stated that he's seen the hc02 in some of them. That may mean that there are several versions. Maybe yours is supposed to have the 00 in it. Try contacting the manufacturer to see if they will tell you what it's supposed to be.

 

[nonprof]

Thanks perry. Ive switched to cmos part but still no luck. I have one more question, can u pls provide the encircled ic from the image above? Is it lm292 or lm211? The installed iic are lm211 and lm6172. Thank u so much!

 

[Perry Babin]

See attached.

 

[nonprof]

Omg.i installed lm211 and put a bypass capacitor parallel to 1k resistor before pin 1 of this board and its working now.dc out gone
.thank u perry!