Digital Deaigns M4A

[caraudioguy2100]

I do not get any rail to rail oscillation on the outputs .

I checked all of the regulators and all seem to work fine .

I checked all the 47mh inductors all are fine .

I get both positive and negative rail voltages .


Any ideas or common issues with this design of amp as to why it won’t oscillate

 

[caraudioguy2100]

I’m gonna try replacing the smd transistors on the audio driver board since some appear a brownish color from heat

 

[Perry Babin]

Black probe on the negative rail, what's the DC voltage on terminal 2 of the 21844s?

Do you have signal reaching the driver board?

 

[caraudioguy2100]

The muting transistor is defective .

It’s marked K30A

So I’m assuming it’s a KSC30A

Do you happen to know if any good replacements for that transistor ?

 

[Perry Babin]

K30A is a jfet. It doesn't read like other transistors.

What do you read 1-2, on ohms?

Then read 2-3 on diode check. Reverse probes and read again.

 

[caraudioguy2100]

Amp has no output I took it out of the amp and the amp plays audio so it’s defective

 

[caraudioguy2100]

Any ideas on what I might be able to use ?

Legs1-2:Ol
Legs2-3:1.455

Proves reversed

Legs1-2:OL
Legs2-3:1.455

 

[Perry Babin]

Do you have any J111 jfets?

 

[caraudioguy2100]

Yes

 

[Perry Babin]

Check one like you checked the k30 and post.

 

[caraudioguy2100]

Legs 1-2:25.2 ohms
Legs 2-3:0.686

Proves reversed

Legs 1-2:24.8
Legs 2-3:OL

 

[Perry Babin]

Post a photo of the circuit around the transistor, good enough to read the circuit board designations.

 

[caraudioguy2100]

Let me know if you need any other photos

 

[Perry Babin]

Some similar amplifiers use a J111 in that location. I don't know if this amp will be OK with it. You need to confirm that the muting circuit is working properly.

 

[caraudioguy2100]

How would I check the muting circuit ?

And install the J111 and what readings do you need

 

[Perry Babin]

The rail voltage needs to build to full voltage before the drive begins. See attached.
http://www.bcae1.com/temp/ampstartup01.mkv

 

[caraudioguy2100]

I installed it and amp has no output again .

So either it won’t work in the amp or something is wrong in the muting circuit

 

[Perry Babin]

What is the DC voltage from leg 2-3 (black on 2) after the end of the mute delay?

 

[caraudioguy2100]

46 millivolts

 

[Perry Babin]

Check R81, D82 and D83.