Punch X360.2 schematic / advice

[Perry Babin]

Black on pin 7?

Can you touch them with your scope?

If nothing accurate (using scope), I'd like to know if they are equal in voltage.

 

[djamps1200]

Before we continue, it looks like since that brief instability from touching pins 1 or 2, I've lost some drive voltage on one side. Drain voltage is still normal looking. See attached.

 

[Perry Babin]

Did any of the PS FETs fail?

 

[djamps1200]

Does not appear so. Waveform on each tab is good. The gate voltage is the same on both sides of the gate resistors so the FET's aren't pulling it down.

 

[djamps1200]

Emitters of Q5 and Q1 are showing the lower voltage. Base voltage on Q1,Q2,Q4,Q5 are all normal.

 

[djamps1200]

If there's a potental issue with Q5 or Q1. I have MMBT3906 on-hand but not MMBT3904. The only NPN's in this footprint I have on-hand are BC847 and MMBT2222A

 

[Perry Babin]

Q5 and Q1 drive opposite banks. If the emitter voltage is low on both, the drive can't be correct on the gates of any of the FETs.

Lift (example below) D10 and D11 and recheck the drive.

 

[djamps1200]

Their emitters are tied together so they would always be the same, am I wrong? According the schematic they drive Q6 and Q10, which are adjacent.

 

[Perry Babin]

You're right.

Did you lift those diodes?

 

[djamps1200]

No change lifting those diodes.

 

[Perry Babin]

Is the base of Q5 going higher than 1v above the emitter of Q5?

Do you have the MMBTA06 or MMBTA56?

 

[djamps1200]

Yes, almost 5V difference. The ones I listed are the only ones I have. I am not a repair tech (am doing this for a friend), normally I am designing/building my own things and stick to a limited supply of parts and build around them.

 

[Perry Babin]

The 2222 will replace the 3904.

 

[djamps1200]

Replaced. Drive waveforms are back to normal. How shall we proceed?

 

[Perry Babin]

Did you try probing pins 1 and 2 with your scope (use 10x on the probe)?

 

[djamps1200]

Scope on 10x mode works better. Pins 1 and 2 are both 4.59 volts with respect to B ground.

 

[Perry Babin]

DC voltage on both ends of R17 and R18?

 

[djamps1200]

Apologies, I tested the wrong chip on the last reply.

Pin 1: 2.21
Pin 2: 2.21
R17: 36.15 / 2.21
R18: -36.5 / 2.21

 

[Perry Babin]

I'd expect the duty cycle to be reduced with those voltages. Can you increase the DC voltage on the B+ line to see if the duty cycle will drop?

Have you checked all of the output transistors to see if any were shorted?

 

[djamps1200]

Duty cycle does change with B+.

9V to 10V, seems maxed out. No change.
10V to 13V, very little change.
13.5V-13.7V, duty cycle drops drastically. Cycle gets so low amp shuts off above 13.7.

Outputs are not shorted. B+ power draw is reasonable at 700ma. Makes audio.