JL Audio 500/1 v1 Rev. 10 Board

[Perry Babin]

Does it swing from ground to 80v?

 

[RiLoWa]

I have ~40VDC from the + output terminal and ~40VDC from the - output terminal to ground.

Was that the right measurement?
OR is it this

With the scope grounded at main ground, my 100Hz signal peaks at ~60V.

 

[Perry Babin]

40v DC of bias is correct.

When you drive the amp to clipping, does it clip very near 0v on the bottom and very near 80v on the top?

 

[RiLoWa]

Yes it clips both sides. I editted post #22 during your response.

Picture shows clipping at ~50% gain setting. Scope set to 50V Div. 2mS.

 

[Perry Babin]

Set the scope to 10v/div instead of 50v/div and set it to AC coupling instead of DC coupling. Does it still only swing plus/minus 30v (±3 div)?

 

[RiLoWa]

Making those changes to the scope and setting it to 1mS Div so the camera could capture more of the display I have this between the + terminal and main ground.

 

[Perry Babin]

To make sure that the scope is calibrated, set the vertical amp to 20v/div and set the coupling to DC. Align the trace with the center reference line ant touch the probe to any point with rail voltage. Does the trace deflect to precisely 4 divisions?

 

[RiLoWa]

Yes, I have exactly 4 divisions or 80V

 

[Perry Babin]

If you slowly increase the level, does it start to clip at the same point shown on the previous photo or does it swing closer to the rails (0-80v) initially?

 

[RiLoWa]

It swings 0-80V initially then clips 0-80V at about the same point using the amplifiers gain. This is taken between the source of Q507 and main ground.

 

[Perry Babin]

To clarify, does the waveform ever swing to the top and bottom of the display if you set the scope to AC coupling and 10v/div?

Or does it always start clipping at about the ±3div points with the scope set as above?

 

[RiLoWa]

To clarify, does the waveform ever swing to the top and bottom of the display if you set the scope to AC coupling and 10v/div?

Yes it does.

Or does it always start clipping at about the ±3div points with the scope set as above?

The key difference is that it swings 80V (8 divisions, AC Coupled) at the Q507 source, but only 50V (5 divisions, AC Coupled) at either output terminal which is the where the last picture was taken from.

 

[RiLoWa]

It would appear that I need to clean up the solder conections on R519 & R520. While checking those, R520 measured intermitantly out of spec. After checking, I must have improved R520's connection, and when I repowered it seems to have cured to problem. I will remove and clean up the leads and report back.

 

[Perry Babin]

At the source of Q507, there is supposed to be a square wave. Is that what you have?

I don't remember exactly which transistors are high-side and which are low side. I'm assuming that 507 is a high-side FET.

 

[RiLoWa]

Yes, 80V square wave. Q504 & Q507 are high side (the two outter outputs).

Pulled those two resistors and they measure in spec. Cleaned them up and reinstalling them seems to have corrected the issue.

I still only have~2.265V across that 5.6V zener. Is that anything to worry about?

 

[Perry Babin]

It's not right and should be corrected if this is for a paying customer. If it's your amp, you'll have to decide if you want to replace it or not.

 

[RiLoWa]

Must be the zener that is defective then?

 

[Perry Babin]

Probably. Lift it and see if it's leaking. Do not apply power to the amp with the zener out of the circuit.

 

[RiLoWa]

Lifting the middle leg, it appears to be leaking.

Looking at Mouser there are several 5.6V zeners in the SOT-23 package.
Do you have a recommendation on a correct replacement part?

 

[Perry Babin]

No. I don't think it's critical.