Metrik MAB 901

[barber02]

It was set at 10x. I’ll redo the test.

 

[barber02]

I’m not sure what’s causing the shadow on the screen. Must the refresh rate of the screen. Anyway.

Probe is at 1x volts per division is 2 Time division is 5us DC coupled.

 

[Perry Babin]

Black probe on pin 7, what's the DC voltage on pins 8 and 11 of the 494?

 

[barber02]

Here is my saw tooth wave at the same setting as above on the Oscope.

 

[barber02]

I get 12.29 volts on pins 9 and 11

 

[Perry Babin]

Does your trace deflect off of the screen if set to 2v as above?

If you set it to 5v/div, does it deflect 2,4 divisions, as it should?

 

[barber02]

Yes at 2 volts per division it does go off screen. It does not deflect off the screen at 5 volts per division. It jumps up 2 divisions.

David

 

[barber02]

I was probing the power supply mosfets and discovered this anomaly. With the amp sitting on my bench with the power terminals to my left. The top row of power supply mosfets output looks like this in the attached photo. The bottom row of mosfets mirror the output of the TL494 so they look normal. The 4 drivers all have The expected wave form that is a mirror of the output of the TL494.
Probe is 1x at 2 volts per division.

David

 

[Perry Babin]

You need to be clear as to which terminals of each transistors you're referring to. I can assume but it wastes time if I'm wrong.

 

[barber02]

Sure thing Perry. I know your trying to help and I’m making it hard for you to. I apologize.

The picture above was taken with the probe grounded to the negative terminal and the probe was on the source leg of any of the top row of power supply mosfets.

With my probe grounded to the same place the probe on source leg of any of the mosfets on the bottom row of power supply mosfets I get this.

Am I not grounding my scope to the right place?

David

 

[Perry Babin]

The source leg of the power supply FETs should read 0 ohms to the primary (main) ground terminal.

With no power supply connected to the amp. Do you read 0 ohms from all PS FET source legs (probe on FET leg at the body of the FET) to the ground terminal?

 

[barber02]

For the drain on the top row of power supply mosfets with the probe grounded to the negative terminal I get this.

Probe 1x and 10 volts per division DC coupled Time divide is 10 us.

 

[barber02]

I get 2.35 meg ohm on all source legs to the ground terminal on the top row of mosfets.

The bottom row all read 0 ohm from the source leg to the negative terminal.

All measurements were taken at the body of the mosfets where the source leg enters the body.

David

 

[Perry Babin]

You either have multiple open ground traces/vias or one large open ground trace.

 

[barber02]

Right you are sir. I just found it. It was hiding under a speaker wire spanning across the bottom of the circuit board. Moved it out of the way and there it was starring at me. In the attached photo the red probe is pointing at it. Learned a good lesson tonight.

Thanks a million for you help Perry. Amazing!

David

 

[Perry Babin]

Don't just bridge it with solder (some people do). Use a couple (or more) lengths of desoldering braid to bridge it.

 

[barber02]

That’s exactly what I did. I used solder wick. It seemed a perfect fit.

David

 

[barber02]

The amp works perfectly now at full voltage. Got the rectifiers back in and getting + and - 56 volts on the rails and it’s stable. Perry fixed from untold hundreds of miles away.

That’s amazing!