Joe--What causes this???

[Gepetto]

Lee looks like it is 1.25MHz using your Tek time base. Much more reasonable for what I would expect. It will probably do this with nothing connected to the inputs, just sitting there powered up.

If this is true probe around to the outputs of each of the op amps. The one with the biggest amplitude is the one that is unstable and oscillating. Alternatively you can connect your scope probe tip to its probe ground with the ground wire forming a small loop antenna. Put your scope on its most sensitive scale and just move the probe loop antenna in close proximity to the board and scan the entire board area. Where you pick up the largest signal is where the oscillation is emanating from.

 

[laatsch55]

Well------that's just F&^%$#@ brilliant!! I guess when you get in the micro range things get pretty sensitive. I love that tactic for a diagnosis. Of course you know where my mind jumped to another application for the loop antenna.

 

[laatsch55]

Joe, how does the oscillation frequency attach itself to the fundamental, and does it's energy remain constant , and does it rob energy from the fundamental?

 

[laatsch55]

Joe, that pic of the Tek was with the sig gen of the AP hooked directly to the scope probe. The 2000 is now hundreds of miles away. But I have another waiting at Fed_Ex to get in the morning.

 

[Gepetto]

Joe, that pic of the Tek was with the sig gen of the AP hooked directly to the scope probe. The 2000 is now hundreds of miles away. But I have another waiting at Fed_Ex to get in the morning.

Wow that is a pretty stong signal Lee if you are still picking it up from hundreds of miles away Sorry couldn't resist.

To answer you other question, it does not rob anything away from the fundamental. The fundamental is likely/probably formed from a digital D/A source or PWM output within the AP signal generator and that 1.25MHz is a function of the DAC or PWM operating frequency. You can look at it similarly to ripple on a power supply output. When I get a chance, I will look at the signal generator output on my HP.

 

[laatsch55]

The 8906 is digital also?


Joe, BTW, when it's served up so professionally, it is hard to resist!!

 

[Gepetto]

Yes it is digital Lee but have not looked at the specifics of the signal generator part of it.

 

[laatsch55]

Ah, ever been inside a Bryston B4??

 

[Gepetto]

Long time ago Lee, why do you ask?

 

[Gepetto]

Ahhh, just noticed your other thread. Sorry to see you have an unplanned project Lee. Don't you hate it when that happens?

 

[laatsch55]

Amen Joe, amen. Went into protect mode, have it about stripped now. My main source of suspicion is now the Philip[s TDA 1085 IC on the secondary power supply board.

 

[mlucitt]

I like that term - "unplanned project" a quite proper euphemism.

Euphemism (noun): The substitution of an agreeable or inoffensive expression for one that may offend or suggest something unpleasant.

Mark

 

[Gepetto]

Really? A TDA 1085? Seems an odd choice for a power amp (but a good choice for a washing machine motor). I want to see that circuit...

 

[laatsch55]

So do I. Looking for even just a schematic right now.

 

[Gepetto]

I suppose that they could use that IC in a preregulator application. It is designed as a motor controller to control an SCR

 

[mlucitt]

Lee has an amp with a permanent press cycle...

 

[Gepetto]

Did you get the schematics Lee?

 

[Gepetto]

Sheet 11 Lee at the bottom. AC control and soft start.

 

[laatsch55]

Could very well be, because all outputs, drivers, diodes, regulators, bridge all check out.

 

[laatsch55]

I believe pagre 12 represents what I have better.