Joe--What causes this???

[laatsch55]

Ceramic on the 33pf?? And try that before the other bypassing??

 

[Gepetto]

Sure, lets go for the throat Lee. The bypassing can wait till later. 33pF ceramic is fine. We can refine it to a film later if you find one.

 

[laatsch55]

Also have some polystyrene in 33 too---maybe----if they go that low.

 

[Gepetto]

sometimes you have to go to Mica when the sizes get that small. I find it difficult to get film in sizes less than about 200pF without searching for a while. Mica would be a great choice if you have them.

 

[laatsch55]

Still there, less amplitude though

 

[laatsch55]

And don't you dare go silently into the night!! Lost the right channel, left is in the 1.8 volt range, right is in the millivolt range with the same numbers.

 

[Gepetto]

you sure you don't have a flakey switch, I noticed this pre has a lot of switches in the control path.

 

[Gepetto]

Lee is the oscillation on pin 1 of Z2 still there with the Ambience switch in the OUT position? Does it change at all when you switch it from the in to the out position?

 

[laatsch55]

Let me check Joe. And I put 3 hours in de-oxiting switches and pots, but could not guarantee anything. Back ina sec.

 

[laatsch55]

Ambience in, oscillation, ambience out no oscillation.

 

[Gepetto]

yup as I suspected Lee...

 

[laatsch55]

Bad Z2 or the Brown-Dog has compatibility issues??

 

[Gepetto]

Lee do you have the PL2000 or the PL2000 II? Want to ensure I am looking at the right schematic.

 

[laatsch55]

Ser II.

 

[Gepetto]

Lee I noticed that they deleted C25 and R46 in the Series II version. The rest of the circuit is the same. I think you have a series I. Those are stability killers for the pin 1 stage of Z2

 

[Gepetto]

OK Series II it is.

 

[laatsch55]

Seriers II.

 

[laatsch55]

I have 3-Series I's of my own here along with a Ser II, if that helps.

 

[Gepetto]

Here is my second recommendation Lee, if you can do it, carefully open the trace between Z2 and the ambience switch connection by putting a small cut in the trace. An Xacto knife blade width will do, just to open the connection between the pin 1 output and the ambience switch. After you confirm the trace is open with your ohm meter, then bridge the open trace cut with about a 200 ohm resistor. Try to make the cut and jumper closer to the Z2 end of the trace than the ambience switch. My hunch is that this will kill the oscillation since you said it got better with the addition of the 33 pf cap.

 

[Gepetto]

Another observation Lee. I'll bet that Z2 pin 13 oscillates more when the ambience pot is at one extreme or the other. Cannot tell from the documentation which end it will be worse on from a CW or CCW perspective.