Joe--What causes this???

[laatsch55]

There appears to be a jumper to clear the traces going to the volume pot from looking at the x-ray from the foil side.

 

[laatsch55]

One of the jumpers that appear in the pic.

 

[laatsch55]

I will check that Joe. It would be the bottom jumper in the pic.

 

[Gepetto]

Probably CW rotation though if I were to guess.

 

[laatsch55]

No change on pin 13 with ambience control maximum or minimum, nor with ambience switch on or off. The same either 4 ways.

 

[Gepetto]

Yes I agree Lee, it looks like there is a wire jumper which would be the ideal position for that resistor.

 

[laatsch55]

No change with jumper to Ambience in and out swith REMOVED. Still have ambience min/max control

 

[laatsch55]

When we are talking gain from pin1, 13 or such are you referring to the parasitic waveform gain or the primary waveform gain???

 

[Gepetto]

The gain of the pin 13 stage is higher (Av=5 for that stage) which make it much more stable. The unity gain stage does not like capacitive loading on it. Because of the low gain in that stage it is not tolerant to much capacitive loading. That long trace is a capacitor. The resistor is an attempt to have the amp drive a resistive load rather than a capacitive load.

 

[Gepetto]

It is a design shortcoming. I think the 4739 is perfectly fine. You said it behaves if you put the ambience switch in the out position.

 

[Gepetto]

No change with jumper to Ambience in and out swith REMOVED. Still have ambience min/max control

You lost me on this comment Lee. Please explain.

 

[Gepetto]

I am talking local feedback gain around the op amp stage.

Pin 13 sees a gain of 270K/47K = 5.7 whereas pin 1 sees 22K/22K = 1 (unity gain)

Unity gain is the least stable configuration.

 

[laatsch55]

With the ambience in/out switc disabled with the jumper removed I still have gain control by way of the min/max pot. Which waveform am I looking for a gain change on, the primary or parasitic??

 

[laatsch55]

I'm getting plenty of change in the primary but the parasitic stays the same width and heighth relative to the primary. When the primary waveform changes the parasitic changes to fit the room allowed for it.

 

[Gepetto]

you should be observing pin 1 of Z2. With the jumper out, it should be nice and stable with no oscillation on it. Then try putting a ~200 ohm resistor in to replace the jumper wire and see what pin 1 looks like after that.

 

[Gepetto]

Not sure I know what you mean by the parasitic Lee?

 

[Gepetto]

time to call it a night for me Lee...

 

[laatsch55]

The oscillation that is riding the primary wave. Nothing so far has affected the oscillation, it stays the same relative to the primary. I am hooked to pin 1, and have been through all this. I can change the strenghth of the primary

 

[laatsch55]

The scope is grounded to the chassis BTW.

 

[laatsch55]

Night Joe. Appreciate you staying up this late. Pic of what I have now.