Joe--What causes this???

[Gepetto]

Lee recommend you change out the 33pF to an 82 or 100pF instead to see the effect.

It seems the addition of the 33pF reduced the amplitude and frequency of the oscillation which is good.

 

[Gepetto]

Lee
By the way, I suspect you found something that has always existed in this preamp. At 6MHz, I bet that is outside the normal hearing range of any member of this forum

 

[mlucitt]

Lee,
I would like to try and duplicate your waveform to see if my PL 2000 has the same issue. Can you give me your setup. Specifically: input frequency, input voltage, one or both channels, load on the output, front panel settings, and anything else that might be pertinent.

Mark

 

[Gepetto]

Hi Mark
Do you have a series 1 or 2? Lee's is a series 2.
Lee is seeing this at Z2 pin 1, the ambience inverting amp output. Any low frequency signal on the input, say 1 KHz and the Ambience control switched in. It does not occur with the ambience control switched out. Pretty straightforward to recreate it on his preamp.

Z2 is a Raytheon RC4739 device.

 

[mlucitt]

Mine is Series 1. I never noticed or observed the oscillation because I don't use the rear channels, but it would still be there.

 

[Gepetto]

You never will notice the oscillation Mark, it is oscillating at 6MHz...unless you tune your shortwave radio into it.

 

[jbeckva]

Not to rob the thread, but hmmm... what's interesting to me is how this is sort of similar to an issue I've seen with the Carver preamps and the Sonic Holography circuit. My issue with that is definitely audible in the form of what sounds like a loud BUZZ whenever I turn it on.

BUT.. and here's the kicker... only with certain amps like the Yamaha M-80 hooked up.

Would think maybe it would be direct coupling versus having a cap inline, but that's not the case. My NAD 2400 works fine, and it has a LAB input that is supposedly not cap coupled.

Hmmm.. interesting... anywhoo ... carry on.

 

[mlucitt]

I meant notice the oscillations on the scope. You would have to speed up the sweep to catch it.

J, if you are hearing a buzz, it is probably related to 60Hz and most likely a bad cap.

Mark

 

[mlucitt]

I am attaching a list of parts for the PL 2000 if it can be any help. I cannot guarantee it is 100% accurate because I took the values off the fuzzy scanned schematic copy I have. If anyone sees an error, let me know and I'll fix it.

Mark

 

[laatsch55]

Mark, Joe says the difference between the Ser I's and II's are the omission of C25 and R76 in the series II. These two comp[onents are in line with pin 1 of Z2 and says Joe, directly affects stability. 1Khz input, 250 millivolts on the sig generator which equates to about 2 volts output. Volts/div is between 50 mv and 5mv. Time base is between 50 and 2 us, not a typo --microseconds.
I have a real clear copy of a series I schematic, want a scan??

 

[laatsch55]

You have tro get the fundamental freq stretched wayyyyyy out to even see it, and I'm thinkin it's a byproduct of the ambience feature, MAYBE, we'll see.

 

[mlucitt]

I can always use a better scan than the one I have now. I'm sure if you post it, they will come.

Have you put the BrownDog chip adapter in yet?

Mark

 

[laatsch55]

Yep, just checked my scope and sig gen in the AP. The oscillation is NOT in the Pre-amp. IT is in my scope or the AP analyzer. Sorry for the chasin my tail guys.

 

[Gepetto]

Hmmm, so how does that explain that it goes away when the ambience is switched out?? Are you certain Lee?

 

[laatsch55]

It DID NOT go away when I switced the ambience out. That's what led me to check the scope directly to the AP sig generator. I'm hooking up ther digital Tektronix to see which piece of equipment is the culprit.

 

[mlucitt]

Do you have a TV station antenna near your house?

 

[laatsch55]

No Mark I don't.

 

[Gepetto]

Did you discover anything Lee? Sometimes RF of this frequency radiates everywhere and it is hard to trace the true origin. You may be a victim of this.

It is hard for me to think that 6MHz propagated through 2 op amp stages with a gain bandwidth of 3MHz.

 

[laatsch55]

Well in that case you were certainly VERY PATIENT with me the other night. It must be hard to see a guy flail around like that, and not say something, Thanks Joe. Have not tracwed it down yet. Good time to though, waiting on parts. I'll warm up the AP and the TYek.

 

[laatsch55]

It appears you were right Joe. The first pic is with the sig gen on in the ap, the second is with the sig gen off.