Dean Markley CD 60 PI issue

[Randall]

Reissue CD60, schematics are nowhere to be found, so all I have is the original drawing to work with.

Amp comes in with no sound other than a loud hum. Pulling V1 and V2 nakes no difference, but pulling V3 stops the noise. Plug into the amp in jack stops the noise and gives me sound. OK, so measure the PI:

Pin 1 - 278v
Pin 2 - 32v
Pin 3 - 35v

Pin 6 - 287v
Pin 7 - 33v
Pin 8 - 35v

Yikes! So thinking I need to figure out where the grid voltage is coming from, and start with lifting the .01/630v on pin 2. Still have 32v on the grid, and I lose the power amp in signal. It gets even weirder as I lift the 82K grid resistor:

Pin 1 - 435v
Pin 2 - 0v
Pin 3 - 25v

Pin 6 - 251v
Pin 7 - 15v
Pin 8 - 25v

And there is ~24v on the connected side of 82K. Then it occurs to me, even though it is not shown on the drawing, surely the amp in jack is before the PI, so if there is a problem with the PI, why would the noise go away coming in thru the Amp In ?

So, I ask, am I wrongly thinking these voltage readings are an indicator there is a problem where there is not? I saw 35v on the grids and cathodes and thought "well that's not good", but now I wonder what I may be missing.

http://music-electronics-forum.com/a...2&d=1248989413

[g1]

When measuring the grid, measure from grid to cathode, not from grid to ground. Then do the math from the cathode voltage you measured normally.
Trying to measure grid to ground will often give erroneous readings.

[Randall]

"When measuring the grid, measure from grid to cathode, not from grid to ground."

I did not know that! Wow, when I measure from grid to cathode I get -1.5v on pin 2, and -1.4v on pin 7.

"Then do the math from the cathode voltage you measured normally."

You lost me here.

[Randall]

As I think about this more clearly, I see that the PI voltages are a red herring, because I get a pop when probing both PI grids, so it not likely I am losing signal here. I now think it is further upstream in the preamp.

http://music-electronics-forum.com/a...9&d=1248989330

http://music-electronics-forum.com/a...0&d=1248989356

http://music-electronics-forum.com/a...1&d=1248989382

[Enzo]

That PI grid reading is a lesson everyone has to learn at some time. It is an example of your meter impedance making a difference. If you try to measure voltage to ground from those grids, your meter and the grid resistor form a voltage divider, so the voltage appears to be a lot lower than it really is.

[g1]

"When measuring the grid, measure from grid to cathode, not from grid to ground."

I did not know that! Wow, when I measure from grid to cathode I get -1.5v on pin 2, and -1.4v on pin 7.

"Then do the math from the cathode voltage you measured normally."

You lost me here.

In this case, your readings were correct anyway. But grid voltages should be considered with reference to the cathode, rather than to ground. It doesn't matter what kind of DC the grid is sitting at if the relationship to the cathode is correct. Here, you had the same rough 1 and a half volts between grid and cathode like we often see with preamp tubes. So the stage is biased correctly.
33V at grid and 35V at cathode is equivalent of 0V at grid and 2V at cathode that we might see at an input stage.
So if you get a wonky grid reading, check grid to cathode voltage, and subtract from the cathode to ground reading. In this case, you had 35V at cathode, and 1.5V from grid to cathode. So grid voltage would be 33.5V.
Note that grid voltages for preamp tubes are rarely shown on the schematic. I think part of the reason is it can add confusion. If plate and cathode voltages are correct, grid voltage usually is too.