My First Phase Linear Restore- New Guy to the Forum

[Gepetto]

Is there a high value resistor, like 220K on the output side of the cap to ground?

Just checked the schematic, no there is not. If putting a 220K there makes it go away then you are just looking at your DMM reading an open circuit, stray fields.

 

[Gepetto]

Just checked the schematic, no there is not. If putting a 220K there makes it go away then you are just looking at your DMM reading an open circuit, stray fields.

That is just not good design. There should always be a bleeder on that cap to prevent static buildup. Otherwise there will be a nasty zap if the pre is connected to a live power amp.

 

[mjstriker]

That is just not good design. There should always be a bleeder on that cap to prevent static buildup. Otherwise there will be a nasty zap if the pre is connected to a live power amp.

So I should look at putting a resistor and off shoot it to ground to eliminate the DC I am reading at the outputs?

And are you suggesting that the readings I am getting might be "normal" for the design of the unit? Haven't tried hooking it up to speakers yet to test sound cause I am worried this excessive DC will be harmful as you previously described.

 

[laatsch55]

That's not normal for a pre-amp.....

 

[laatsch55]

Yes, put one there temporairly ....

 

[Gepetto]

So I should look at putting a resistor and off shoot it to ground to eliminate the DC I am reading at the outputs?

And are you suggesting that the readings I am getting might be "normal" for the design of the unit? Haven't tried hooking it up to speakers yet to test sound cause I am worried this excessive DC will be harmful as you previously described.

Not normal for a preamp but perfectly normal for a cap with one end floating with respect to any ground termination

 

[mjstriker]

Not normal for a preamp but perfectly normal for a cap with one end floating with respect to any ground termination

Since the two caps are back to back (positive side facing in) should I attach the resistor to the final lead (negative) going to the outputs or at some other point in the chain?

 

[Gepetto]

Since the two caps are back to back (positive side facing in) should I attach the resistor to the final lead (negative) going to the outputs or at some other point in the chain?

Yes, as you described, effectively tip to shell on the RCA jack Mike.

 

[Gepetto]

Since the two caps are back to back (positive side facing in) should I attach the resistor to the final lead (negative) going to the outputs or at some other point in the chain?

Hi Mike
Did you get it done?

 

[mjstriker]

Hi Mike
Did you get it done?

Took me a little while but I finally was able to make the trip to Rat Shack and pick up some resistors. Threw them in and all the DC at the outputs went away!! I still have a voltage reading in between the two capacitors in series (around 1 V each channel). Seems like I am heading in the right direction.

I also get an increase in DC at points 4 and 12 on the new IC when I move the bass knob for the left and right outputs, respectively. Still trying to narrow where the stray DC is coming from on that but if I keep both knobs all the way to the left it is fine. I tried setting the switch to flat but it makes no difference.

On a side note the WOPLD 500 is doing awesome. Picked up a oscilloscope a few days ago and have been getting my feet wet on how to take output readings. I am able to get the sine wave and measure various frequencies with a tone generator I downloaded on my iPhone. Trying to learn what information I can get from using the oscilloscope and maybe use it to help diagnose why the pre-amp is giving me such a headache.

 

[Gepetto]

Took me a little while but I finally was able to make the trip to Rat Shack and pick up some resistors. Threw them in and all the DC at the outputs went away!! I still have a voltage reading in between the two capacitors in series (around 1 V each channel). Seems like I am heading in the right direction.

I also get an increase in DC at points 4 and 12 on the new IC when I move the bass knob for the left and right outputs, respectively. Still trying to narrow where the stray DC is coming from on that but if I keep both knobs all the way to the left it is fine. I tried setting the switch to flat but it makes no difference.

On a side note the WOPLD 500 is doing awesome. Picked up a oscilloscope a few days ago and have been getting my feet wet on how to take output readings. I am able to get the sine wave and measure various frequencies with a tone generator I downloaded on my iPhone. Trying to learn what information I can get from using the oscilloscope and maybe use it to help diagnose why the pre-amp is giving me such a headache.

Hi Mike
The middle voltage of those 2 caps is indeterminate as they are technically floating so nothing to worry about there. That midpoint is floating in space somewhere

How much voltage are you measuring on 4 and 12?

 

[mjstriker]

Hi Mike
The middle voltage of those 2 caps is indeterminate as they are technically floating so nothing to worry about there. That midpoint is floating in space somewhere

How much voltage are you measuring on 4 and 12?

It is constantly fluctuating and peaks at about 1V when the nobs are all the way to the right. It goes up really fast but I don't let it sit there for too long so don't know if it will go higher