WOPL 700B Rebuild Left Channel Buzz

[jskylab]

Hey Everyone,

I have been battling this newly rebuilt 700B for a while now and it's driving me crazy!! Everything had been great, from the new backplane, control pcb, chassis, etc.. Bias has always been good and it sounds great except for some low level buzz on the left channel. I have recently disconnected the DC to chassis gnd connection at the left backplane and thought that disconnecting it helped but alas it didnt. So now I'm back to where I started....copper buss at inputs, no mode switch, bypassed volume pots, star ground wiring. Even with the input coax disconnected from the control board the channel still buzzes. And when I have it open I can place my hand over the left channel of the control board and effect the buzz (I am obviously acting as a shield from some interference). Any thoughts or suggestions of what to check next? One more thing, I even tried a new control board and have the same issue. Thanks so much!!

Jeff

 

[Gepetto]

Hey Everyone,

I have been battling this newly rebuilt 700B for a while now and it's driving me crazy!! Everything had been great, from the new backplane, control pcb, chassis, etc.. Bias has always been good and it sounds great except for some low level buzz on the left channel. I have recently disconnected the DC to chassis gnd connection at the left backplane and thought that disconnecting it helped but alas it didnt. So now I'm back to where I started....copper buss at inputs, no mode switch, bypassed volume pots, star ground wiring. Even with the input coax disconnected from the control board the channel still buzzes. And when I have it open I can place my hand over the left channel of the control board and effect the buzz (I am obviously acting as a shield from some interference). Any thoughts or suggestions of what to check next? One more thing, I even tried a new control board and have the same issue. Thanks so much!!

Jeff

Hi Jeff
Pls send pictures that I can actually zoom in on

 

[jskylab]

 

[Mohawk]

Hmmmmm, maybe not enough heat on the pem nuts....

 

[Gepetto]

One thing that I notice immediately is that your orange wires connected to pin 5L and 5R of the control board should be routed to the single point ground, the bus bar between the caps and not to the backplanes (please refer to pages 46 and 47 of the attached). It would also be helpful if detailed photos of the input jacks were provided to ensure all is proper there.

 

[jskylab]

Thanks Gepetto! Don’t know how I didn’t catch that. I’ll move them over and see what happens! I’ll get a pic of the input wiring as well.

Jeff

 

[Hexis22]

It would also help to clarify how you terminated the other end of all the GND connections at the DC protection board Phoenix connector.

For example, you really should run separate grounds for each channel output back to the buss bar. It appears you added a Black wire to loop both together.

 

[jskylab]

Well rerouting the grounds from 5L and 5R to the buss bar didn’t fix it. The right channel is dead quiet but the left is still buzzing. Going to look at the watts board grounding next.

 

[Gepetto]

Well rerouting the grounds from 5L and 5R to the buss bar didn’t fix it. The right channel is dead quiet but the left is still buzzing. Going to look at the watts board grounding next.

I cannot tell what the AC wire whip you have coming down the center (2 black wires) but I can tell you that they emit an electromagnetic field so it is good to have them well separated from everything else and tightly paired together for the entire run. I am not sure where those runs are heading. They are biased to the left side and you indicated that your hand affects the field.

 

[jskylab]

I think progress has been made! Everything is much quieter, albeit I do have the front panel off right now. Not sure where to get the meter drive feed off of the watts board. What I did was ran all three grounds separately from the watts board to the ground buss. I also rerouted the ac wiring to bring it up and behind the caps to the left side, which got rid of the center ac wiring that would have gone to the power switch. Also ran the watts ac lines along the center with the mains ac line. Like I said, all is quiet but I don’t have the front panel hooked up. The watts directions say to derive the meter drive signal from the watts board but I don’t m is where. I have removed the meter drive lines coming off the backplane. And lo and behold if I bring my mains ac which is hanging off to the left over in front of the control board I get the same buzz I was getting before.

 

[Gepetto]

I think progress has been made! Everything is much quieter, albeit I do have the front panel off right now. Not sure where to get the meter drive feed off of the watts board. What I did was ran all three grounds separately from the watts board to the ground buss. I also rerouted the ac wiring to bring it up and behind the caps to the left side, which got rid of the center ac wiring that would have gone to the power switch. Also ran the watts ac lines along the center with the mains ac line. Like I said, all is quiet but I don’t have the front panel hooked up. The watts directions say to derive the meter drive signal from the watts board but I don’t m is where. I have removed the meter drive lines coming off the backplane. And lo and behold if I bring my mains ac which is hanging off to the left over in front of the control board I get the same buzz I was getting before.

If you want a quiet amp, wire dress matters. Ask Mark, Perry, Eric, Glen, Lee, George, Gene, Matt, Jim, John and anyone else I inadvertently left out? You can get over 108dB SNR if you are very careful with wire routing.

 

[Gepetto]

BTW, are you Jeff from the west coast???

 

[George S.]

You can get audio signal to drive your VU meters off the DCP (Watts board) at the speaker jack connections on the board. Terminate each meters signal wire with a small ring terminal and place under the screw or nut that attaches the board to the output (speaker) jacks.
Joe showed us this a good while back.
The 60 Hz hum may be reduced when you bolt/ground the face plate to the chassis.
AC runs do need to be kept separate from DC runs.
I keep the DC across the bottom and AC across the middle or top depending if it's a 400 or 700.
No hum in any of my three builds. Dead quiet.

 

[George S.]

Here's a old photo of a PL700S2. It's had a lot more work since, but it shows the meter single drive wires, red is right, white is left, coming off the DCP.
The advantage is getting the fine AWG wire connections off the backplanes and to a more user friendly area.
Zoom in to the top of the DCP. Red and white wires.

 

[mlucitt]

Referencing Post #7, no Grounding jumpers are allowed here. Every Ground wire originates at the STAR Ground (between the main capacitors).
That includes the Gray and White Ground wires stuffed into the Phoenix Terminal Block, one wire per position please.

 

[Gepetto]

Referencing Post #7, no Grounding jumpers are allowed here. Every Ground wire originates at the STAR Ground (between the main capacitors).
That includes the Gray and White Ground wires stuffed into the Phoenix Terminal Block, one wire per position please.

And that is a lot easier to do with the bus bar upgrade...because this means a lot of wires emanating from the bus bar.

 

[George S.]

Yup, note the number of ground wires soldered to that original copper buss bar on my 700 photo.
Extra holes were added and soldering was done with a Weller Expert soldering GUN.
Run single ground wires and use Joe's aluminum buss bar if you desire easier construction and easier ability to take it apart in the future.

 

[Hexis22]

Highly recommend you invest in the White Oak bus bar upgrade, or at least separate the ground wires by drilling additional holes in the copper bus bar.

That mass of wires is not sufficiently soldered to the copper bus bar.

 

[Hexis22]

Here is what the WO bus bar upgrade looks like installed.

 

[jskylab]

Yes, I am on the west coast, SF to be exact. And that copper buss bar is definitely not my best work! Hexis, your amp looks stellar and its what I'm referencing to get this all cleaned up. Didn't know about the WO buss bar, bridge kit and newer version of the light board until now. Anything else I'm missing?

Jeff