PL700II Hum/Buzz volume control issue

[RadioMan]

I have PL700II that has been fully recapped and serviced but have a strange issue with hum/buzz sound.
With a preamp connected when I rotate both volume pots on the amp to 50% I get hum/buzz sound.
When I turn the pot all the way up the noise goes away. The same is true when the posts are fully closed the amp is totally silent?

When nothing is connected to the inputs I get hum/buzz as soon as I crack open the volume pots all the way to 100%


Anyone have any ideas?

 

[Gepetto]

Did it do this before you had it recapped and serviced?

 

[RadioMan]

It was dead when I got it now it works great with full out put but have this noise issue.
Has new WO filter caps installed.

Seems like a ground loop but don't know how that would happen?

 

[Gepetto]

It was dead when I got it now it works great with full out put but have this noise issue.
Has new WO filter caps installed.

Seems like a ground loop but don't know how that would happen?

That is the most likely culprit. Did you do the service or take it to someone?

 

[RadioMan]

All I did was change electrolytics. And the 2 7.5K power resistors as they were open. I did not move any wire harnesses.

The amp it totally silent with the pots closed.
And with the pots all the way up connected to the preamp the amp sound great and is quiet?

Totaly Lost

 

[laatsch55]

Non-shorted open inputs can get noisy...;

 

[Gepetto]

Some pictures might help us help you. Insides...

 

[George S.]

Non-shorted open inputs can get noisy...;

And missing RCA insulating grommets.

 

[RadioMan]

What pictures would be most helpful and I'll send them soon?

Just to clarify, So for example when I short the RCA inputs and move the volume pots to ½ way or 50% I get the most buzz. When the pots are fully open or fully closed the buzz goes away.
It should not make any noise no matter what position the pots are in when the inputs are shorted?

Insulating grommets on the RCA connectors are there and in good condition.

As I said other than this weird noise issue the amp seems to be working fine and sounds amazing but I want to be able to use the volume pots on the front of the amp without any noise.

 

[George S.]

Here's how a PL attenuator/volume pot is wired.
On the pot, the center lug is the wiper. This gets wired to the control board and is the signal from the pot. The wiper ALWAYS goes to control board signal connection.
That leaves the two outer lugs.
One lug gets the two ground wires. One ground from the RCA, the other ground to the control board.
The remaining outside lug gets the signal from the RCA.
So, check that these are wired correctly.
And you can easily check that the pots are working correctly with a ohm meter. I would disconnect the signal wire from the board. One DMM lead to that wire, other lead to the center contact on the RCA jack. Rotate the pot and observe values. You should see a few ohms on the low setting and then up around 100K ohms on a 100K pot.

 

[RadioMan]

I check everything as you stated and all check out fine.
Pots read 100K as they should and have continuity throughout there range no dead spots. All wires look to be connected correctly in fact it looks all stock from the factory.
I even re-soldered all the grounds in case of cold solder joint and still no luck.

 

[George S.]

Weird. I don't have a answer.

 

[RadioMan]

This is the craziest thing have experienced in a while. The amp works perfect if you keep the pots open all the way. Full power No noise sounds great.
I see all the audio ground wires tie together near the input connectors, all looks goo there.
I have looked for broken traces in the control board, loose wires, cold solder joints etc...
The amp looks like it still has all the factory cable ties on the wiring harnesses so no signs of wiring errors or tampering.
I am lost with this one!

 

[George S.]

A thought.
Since it manifests equally on both channels, could it be in the power supply.
First check that storage cap polarity is correct.
Upper cap has negative to buss bar.
Lower cap has positive to buss bar.
Upper cap positive is B+ wiring connection.
Lower cap negative is B- wiring connection.
Check your B+ and - rails with DMM negative lead on buss bar, positive on B+. Should be +100 VDC. Now move positive lead to B-. Should be -100 VDC.
If this is correct, then post a good photo or two of all the installed output transistors. Perhaps the configuration or devices aren't correct and somehow it's causing oscillations.

 

[Gepetto]

The input circuit on the PL700B and PL700 S2 is most vulnerable to noise pickup when the attenuation pot is at its midpoint position. With your inputs shorted, you essentially have the input to the control board with a 50K ohm resistor to ground on the ground side of the pot and a 50K ohm resistor to ground on the signal input side of the pot.

What is the position of the input switch? Normal or direct? Does the position of that switch make any difference in the buzz you are hearing?

Pictures around the input jacks and that direct-normal switch would be helpful. But some overall closeups of the innards will also be helpful as some of our members with eagle eyes may spot a problem.

If it is quiet at both pot extremes, that generally means you wiring is picking up signal noise and the amp is amplifying it.

 

[RadioMan]

Switch in normal. Position makes no difference.

Anyway here are some pics…

 

[Gepetto]

Switch in normal. Position makes no difference.

Anyway here are some pics…
View attachment 75076 View attachment 75077 View attachment 75078

What is the position of the input switch? Normal or direct? Does the position of that switch make any difference in the buzz you are hearing?

 

[RadioMan]

What is the position of the input switch? Normal or direct? Does the position of that switch make any difference in the buzz you are hearing?

Switch in normal. Position makes no difference.

 

[George S.]

2nd photo, the upper green cap, left lead beside solder joint. Is that a break? Probably not, but that's only thing I see. Very nice clean unit.

 

[e30m3mon]

Good eye, George! Sure looks like some cold solder joints, at a minimum. I'd take a minute and put my magnifiers on...and reflow for starters.