1987x Buzzing/humming - Filter Caps?

[Splitslim]

As I'm slowly getting acquainted with this amp, I'm coming across new issues which need to be addressed.

This 1987x has a master volume installed pre-PI, right after the effects loop between W3 and W4. The pot is installed where the indicator light used to be. It's got twisted leads to the pot (not shielded unfortunately) and the pot terminals are rotated 90 degrees away from the standby switch.

I'm getting a lot of buzzing when turning up the master volume. Even with the input volumes at 0, the buzzing is there throughout the entire sweep of the master. The presence pot is very scratchy as well. Here is a video;



I pulled V3 and it was whisper quiet. When I individually pulled V2 and V1 with V3 in, the noise was still there. That leads me to believe it's in the preamp section.

I don't think this sounds like 60hz hum which would be a grounding or shielding issue right?

From examples I'm finding on the internet, it sounds like the filter caps need to be replaced. They don't show signs of degradation, but what else could it be?

 

[tschrama]

My first thought.....hum from bad shielding .. as it tops at about 70% of the pot travel, when it's inpedance it maximal. not preamp related. But relatied to the wiring around the MV.

btw... these pics dont help. Share some pics of the entire guts in one shot..

 

[Guitar-Rocker]

Sounds like you are getting DC voltage on the Presence pot.

 

[Splitslim]

Here is the MV pot beside the master switch - the terminals are now 90 degrees to the right away from the standby switch (no change in noise). I notice that the pot isn't grounded to the ground terminal, is that correct?

 

[neikeel]

It is grounded to the preamp rail via that black wire. I would not ground it there I would run the wire to the preamp filter cap lug, not the bus rail ( I normally disconnect the presence pot from the bus rail and run the ground wire (in the loom) to the pi filter cap ground lug too.
Try these simple things first.
I don’t think that being close to the standby is an issue.

 

[Splitslim]

So it seems like a grounding issue rather than a filter cap issue in your opinion?

 

[myersbw]

Well...you said this was after the FX loop? Patch the FX loop and see if it diminishes or goes away entirely. Reason I say that is I'd ASSUME the MV was functioning fine when it left the installer's hands...inferring that maybe the FX loop jacks may be showing oxidation issues...?

 

[tschrama]

you need a shielded wire from the MV middle lug.

Cann't make out the gnu scheme from those pics. But since the buzz is loudest at MV highest impedance: that is a clear give away that it is capacitive buzz coming in.

BTW is that case of the MV grounded? Doesn't look that way. Solder the GND lug to the pots' case!!

 

[ampmadscientist]

"I pulled V3 and it was whisper quiet. When I individually pulled V2 and V1 with V3 in, the noise was still there..."

Pull V1 and V2 preamp tubes - both at the same time. BOTH tubes are out.
Leave V3 in.
Is the noise still there? Yes or no?
You are not making yourself clear on this. Be specific.

WHAT is the result of that test?

 

[neikeel]

The MV pot is a PEC stainless and soldering it is very difficult and IMO totally unnecessary as long as it is grounded to a proper chassis lug.
I guess we wait for the tube pulling test, but I suggest implementing the easy grounding corrections first (and do we assume it was quiet when first modded.....).
Forgot to ask - are the output tubes reasonably matched?

 

[Splitslim]

I'll test as requested and report back tonight/tomorrow.

Thanks!

 

[ampmadscientist]

The MV pot is a PEC stainless and soldering it is very difficult and IMO totally unnecessary as long as it is grounded to a proper chassis lug.
I guess we wait for the tube pulling test, but I suggest implementing the easy grounding corrections first (and do we assume it was quiet when first modded.....).
Forgot to ask - are the output tubes reasonably matched?

It is very easy to solder to stainless steel, when you have good equipment. I do it all the time.
The ground buss is soldered to the back of the pots because the pots are crimped together.
Crimped connections oxidize and fail. (oxygen is between the metal)..and the grounding will be lost eventually.
(pots can come loose from the chassis)
And that is probably why the ground buss is soldered.
Not so unnecessary after all.

BUT
you DO have your MV pot right next to the power switch!
Which means that the AC magnetic field "could" enter the audio path.

What I am getting at here is: filament buzz.
Hum or buzz does not mean the absence of a ground wire...necessarily.

Filament buzz is really common in Marshall amps because of the filament wiring layout.
Removing V1 V2 (at the same time) can indicate if this AC filament buzz is being amplified in the preamp.

"...I pulled V3 and it was whisper quiet...."
That is telling me that: the capacitors are OK.
If the capacitors were bad, there would be (pretty noticeable) hum even with the phase inverter removed.

"...I pulled V3 and it was whisper quiet...."
That is also telling me that the power tubes are matched.
If the power tubes were out-of-match, the hum would be there, with just the power tubes only.

So, let's get V1 V2 pulled out. (at the same time) PI is in... and see if the noise goes away.
Because
you are looking at filament AC buzzing or you are looking at high frequency oscillation in the preamp.
(HF oscillation causes a buzzing noise)
You cannot hear the high frequency...but you can hear the power amp peaking out from the oscillation.

When the power amp is peaked out
the output tubes are fully ON.
Then you hear the ripple in the power supply (HUM). This hum is normally masked by the output tubes shutting down between guitar notes. (the bias shuts the tubes off between guitar notes)
But when the PA peaks (from HF oscillation) constantly with no music input - then you DO hear the hum.

 

[neikeel]

It is the solder lug of the pot that is important.
A decent bus is ideal
But I can’t be arsed to argue with you.

 

[Splitslim]

Here are the results of the testing tonight.

I pulled both v1 and v2 at the same time. Noise was still there. Removed input lead and jumper, noise still there. Patched the FX loop, noise still there;



This was interesting. Under the same test conditions, I toggled activating and bypassing the FX loop with the back button. When active and the master on full, the noise was gone. As I turned the master down, the noise returned. Once I bypassed the FX loop with the back button, the noise returned during the entire sweep of the master.



Looking forward to your feedback.

Thanks in advance.

 

[ampmadscientist]

Here are the results of the testing tonight.

I pulled both v1 and v2 at the same time. Noise was still there. Removed input lead and jumper, noise still there. Patched the FX loop, noise still there;



This was interesting. Under the same test conditions, I toggled activating and bypassing the FX loop with the back button. When active and the master on full, the noise was gone. As I turned the master down, the noise returned. Once I bypassed the FX loop with the back button, the noise returned during the entire sweep of the master.



Looking forward to your feedback.

Thanks in advance.

So it's not filament buzz, but it sounds like an open connection to the PI...like the PI input is not hooked up to anything.
Take the master volume out of the circuit and put it back to stock, then test it.

 

[Splitslim]

If I understand the schematic correctly and the connection points of this master volume, it seems to be connected before the fx loop after the tone stack in the circuit. The middle terminal green lead on the pot goes the pcb and connects to BN4. The right terminal blue lead on the pot goes to the fx loop card and connects at W3. W4 at the fx loop card brings the signal to the PI after the fx loop.

Does this make sense or am I wrong?

 

[Splitslim]

I'm wrong, W3 at the FX loop card is the input into the PI after the FX loop, so this master is after the tone stack and FX loop, right before the PI.

W4 is the input into the FX loop from the tone stack.

Bad pot or lack of shielding at the pot?

 

[myersbw]

So...did you clean the FX loop jacks with DeOxit or similar? Give it a go, if not already.

 

[Guitar-Rocker]

Jumper W3 and W4, which bypasses both the Master and the FX, and see if everything is normal. If that fixes it, then try jumpering around just the FX and check again.

 

[tschrama]

problem and solution was identified.. go install shielded wire and make sure the pot casing is grounded via the tonestack ground bus.