Can't Hear Presence Control. '73 Circuit Help (Really Probably Solved This TIme!)

[Mjh36]

6/16/23: Solved. Had the LarMar wires going to the wrong power tubes. Switched them and presence adjustment came back! That's all that it was. Quite the oversight on my part but I sure learned a lot.


Ok you've seen I've recently got this '73 50 watt. I've done a couple things to it but nothing too intrusive. I can't recall if I heard the presence control working before, but I think it was working when I got it. Maybe not and I was hearing things, I can't say for certain. Currently I'm not hearing a tonal shift when I turn the knob. That's where I'm at now and I'm trying to diagnose it but I need a little more circuit help as far as measuring.

First off, it did come with a Dual 100k pot master volume mod with no safety resistors as pictured here. I changed it to a LarMar 250k with safety resistors, but right this moment it's a Frondelli MV just to try it out. I read that affects the Presence control at lower volumes. Even with both the LarMar or Frondelli MV turned up all the way, but with a bit of attenuation for hearings sake, I still didn't hear any noticeable shift.

I sprayed a little Deoxit D5 just to try, no change.

Is there ways I can check the Presence components while in-circuit. I get flashing zeroes when I try to measure the capacitance in-circuit. All connections are stock and seem solid going all the way to the 4 Ohm tap. Appears to be the Mark 2 presence circuit. There are resistance readings on the pot as I turn the dial so I believe the pot is OK. Anything I can try? Any testing tricks or measurements I can do that I don't know about?

 

[Mjh36]

Am I right in assuming that the presence circuit is kind of a localized thing I can focus on? The pot/resistor/capacitor, the NFB resistor, and the 2 purple wires leading to the 4 ohm tap. Should I need to broaden my scope and consider other areas of the circuit? Everything else works fine, amp sounds great.

I'm going to try and piggyback another .68uF cap on the stock one and see if that changes anything.....

Well that didn't do anything. Also checked continuity and everything seemed fine.

How subtle is the presence control? I should hear something obvious changing right?

 

[mickeydg5]

What happens when you turn the MV all the way down?

 

[Mjh36]

What happens when you turn the MV all the way down?

MV all the way down there's no sound. Dead quiet. Both with the MV set high or low I don't notice any use of the presence knob coming back, atleast that I can hear. It's either extremely subtle or nonexistent. I'm trying to figure out what I'm missing here.

 

[Jon Snell]

Under no signal conditions, what is the voltage across the 4k7, on the presence control? Does it vary with the control?

 

[Mjh36]

Under no signal conditions, what is the voltage across the 4k7, on the presence control? Does it vary with the control?

I'm reading about 10.20 VDC across the 4k7 resistor. And no change when I turn the presence knob, it stays the same.

 

[Jon Snell]

In that case, with a signal passing through the amplifier, I usually find either a CD that I know or the radio etc, place a 220n capacitor across the 4k7, the sound should change to more trebly. If it does, remove the 220n and short out the wiper of the pot to the top of the pot (4k7 top, where 10 volts is). The sound should change. If it does, the pot is faulty, if it doesn't the mustard cap has failed. [Very unlikely the latter].

 

[circusinthesky]

I'm over fifty. I can no longer hear the presence control on any vintage Marshall circuit.

 

[RadioAD5GB]

Ok you've seen I've recently got this '73 50 watt. I've done a couple things to it but nothing too intrusive. I can't recall if I heard the presence control working before, but I think it was working when I got it. Maybe not and I was hearing things, I can't say for certain. Currently I'm not hearing a tonal shift when I turn the knob. That's where I'm at now and I'm trying to diagnose it but I need a little more circuit help as far as measuring.

First off, it did come with a Dual 100k pot master volume mod with no safety resistors as pictured here. I changed it to a LarMar 250k with safety resistors, but right this moment it's a Frondelli MV just to try it out. I read that affects the Presence control at lower volumes. Even with both the LarMar or Frondelli MV turned up all the way, but with a bit of attenuation for hearings sake, I still didn't hear any noticeable shift.

I sprayed a little Deoxit D5 just to try, no change.

Is there ways I can check the Presence components while in-circuit. I get flashing zeroes when I try to measure the capacitance in-circuit. All connections are stock and seem solid going all the way to the 4 Ohm tap. Appears to be the Mark 2 presence circuit. There are resistance readings on the pot as I turn the dial so I believe the pot is OK. Anything I can try? Any testing tricks or measurements I can do that I don't know about?

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Okay. First thing. Realize that the presence control is part of the feedback loop from the OT and goes back to the PI stage. Thus - it's {relatively} inactive at low volume levels, and becomes increasingly more active as your playing volume rises. Same with a "resonance" control (that lots of people add - self included) - all part of the feedback loop.
Want to make it more active, first determine which tap it's taken from on the OPT. 16ohm? 4ohm? Contrary to common schematics, actual circuitry didn't (still doesn't) automagically line up with the drawing. If your NFB loop is tapped at the 4 ohm tap, move it to the 16 ohm tap if you want more activity. See what that does - but also pay close attention to {what else} it does. After that, you have little choice but to start fiddling with the resistors in the NFB circuitry - assuming there's nothing stupid like a bad pot, shorted cap etc.
My clone is substantially modified in this area ( added the resonance control but also changed the cap/resistor values to shift the focus of the intended frequency that the controls affected)
E=I*R Basic Ohm's Law
If you want to get to fiddling with targeting frequency ranges you need to then consider capacitive and inductive reactance ( in audio circuits, usually the only place inductive reactance is fitting the filter choke in the power supply - it's more of a radio gig)
Capacitive reactance is expressed in OHMs - but you'll need your A/C analysis hat on since frequencies are dynamic and ever-changing. The basic formula is (10-1)Xc=1/ωC=1/2πfC In most cases, this can be reduced to a simpler equation: XC = 1/(2πfC) where XC is (capacitive reactance in ohms), = 1 / ( 2 * PI * Frequency * Capacitance) PI is Pi. ( 3.141519265............. )

This probably takes you well beyond where you want to go.......

 

[XTRXTR]

With an O'scope you should be able see the waveform change when you dial the presence up or down. Spike edges should round down and back up.

 

[paul-e-mann]

Ok you've seen I've recently got this '73 50 watt. I've done a couple things to it but nothing too intrusive. I can't recall if I heard the presence control working before, but I think it was working when I got it. Maybe not and I was hearing things, I can't say for certain. Currently I'm not hearing a tonal shift when I turn the knob. That's where I'm at now and I'm trying to diagnose it but I need a little more circuit help as far as measuring.

First off, it did come with a Dual 100k pot master volume mod with no safety resistors as pictured here. I changed it to a LarMar 250k with safety resistors, but right this moment it's a Frondelli MV just to try it out. I read that affects the Presence control at lower volumes. Even with both the LarMar or Frondelli MV turned up all the way, but with a bit of attenuation for hearings sake, I still didn't hear any noticeable shift.

I sprayed a little Deoxit D5 just to try, no change.

Is there ways I can check the Presence components while in-circuit. I get flashing zeroes when I try to measure the capacitance in-circuit. All connections are stock and seem solid going all the way to the 4 Ohm tap. Appears to be the Mark 2 presence circuit. There are resistance readings on the pot as I turn the dial so I believe the pot is OK. Anything I can try? Any testing tricks or measurements I can do that I don't know about?

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View attachment 131433

Your master volume mod can render your presence and EQ useless, theres nothing wrong with your amp. Turn the master volume all the way up to bypass it and use an attenuator to control volume and your presence and EQ will work better.

 

[Mjh36]

Your master volume mod can render your presence and EQ useless, theres nothing wrong with your amp. Turn the master volume all the way up to bypass it and use an attenuator to control volume and your presence and EQ will work better.

Yeah you're right I mentioned that in the first post I've tried with the master cranked and a little attenuation in hopes it would work better, but I wasn't hearing any difference either.

I'm just starting to dive into the amp and see what I can find today. A quick test with the 220n exampled above I don't think yielded anything I could hear, but I need to redo it here in a minute.

 

[Mjh36]

With an O'scope you should be able see the waveform change when you dial the presence up or down. Spike edges should round down and back up.

Gotta get me one of those soon. I have a waveform function on a multimeter but I'm not sure it's sufficient for the task.

 

[Mjh36]

I'm over fifty. I can no longer hear the presence control on any vintage Marshall circuit.

This could very well be the case! I do have a little tinnitus but so far it's not affecting high end too much. I can hear the presence controls loud and clear on my two Ceriatones. Never played a vintage one until now though and I just don't have a benchmark to compare it to. Is it that subtle of a control?

 

[FleshOnGear]

Never played a vintage one until now though and I just don't have a benchmark to compare it to. Is it that subtle of a control?

In my experience, no it’s not subtle. I have a ‘72 with a 100k feedback resistor from the 4-ohm tap, and the action of the presence control is definitely audible.

 

[Mjh36]

In that case, with a signal passing through the amplifier, I usually find either a CD that I know or the radio etc, place a 220n capacitor across the 4k7, the sound should change to more trebly. If it does, remove the 220n and short out the wiper of the pot to the top of the pot (4k7 top, where 10 volts is). The sound should change. If it does, the pot is faulty, if it doesn't the mustard cap has failed. [Very unlikely the latter].

Ok I tried that out using Peter Cetera's Glory of Love on repeat off my phone for the input signal. I basically can't say I heard a difference doing any combination of those things. Hmm.
I have a 220n 100v I used, but I also tried this with the .68uf 200v I have. Across the 4k7 I couldn't hear any change. Turned the knob, no change.
Shorting the wiper to the resistor didn't do anything that I could hear either. Turned knob, nothing.

So is that first test about constant 10VDC across the 4k7, that seems to be nominal I assume? Atleast that part is correct or..?

 

[Mjh36]

In my experience, no it’s not subtle. I have a ‘72 with a 100k feedback resistor from the 4-ohm tap, and the action of the presence control is definitely audible.

Thanks, I just wanted to make sure. This is really stumping me. I should hear some sorta change with the things I tried above I would think.

 

[FleshOnGear]

Maybe your presence control is stuck all the way on?

 

[Mjh36]

Maybe your presence control is stuck all the way on?

Well that would be cool cuz that's how I like it set. But I can't tell.... I mean the amp sounds good I get enough treble through the eq, but I don't want to miss out and just would like to know what's up with the presence anyway.

I got an idea, without desoldering anything, can I alligator a whole separate presence circuit for testing? Take alligator from the NFB resistor and have it go to a pot/cap arrangement and test what that does?

 

[XTRXTR]

"I got an idea, without desoldering anything, can I alligator a whole separate presence circuit for testing? Take alligator from the NFB resistor and have it go to a pot/cap arrangement and test what that does?"

You should also add a new cap to that new pot for testing in that manner.
That would give two possible outcomes, I think:
1 no difference which means the current pot is shorted to ground
2 big difference which would mean the current pot is open (given that currently nothing happens)
3 You can't hear any difference lol