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

[FleshOnGear]

No. Reading resistance there as mentioned by @dtier earlier places the 100k in parallel with everything in the presence circuit which should give a lower value than the 4.7k resistor, something lower than about 4.4k in measurement. It was just a quick test to see if a proper value came up.

@Mjh36
Honestly the problem with the circuit which is what I thought from the get go is that the .68uF capacitor is far too large for the 5k potentiometer value.
I just wanted to see if all was intact by performing that quick feedback resistor resistance measurement in circuit.
You should replace that .68uF with a .1uF value which will give you about eight times more useable range for that circuit.
That big ole capacitor is passing too much frequency even when the PRESENCE is to zero.

I have no idea why Marshall people used a .68uF right there. It makes no sense and that value was quickly dropped from their lineup.

Oh, crap. Is it really a 0.68uF? All this time I was thinking it was a 0.068uF. I feel dumb now. I agree that 0.68 is way too big, and would explain why it doesn’t sound like a proper Presence control. It would also explain why some of the tests @Mjh36 did yielded no real results.

 

[Pete Farrington]

A high value presence cap eg 680nF makes the presence control have more effect, not less.
It brings the boost down into the low mids / high bass.

My guess is that the 680nF cap may be dead, open circuit.
If feasible, I suggest to consider taking it out and trying it on the appropriate capacitance test range of the multimeter.

 

[dtier]

Your readings on the pres pot look normal for a 5k pot. You are right that if the cap failed the presence pot would have no effect. Its not likely but if it is failed open you can put a known good cap across it and it would work. If it failed shorted it will read 0 ohms with your meter.

I have LARMAR type PPI's on three 70s heads and the pres controls are all noticeable even at lower volumes.

I had to try out low settings again on the PPI master today. I usually only use them above 4 with an attenuator if necessary, and I always keep pres control low. I can't really notice the pres control below 4 which makes sense. Mine use .1uf. I never tried .68 but I'm gonna now.

I also wear hearing aids. If I take them out I can't hear pres control at all as was mentioned earlier. We all have varying levels of high frequency hearing response.

 

[Gblev]

Yeah that's my understanding as well, so I'm doing all these tests with the master on 10.

I'm soldering in the bias resistors right now to go non-MV, but I'm unsure if I'll here a difference if I couldn't hear presence with the ppmiv dimed.

That's how mine works, and with you not hearing it, that's a mystery for sure. Have you tested the pot to make sure it's working correctly?

 

[mickeydg5]

A high value presence cap eg 680nF makes the presence control have more effect, not less.
It brings the boost down into the low mids / high bass.

My guess is that the 680nF cap may be dead, open circuit.
If feasible, I suggest to consider taking it out and trying it on the appropriate capacitance test range of the multimeter.

No it does not. The larger capacitor value practically forces a large amount of presence because it is always passing the majority of the feedback signal to ground. The RC shelf is much to low with the larger capacitor and diminishes the range of the PRESENCE control operation.
The whole PRESENCE circuit is based on the limits of negative feedback higher frequencies being sent back into the amplifier itself.

 

[mickeydg5]

Oh, crap. Is it really a 0.68uF? All this time I was thinking it was a 0.068uF. I feel dumb now. I agree that 0.68 is way too big, and would explain why it doesn’t sound like a proper Presence control. It would also explain why some of the tests @Mjh36 did yielded no real results.

Yes, that big ass barrel looking thing is a .68uF as indicated in his posted schematic.

 

[Mjh36]

Hey gang, finally got some time to work on it a little bit. Still no luck. I made a summary video going over what I've been doing if you care to check it out.
Refreshed my ears with playing the Ceriatone AH-100 earlier which has almost the same circuit. 5k/.1uf with a variable 100k/4 ohm tap nfb. Definitely can hear the presence sweep, dark to bright like it should sound, no problem. But still no shift in presence with the Marshall with all that I've done.

 

[Alshaheen]

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?

I have a ceriatone jtm45 about 2011 build and I also can't hear presence ....it has ppimv and the tone knobs are subtle too

 

[Mjh36]

Honestly the problem with the circuit which is what I thought from the get go is that the .68uF capacitor is far too large for the 5k potentiometer value.
I just wanted to see if all was intact by performing that quick feedback resistor resistance measurement in circuit.
You should replace that .68uF with a .1uF value which will give you about eight times more useable range for that circuit.
That big ole capacitor is passing too much frequency even when the PRESENCE is to zero.

I have no idea why Marshall people used a .68uF right there. It makes no sense and that value was quickly dropped from their lineup.

This is interesting. And I think when they transitioned to .68uF on the pot they eventually switched from 5k to 25k for more usable range?

Still stumped that I can't hear any shift even with the 5k/.68uF.

I downloaded all of Ceriatone's/Metro's layouts and 5k/.1uF is the main theme. But there's all combinations. The Plexi 51 All Access has a 5k with .1uf/.68uf on a switch. It's gotta do something.

 

[Mjh36]

A high value presence cap eg 680nF makes the presence control have more effect, not less.
It brings the boost down into the low mids / high bass.

My guess is that the 680nF cap may be dead, open circuit.
If feasible, I suggest to consider taking it out and trying it on the appropriate capacitance test range of the multimeter.

I thought so as well, the .68uf being a shift down into some midrange? While the .1uf is all highs? Either way I should hear both one would think.
And I agree I think this is what I'll have to do, though I was trying to avoid it. I have to get them out of circuit for more answers.

 

[Mjh36]

That's how mine works, and with you not hearing it, that's a mystery for sure. Have you tested the pot to make sure it's working correctly?

I need to test it out of circuit I think to confirm.

 

[dtier]

I need to test it out of circuit I think to confirm.

Just one thought, could the 4 ohm tap be shorted on the trans? Hard to test with a meter, it won't read 4 ohms, maybe less than one ohm. You could however play on the 4 ohm tap regardless of your speaker impedance to make sure there is signal there. If you haven't already.

Dave

 

[Mjh36]

Just one thought, could the 4 ohm tap be shorted on the trans? Hard to test with a meter, it won't read 4 ohms, maybe less than one ohm. You could however play on the 4 ohm tap regardless of your speaker impedance to make sure there is signal there. If you haven't already.

Dave

Good idea I was wondering if it could be something with the 4 ohm, but I gave it a try and it passes signal fine.

 

[mickeydg5]

This is interesting. And I think when they transitioned to .68uF on the pot they eventually switched from 5k to 25k for more usable range?

Still stumped that I can't hear any shift even with the 5k/.68uF.

I downloaded all of Ceriatone's/Metro's layouts and 5k/.1uF is the main theme. But there's all combinations. The Plexi 51 All Access has a 5k with .1uf/.68uf on a switch. It's gotta do something.

Just because others copy it doesn't mean it is adequate.

Even if there is a little resistance in the potentiometer that will drop the top range of the filter circuit. The PRESENCE control is a variable filter circuit.

Do some testing on the PRESENCE potentiometer to make sure all is good there.
With power of the amplifier OFF:
*** note: measure directly from the metal of the tabs, not the solder joints
1) measure resistance between the two potentiometer tabs the .68uF lead connects to in unison. It should always be zero ohms.
2) measure resistance between the two potentiometer tabs, the middle tab at the end of the capacitor lead and the end tab at the lead of the 4.7k resistor and purple wire with the potentiometer in two positions - all the way down and all the way up.

 

[mickeydg5]

I need to test it out of circuit I think to confirm.

You can test in circuit. See post above.

 

[Mjh36]

Just because others copy it doesn't mean it is adequate.

Even if there is a little resistance in the potentiometer that will drop the top range of the filter circuit. The PRESENCE control is a variable filter circuit.

Do some testing on the PRESENCE potentiometer to make sure all is good there.
With power of the amplifier OFF:
*** note: measure directly from the metal of the tabs, not the solder joints
1) measure resistance between the two potentiometer tabs the .68uF lead connects to in unison. It should always be zero ohms.
2) measure resistance between the two potentiometer tabs, the middle tab at the end of the capacitor lead and the end tab at the lead of the 4.7k resistor and purple wire with the potentiometer in two positions - all the way down and all the way up.

Hey thanks here's what I got:

Test 1) Reads about 0.1 ohms at all positions

Test 2) Presence down = 5.9k ohms ----- Presence up = 11 ohms

 

[mickeydg5]

Hey thanks here's what I got:

Test 1) Not quite zero, reads about 2 ohms at all positions

Test 2) Presence down = 5.9k ohms ----- Presence up = 19 ohms

That seems fairly normal.
The 2 ohms reading is because of your contact points and meter leads to the multmeter.
5.9k tells me the potentiometer has a "+" tolerance of about 20%.
19 ohms is good for a zero point.

Another test?
With the amplifier OFF:
Short out the .68uF capacitor with a jumper wire or something, you know wire with alligator clips at each end.
Now put the meter across the 4.7k resistor to measure resistance.
Measure with PRESENCE potentiometer in two positions - all the way down and all the way up.
What do you get?

 

[Mjh36]

That seems fairly normal.
The 2 ohms reading is because of your contact points and meter leads to the multmeter.
5.9k tells me the potentiometer has a "+" tolerance of about 20%.
19 ohms is good for a zero point.

Another test?
With the amplifier OFF:
Short out the .68uF capacitor with a jumper wire or something, you know wire with alligator clips at each end.
Now put the meter across the 4.7k resistor to measure resistance.
Measure with PRESENCE potentiometer in two positions - all the way down and all the way up.
What do you get?

Presence down = 2.56k ohms -------- Presence up = 17 ohms

Does that sound about right? I think so, parallel resistance right?

I also adjusted the test numbers in the previous post, I was using alligator clips adding a little resistance too.

 

[mickeydg5]

Presence down = 2.56k ohms -------- Presence up = 17 ohms

Does that sound about right? I think so, parallel resistance right?

I also adjusted the test numbers in the previous post, I was using alligator clips adding a little resistance too.

All is done ok.
The values are correct.
By shorting the capacitor you were able to get a good DC resistance reading across the paralleled 5k pot and 4.7k. So 2.4-2.5k or so reading is good.

Since everything checks out in the feedback and presence circuits the only thing you can do is either:
1) place the negative feedback wire feed on the 16 ohm output tap and see how it affects the range of the PRESENCE
2) either place a .1uF inline with the .68uF or just replace the .68uF with a .1uF

Try it out.

 

[Mjh36]

All is done ok.
The values are correct.
By shorting the capacitor you were able to get a good DC resistance reading across the paralleled 5k pot and 4.7k. So 2.4-2.5k or so reading is good.

Since everything checks out in the feedback and presence circuits the only thing you can do is either:
1) place the negative feedback wire feed on the 16 ohm output tap and see how it affects the range of the PRESENCE
2) either place a .1uF inline with the .68uF or just replace the .68uF with a .1uF

Try it out.

Yeah I hear ya. I made an order last night for some parts. 5k pot, 25k pot, new 4k7, a .1uF cap, just everything in case I need it. Unless there is some other mystery section of the circuit that could affect presence, that seems like all I can do. And I've been really hoping I didn't have to mess up these solder joints, but oh well. I'll try all those things out.

Do you know what my resistance should be across the .68uf capacitor in circuit? Because I get:
4.34k presence at 10
10.2k presence at 0
But my other .68uF Synergy cap I ordered and using for testing, out of circuit resistance reading, is overload.

Nevermind I get it, it's reading the resistors, I gotta get the cap out of circuit.