Treble Control Dead...!!

bertus

Joined Apr 5, 2008
22,270
Hello,

The mid signal is influenced by the mid pot (R42) in combination with C22, this forms a ballast for the mid range signals.
The Bass signal is influenced by the bass pot (R41) in combination with C21, this forms then the ballast for the bass signals.
The High signal goes though C20 to the treble pot (R40) this pot is acting as a kind of balance between the treble signal and the combined bass and mid signals.
I hope this clearifies the working of this type of tone control.

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello,

The mid signal is influenced by the mid pot (R42) in combination with C22, this forms a ballast for the mid range signals.
The Bass signal is influenced by the bass pot (R41) in combination with C21, this forms then the ballast for the bass signals.
The High signal goes though C20 to the treble pot (R40) this pot is acting as a kind of balance between the treble signal and the combined bass and mid signals.
I hope this clearifies the working of this type of tone control.

Greetings,
Bertus
Now for the parts: C20 is 470pf in the schematic, but I don't know the voltage or other factors that count. I found 470pf, 500 WVDC, dipped silver mica caps(many other values) from http://www.rfparts.com/caps_dippedmica.html

Also, I've heard a lot of people complaining about excessive bass(boomy) on this amp(which I thought was just due to my C20 issue). Most say they need to reduce bass to almost minimum to eliminate this boominess. Since C21 is .47uf, would a different value make a difference here as a mod...?
 
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bertus

Joined Apr 5, 2008
22,270
Hello,

If C20 is "open", no high signal will pass through it and the sound will be out of high tones.
The capacitor you found for C20 is one of high quality.
You have the cheap ceramic and the little bit more expencive silver/mica you named.
The voltage of the capacitor must be at least 2 * the voltage supply, so 500 Volts is no problem.
For C21, if you make the value higher, it will effect lower frequencies.

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello,

If C20 is "open", no high signal will pass through it and the sound will be out of high tones.
The capacitor you found for C20 is one of high quality.
You have the cheap ceramic and the little bit more expencive silver/mica you named.
The voltage of the capacitor must be at least 2 * the voltage supply, so 500 Volts is no problem.
For C21, if you make the value higher, it will effect lower frequencies.

Greetings,
Bertus
If C20 is "open", should the balance of other mid and low frequency signal from input still pass through this stage(across C20), not counting the other paths through the mid & bass pots, which we have agreed does get to the next stage, through R40, unless either, or both R41/R42 are set counterclockwise to "0"(grounding the entire tone section=no sound), or is the C20 path totally dead to "any signal" able to pass at any time. If this is true, it would explain a lack of reasonable high frequencies, and an excess of mid & lows in this state.
Do you feel fairly confident, thus far, that the treble pot itself is not an issue, since I did read 0 to progressively increasing R values, albeit a few erratic jumps while measuring(not perfectly smoothly increasing readings)while on the pcb, but even if it wasn't perfectly fit, I would still have gotten some tone change(varying R verified) if C20 were fine......correct....?
These are more of a confirmation of my logic than anything else, while waiting to get parts........so I hope you don't mind the extra questions.
 
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bertus

Joined Apr 5, 2008
22,270
Hello,

Yes, you are correct.
If C20 is fuctioning and R40 is sometimes lacking contact, there will be positions where you have the high tones coming through.
At the places where R40 is lacking contact you will hear nothing.

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello,

Yes, you are correct.
If C20 is fuctioning and R40 is sometimes lacking contact, there will be positions where you have the high tones coming through.
At the places where R40 is lacking contact you will hear nothing.

Greetings,
Bertus
1) That's what I thought about the treble pot, but a little unclear on the answer to whether an open C20 would pass "no signal", or just a partial signal with "no treble/high frequency" when R41 or R42 are not set to "0".

2) Also a prior question regarding tone caps/pots. Are they "cut only" in actuality, or are they boosting and cutting(t,b,m) from center=5(flat).... I think guitars are considered "flat" at 10, and treble is cut only. Is it different in these amplifiers...?.........
 

bertus

Joined Apr 5, 2008
22,270
Hello,

In this tone control circuit is used that a capacitor forms a lower resistance (Xc) for a higher frequency. (and so damping higher frequencies).


Reactance of a 100 uF capacitor:
Frequency (Hertz)Reactance (Ohms)
60 26.5258
120 13.2629
2500 0.6366

When a pot like R41(bass) or R42 (mid) in series with such a capacitor the effect of the capacitor will become stronger when the resistance of the pot becomes lower (closer to 0).
For the treble C20 gets also a lower value at higher frequencies , enhancing the high tones. (it is in series with the signal)

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello,

In this tone control circuit is used that a capacitor forms a lower resistance (Xc) for a higher frequency. (and so damping higher frequencies).


Reactance of a 100 uF capacitor:
Frequency (Hertz)Reactance (Ohms)
60 26.5258
120 13.2629
2500 0.6366

When a pot like R41(bass) or R42 (mid) in series with such a capacitor the effect of the capacitor will become stronger when the resistance of the pot becomes lower (closer to 0).
For the treble C20 gets also a lower value at higher frequencies , enhancing the high tones. (it is in series with the signal)

Greetings,
Bertus
Wow!! that's pretty heavy duty stuff, but informitive. I think my biggest problem at this point is trying to imagine signal flow(direction). I guess it's not just a simple matter of just getting from one place to another without external forces. It seems that signals are either flowing in all directions, or is it just that because of resistance(fixed or variable), capacitance, voltage changes, etc. somewhere down the line, or on a side path, all have an affect on the signal being altered before it can make it's final destination.

A crazy analogy being a group of people(input signal) on a hiking trip. If the path were straight, level, and no obstructions or weather problems, that would be a straight wire with no resistance, voltage changes, etc,,,,,,(no amplification, tone, etc...)

Now an amplifier circuit would be the same group of people(input signal) on a trip with resistance of winding roads, steep grades, narrow paths, varying weather.........following multiple paths where people split off, encounter more obstacles, changes, sometimes regroup again at different points.........capacitors being need to rest, store energy, get refreshed/altered..........amplified by multiplying the population(voltage potential) and going downhill on steep grades, bigger population gaining momentum(current), etc........all combining and affecting each other before they reach their final destination(completely altered, being bigger, stronger, altered=volume/tone).
I apologize for being a caveman here, but if I can picture it in some way, it would really help to get a grasp on what's really going on with these circuits, parts, voltages, current, resistance, capacitance, etc.....
Feel free to correct, or clearup my analogy..........
 
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Thread Starter

wolf1419

Joined Nov 1, 2008
117

Thread Starter

wolf1419

Joined Nov 1, 2008
117
I had to order some parts, but the silver mica caps place wasn't working for me, so I ended up ordering the the stock ceramic caps 471pf, 500v, some misc parts for other projects, and a simple Mastech 390-735 Capacitance Meter(0.1pf-20kuf) and figure it will help me with testing and learning. It's not an expensive one($45.), but hopefully useful....I checked min & max capacitance values in my schematic as a gauge(many multi meters have capacitance testing, but not within all the ranges needed).
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello,

Strange that the silver/mica did not work for you.
In the EDUCYPEDIA there are much more pages on components than the Capacitor page and resistor page I already gave you.
Here is the general electronics page.
http://www.educypedia.be/electronics/electronicaopening.htm

Greetings,
Bertus

PS there seems to be a time difference of about 6 hours between us, so there are timeslots to meet eachother.
Not going with silver mica caps was more to do with their high shipping charges for such a small item and then I decided to go with a known supplier that had many other items that I wanted and got free shipping. I figured the ceramics would be o.k. as a stock replacement just to get it working properly.
This is the meter: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=390-735
One guy loves it/one doesn't. Hope it works for my applications, or they do have $$ back if it doesn't. I probably should have asked first to see what you recommend in a low priced unit.
 

bertus

Joined Apr 5, 2008
22,270
Hello,

For audio applications the ceramic type will probably work well.
There are so many types of capacitors that you can write a book about it.

Let me know when the parts arive and we will continue.

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello,

For audio applications the ceramic type will probably work well.
There are so many types of capacitors that you can write a book about it.

Let me know when the parts arive and we will continue.

Greetings,
Bertus
Thank you, and I'll keep reading in the meantime-great site you shared!!
 

bertus

Joined Apr 5, 2008
22,270
Hello,

I have noticed that some link on the EDUCYPEDIA are not active anymore.
Most of them can be found back with teh WAYBACKMACHINE.
http://www.archive.org/web/web.php
Copy the url in the search field and click "take me back".
This will produce a page with dates where the page still was there.
By using the dates you can find back lots of "lost pages".

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello,
It's been a while, I've done "some" studying, but not enough to make everything click in by a long shot. Anyway, I received my 471pf caps & capacitance meter. I tested the new ceramic(orange)caps and they ranged from 435-485pf, so the meter seems to be o.k.
I got back into the chassis and even though it is said that you must remove one leg to check a cap, for haha's I just tried it without removing and it(green cap) was right in there at 475pf, so I think it's good(infinite continuity reading-not shorted, proper capacitance). I decided to test each leg to ground and one leg is shorted to ground regardless of the treble pot position 0-10. If I understand properly, this cap allows only the upper frequencies to pass through the cap and mix with the mid & bass circuits before going to the next circuit.
Should one leg on this cap be shorted to ground(doesn't seem like it should which would suggest a different problem), or variable with the treble pot. It's still a bit of a mystery knowing what I should expect when measuring certain components "on the board", which is not completely removed due to many connections hindering full removal if not totally necessary(i'm saving that as a last resort).
 

bertus

Joined Apr 5, 2008
22,270
Hello Wolf,

Long time, no see. Welcome back.
What experience did you have upto now?
I understand that you have measured the 470 pF cap, as it seems to be O.K.

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello Wolf,

Long time, no see. Welcome back.
What experience did you have upto now?
I understand that you have measured the 470 pF cap, as it seems to be O.K.

Greetings,
Bertus
I have done a lot of reading regarding resistance, voltage, and current relationships(oms law), definitions, measurements, resistors, pots, capacitors and IC's. I have worked with some schematics, symbols, finding the parts on the board, and some field measurements.
Now my current question is about that C20 appearing to measure fine on the board, but one leg shorted to ground regardless of pot position 1-10. Is that what I should be getting..?
I would think not.....meaning no sound through the treble circuit...
That would leave the pot, which also tested o.k. ...I don't see anything else in the immediate path..........I'm stumped!
 

bertus

Joined Apr 5, 2008
22,270
Hello,

Here is the drawing again.



You say that point B is grounded.
In that case the treble pot is almost a volume regulator, when the pot is also on ground.

Greetings,
Bertus
 

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