AMT CN-1

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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tuck
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Post by tuck »

Hi everyone,
after all those years I opened the AMT CN-1 and tried to trace it.

The I/O board
io.JPG
The cabsim board
cabsim.JPG
Schematic as it is on the pcb
AMT_CN1_PCB.pdf
(51.9 KiB) Downloaded 57 times
Schematic cleaned up
AMT_CN1_CLEAN.pdf
(69.13 KiB) Downloaded 79 times
Does it make sense or is it far away from correct?
The Headphone out jack contains a dpdt switch (six pins beside the jack).

The zip contains two Kicad folders but beware, both have the same file names as I copied the folder to clean up the schematic.
amt_cn-1.zip
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bajaman
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Post by bajaman »

2SK208R are N jfets not npn bipolar transistors
How does it sound ? - looks a fairly simple circuit
what value are the 4 potentiometers ?
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tuck
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Post by tuck »

The value must be printed on the bottom, I cant see it anywhere. (Edit: did not desolder them)
2SK208R j-fet right!
It sounds good. The Size Cab pot is kind of subtle but with the rest you can mess up your sound pretty nice :-)
The headphone out has a noticable hiss but it is ok.
https://amtelectronics.com/new/manuals/ ... ENG_V2.pdf

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temol
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Post by temol »

C19 -shouldn't it be connected like this?
C19.jpg

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Post by bajaman »

Hi Temol
NO - i to have pondered this - the connection is correct as Tuck has drawn it BUT there is definitely some connection via some resistance to ground missing from that first jfet source ;-)
Also - the jfet does not bias up correctly unless there is some resistance connected to the v+ supply rail - i tried lots of different value resistors with R38 RV3 and R39 but could only get 500mv dc at the source when using a 30k load to ground
More investigation is neccessary - the rest of the circuit up to the out seems to be biasing and working correctly - didn't bother with the headphone section !
bajaman
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tuck
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Post by tuck »

The headphone section is th emost important!!! :-) At least for me when playing on the couch.
I will check the ground connections again. Resistor to ground always with a via. tht components to ground are connected to the backside ground plane.

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Post by bajaman »

I have mod to lower the hiss in headphone section - but first we need to know how that first jfet is biased and what is the missing component that he source connects to ;-) potentiometer values would be helpful, but not essential !
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Post by bajaman »

I think there is a trace on the other side of the board that connects pin 3 of the size control (my guess: 1M) to terminal 1 (the source) of the first jfet
Doing this and simulating makes the jfet bias up around 3.5v on the source and the size control works like the AMT owners manual drawing
All 150j capacitors are most likely 150pF, C19 is most likely 4.7nF position pot is most likely 250k and the magnet and turn pots most likely 100k
a very simple circuit that simulates well
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Post by temol »

I'm playing with the circuit in Ltspice now. Looks like bajaman is right about missing connection.
But I'm not sure about proposed size pot value. With 1M, the resonant peak sits around 90Hz. In the manual it's around 100-110Hz. 100k pot = 110Hz, 500k= 100Hz

1M
1M.jpg


500k
500k.jpg


100k
100k.jpg

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Post by bajaman »

ok 500k then ;-)
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Post by tuck »

You are right, Size pin 3 goes to source
sizepot.PNG
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tuck
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Post by tuck »

C19 is 47n
The blue one
IMG_2512.jpg

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Post by bajaman »

Hey Tuck
Can you place a mutimeter across those pots (size and position on minimum, magnet and turn on maximum) and let us know the readings please ?
Many thanks
bajaman
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Post by bajaman »

strange that he would choose a 47n capacitor in that position - but that is what is there !
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Post by bajaman »

R35 is 275 (2.7M ?) not 272 as shown on the schematic
R34 is 512 (this would make it 5.1k) - I am not convinced that this is it's value though - i have struck AMT stuff in the past where the smd code was not the actual value of the part !
R39 is 205 which would make it 2M
I am wondering if the three 275 resistors are - it doesn't seem right for them to be 2.7M ????
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Post by temol »

2.7M seems to be the correct value.
AMTCabSim.jpg

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tuck
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Post by tuck »

bajaman wrote: 12 Jul 2024, 21:17 Hey Tuck
Can you place a mutimeter across those pots (size and position on minimum, magnet and turn on maximum) and let us know the readings please ?
Many thanks
bajaman
SIze 99,9k
Magnet 96,6k
Turn 23,14k
Position 491,1k

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Post by lrgaraujo »

temol wrote: 12 Jul 2024, 20:53 looks a bit similar to the CN-1 https://media.amt-sales.com/the-guitar- ... tor-kampf/
have you simulated this circuit, Temol? I've tried it with ltspice's j201 and for some reason I can't make the resonance peak and cutoff freq pot doesn't work as it should

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Post by temol »

lrgaraujo wrote: 13 Jul 2024, 16:27
have you simulated this circuit, Temol? I've tried it with ltspice's j201 and for some reason I can't make the resonance peak and cutoff freq pot doesn't work as it should
I tested it a long time ago. On breadboard and ltspice.
amt.jpg

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