Marshall AVT100 FX not adjusting

[jp3zone]

I have an AVT100 with a peculiar problem in the dfx circuit. The fx work, you can select different effects and vary the amount mixed into clean signal too. The problem is the Adjust control does nothing at all. I pulled the adjust VR17 (b20k) pot to check & it measures fine.

Where else should I look? Could it be that the fx module itself has gone bad, and if so where can I get one online?

[soundguruman]

That's pretty hard to say, since the schematic does not show a hint of the effects wiring.

[Jazz P Bass]

Yeah, you will need to access the correct schematic.
AVT100FX.
The one you supplied is a plain jane AVT100.

[jp3zone]

The schematic is correct, trust me

If you look carefully at the third page of the second attachment, you'll see cnb1 connector. This is where the dfx module plugs in. Marshall doesn't supply the dfx schematic, so this is the complete schematic for the rear board.

I've checked the vdc to the dfx pcb and it's spot on... Haven't really seen one of these fail before.

[Jazz P Bass]

Oops
The spaghetti amp!
What does the encoder on 60-02/ pg 3 do?
Can you please advise where VR17 is.
Which schematic.
Which page?
(this isn't Marshall Tech Support)

[jp3zone]

Ok, the encoder is used to select the type of effect - chorus, delay etc. and it works, as does the dfx mix control. VR17 is top left on page two of the 1st attachment, and it controls the intensity or delay time of the different fx. At the moment vr17 has no effect on the, err effect.

[Jazz P Bass]

Thankyou for the redirect.
I am confused by the "Clean Adjust Label".
Is there any signal riding on CNF7 (VR17) or is it a voltage adjust that goes to the DFX board.
CNF7 pin 1 is ground.
Pin 2 is the wiper which goes to the DFX connector, pin 14.
Pin 3 goes to DFX pin 12 and it looks like to +5V.
There are caps on pin 2 & 3.
Are they o/k, not loading anything down?
I would be curious as to what VR17 is doing down on the DFX aboard itself.
If it is a voltage level to a microprocessor & it reaches the MP, than that proves the dfx board is the problem.

[Enzo]

Just looking at it, it makes me think VR17 is just making a control voltage. One end is grounded, and the top of hte pot comes from a point with a cap to ground that would kill most signals. C213,214. SO is there a DC voltage across the ot and is there a variable DC at the wiper?


Of course I could be totally wrong.

[Jazz P Bass]

The varying Vdc level would make a nice input for a microcontroller.

[jp3zone]

The pot does indeed vary the voltage. And there is dc across it. So replaced the fx & alls well, seems the old dfx was damaged after all.

Thanks for the input guys.
Later

[doctor]

The pot does indeed vary the voltage. And there is dc across it. So replaced the fx & alls well, seems the old dfx was damaged after all.

Thanks for the input guys.
Later

Hi all,

In order to assemble this puzzle the DSP board schematic is needed (see attached).

The VR17 is connected as following (we need review 3 pages together):

1. VR17 upper contact–CNF7:3–CNB7:3–R250 [1K] –R239 [3K3] –CNB1:7–J1:7 [+5V].
2. VR17 mid contact–CNF7:2–CNB7:2-R242 [1K]- CNB1:14–J1:14 [EDIT_A]-U4:61 [ADCIN1]
3. VR17 low contact–CNF7:1–CNB7:1-GND.

Thus the VR17 supply DC control voltage (from 0 to +3.5V) to controller’s ADC input No:1.

Check:
1. Whether the voltage at the VR17 upper contact is approximately +3.5VDC.
2. Whether the VR17 low contact is connected to GND.
3. Whether the voltage at the U4:61 (DSP board) monitors the voltage at the VR17 mid contact.

[Enzo]

REplacing the DFX card solved it for him, so the trouble was on the little board.

[Jazz P Bass]

Thanks for the DSP schematic.
Nice to have.

[Barrym]

Hi all,

In order to assemble this puzzle the DSP board schematic is needed (see attached).

The VR17 is connected as following (we need review 3 pages together):

1. VR17 upper contact–CNF7:3–CNB7:3–R250 [1K] –R239 [3K3] –CNB1:7–J1:7 [+5V].
2. VR17 mid contact–CNF7:2–CNB7:2-R242 [1K]- CNB1:14–J1:14 [EDIT_A]-U4:61 [ADCIN1]
3. VR17 low contact–CNF7:1–CNB7:1-GND.

Thus the VR17 supply DC control voltage (from 0 to +3.5V) to controller’s ADC input No:1.

Check:
1. Whether the voltage at the VR17 upper contact is approximately +3.5VDC.
2. Whether the VR17 low contact is connected to GND.
3. Whether the voltage at the U4:61 (DSP board) monitors the voltage at the VR17 mid contact.

I realise this is an old post, Would you have the AVT100 main board diagram too please?

I have an Vallvestate 2000 that is popping/crackling, it doesnt seem to be the pots or switches, more likely a capacitor failed, Are switches also available as the OD1 switch is sticking on the preamp, the footswitch works ok, but can't switch when the OD1 preamp switch sticks.

Thanks for any help

[Jazz P Bass]

Here is the AVT100 schematic.

AVT100.zip