VOX AD60VT Drive potentiometer acting ... (pedals selections)

[jean-marc]

Hello all; My first post ! : Just acquired a Vox AD60VT first generation ( blue cloth ) On the pedals switch selection ,left side of controls amp, when i use the DRIVE potentiometer it is acting strange : lets say for simple : select Fuzz pedal , Level control full CW :If the Drive pot is not near full CW there is no pedal effect, only plain sound without effect; If I turn the Drive full CW the effect (fuzz for example ) comes on only when the pot is almost at full CW ;like almost an on-off situation .. Maybe a dirty potentiometer but I wonder ...I am not sure the pot will act like this if it was in need of cleaning, but maybe .. This is not related to the LEVEL control which is fine . Anyone did recall a problem like this ?
Thanks for your help . J-Marc

[The Dude]

It sounds like possibly the pot itself or a solder connection on the pot. Check solder connections. Check pot resistance both end to end and ends to wiper to make sure it's working correctly.

[Enzo]

Have you tried reinitializing the amp? It is after all just a big computer. I forget how the manual calls it, maybe "restore factory settings".

[The Dude]

Schematic link:

https://usermanual.wiki/Collections/...anual.pdf.html

[jean-marc]

Well, I am a little bit lost here,...These 10K pots (drive and level and some others ) read 570 ohms from CCW then raising to about 11K, then at full CW about 2 ohms... The Level pot : from 570 to 2K max, then 2 ohms at full CW ; About the same for some others (delay, etc ) I just dont get it : how come these pots goes down to 2 ohms at full CW ? I am lost ! Please help ! Thanks

[Enzo]

They read 570 ohms between what and what? To ground?

Look at your schematic. Page 11 on the file Dude posted above. Upper left area. Those are your pots. Note they are all wired in parallel between ground and +5vDC. That is 14 10k pots in parallel. SO they will never read 10k. One end of each is in fact grounded, so ther is your 2 ohms. You want to know if they work? Read the wiper, does it change up and down as you move the control? Then the control is likely working.

These are not volume controls, these controls send a variable DC voltage - between zero and +5v into a digital IC. and from there right into the central processor. These amps are basically computers with a power amp for the speaker. SO if you expect the controls out front to behave like the amp was a Fender inside, you will be disappointed.

I asked earlier, have you tried reinitializing the amp?

AD60 / AD120

Initialization

• Switch the amp off.
• Switch the amp on while holding down the UP and DOWN BANK buttons, and TUNER/CANCEL button. If you've done this correctly, a "P" will be flashing in the BANK Display.
• Press the WRITE Button and watch the BANK and the four CHANNEL LEDs do a little dance as they indicate each one of the 32 programs being reset to its factory preset. Once they both stop "dancing," you're done. The amp will automatically enter Program Select Mode with Program 1 - 1, "Flying High," selected.

Mounting the "Amp Type" knob


The Amp Type knob is not keyed. So mounting it correctly requires knowing what Amp Type is currently selected. The factory setting for patch program 1 - 1 has an amp type of "UK Modern". The following procedure uses this fact combined with the Original Value LED to find the correct orientation for the knob.

• Reset the unit.
• With patch 1 - 1 selected, turn the Amp Type knob until the "Original Value" LED lights.
• When this LED is lit, install the knob with the arrow pointing at "UK Modern".

[jean-marc]

Yes you are right Enzo; I was not able to access the schematics before , now i got it :
So to resume : I have about 510 ohms between wiper to one end of potentiometer ; Yes all pots goes from low values (510) , raise to mid position of pot then goes down to low values : seems all fine ;
EXCEPT the Drive potentiometer : this one goes from low value, 510, and raise all the way past the mid position to about 10.8 K very near full CW : just BEFORE full CW goes down to the low value of about 2 ohms ;
So this Drive pot is the only one not going down after mid position and continue to raise.. Does this make sense for this particular potentiometer ?? Maybe ? Actually if you read my previous post , this is the one, Drive, on which i do not get any effect by varying it, except at FULL CW ; LEVEL pot is fine.

And finally, yes I did the re-initializing procedure, without any change, But i will redo it again because I prefer to double check.. I will let you know ...

[Enzo]

SO pull that pot and measure it sitting free. Sounds to me like the element is open right near one end.