1 Bank of 8 faders not working, Any Ideas ?

[funk]

hello All, Just incase anyone knows the fault before i unsolder the faders, Has anyone come across this before ?

When you power on all faders default to 0.

But bank 1 ( 1-8) of faders are all stuck down to 0 when you try and push them up they just slam back down to 0,
i have tried swapping the two faders banks around from 1-8 to 9-16 and the same fader bank does the same thing, the other works fine on both ribbons, so it is the fader bank.

when on if you look at to photo's faders 5 6 7 the middle leg of the top NPN 2n6039 darlington pair ( the other bottom one being a 2n6036 pnp )is getting hot in about 10 seconds after turning on, they seem to get hotter from 1 being warm to 7 very hot, you can see the dark areas on the middle leg of the top one but only every other one,
I have done checks on them all against the good bank and all seem fine,
I have checked all the caps these seem fine and all the grounds seem ok,
All the fader motors are ok same ohms on the coils and the fader tracks are all ok,

The legs with the burning on faders 5,6,7 are all at transistors Q2 leg 2 at the 12v input ( nothing to do with the pen scribble and arrow on the paper ignor that )
At a guess i would think the pair of transistors are just leaking the 12v straight to ground through q3 2n6036, there is no shorts here to any of the other legs so maybe something holding open the gate when powered up ?

So,...any ideas please before i strip them down and take them out to check ? ?

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[Jondav1120]

Hi Funk,

What happens if you turn the "fader motors off" on the control surface? This should turn Q1 and Q6 off and remove the control signal from the bases of Q2-5.

Regards

John

[funk]

Hi John, tried that and it still holds the faders down
till the fader motors heat up, then have to turn off.

When powering on then checking the voltages to the bases of Q2 and Q3 on both boards after the faders have reset to 0, the good board the voltages go to 0 but on the bad board the voltage to the bases of Q2 and Q3 stay at about 10v, so keeping q2 and q3 open letting in the 12v to the coils of the faders ( this happens to all the faders) hence not being able to move the faders up, while the voltage is constantly keeping them down.
So it is looking like as your thinking a control issue ?

[Jondav1120]

Hi Funk,

Just having a look at the circuit diagram...trying to work out what is common to all the faders, which would cause them to lock down.

Regards

John

[Jondav1120]

Hi Funk,

Had a look at the circuit diagram....two possiblities:

1. check the output voltage of U3. This feeds the faders and the D-A converter with a stable 5V supply, and if this goes bad it would affect all faders on that board.
2. It's possible that U908 (D-A converter) has failed.This post has a lot of info on the D-A converter problem.

Regards

John

[funk]

Cheers will have a look, also read in another post about the AD8803 8 chan control chip on the board, tempted to swap the chip to find out.
Cheers keep you posted

[Bruce Graham]

Hey Funk;

The problem as discribed is usually the fault of tghe AD8803 on the 8-way fader boards, and AD8804 in the 9-ways fasder boards. These are Analog to Digital OP amps that controls the fader motors.

If you are good at solder Surface Mount parts, it's easy. If you are good at it any electronic service place can do the job. I think I've changed 6 by now.

If they can order the parts have them do that. They may get them faster themn you canm from ebay. I suggest you have them order a few extra's as, in my experinece, they do go from time-to-time.

So, for clarification, when you said "goes to 0" do you mean "to the bottom of the fader" representing Off? Just wondering.

You will find that the monitor is still correct, so you can still do stuff while the fader motors can be turned off.

Best of luck.

Cheers
Bruce

[funk]

Hi All does anyone know if the AD8803AN will do ? is this the same as the AD8803A ?
Thanks

[Jondav1120]

Hi Funk,

See screenshot of relevant section of datasheet for the AD8803. Sorry about the picture...you will need to scroll across to the right for the relevant bit!

AD8803.JPG (Array KiB) Viewed 1846 times

The AD8803AN is correct functionaly, but packaged in a PDIP16 (Plastic Dual-in-Line) package. This is not what you want! The AD8803 used in the D8B is the SO16 (surface mount) package which according to the data sheet is designated AD8803AR.

Trust that helps!

Regards

John

[funk]

Hi John thank you for that , i have that data sheet, how did i miss that ....Dowww, thanks again.