Ultimation fader - TR6 VN0610L fries

[kilmister]

Hi folks!

Very often I find that TR6 VN0160L fet has fried at Ultimation fader card causing fader to crowl upwards.
Function of the fet is to be Motor Disable switching fet after the touch sense circuit. Please refer to Ultimation Service Manual 82E353 sheet #1.

I have replaced fried fets with 2N7000 which has worked fine, but I wonder what the heck fries those fets so often.
Has anyone else come across with same problem?
Any ideas?

Cheers,
Paavo

VN610L Datasheet
2N7000 Datasheet

[Artur D'Assumpção]

Hi Paavo,

I suspect these get really hot and just fry. It's a very compact unit and it gets hot. The temperature variations from turning on/off the console may affect drastically the electronics.

This is the think I suspect most.

What's the symptoms of that failure?

Cheers,

Artur

[kilmister]

Artur, the symptom is a fader which goes slowly up.

I doubt temperature because TR7 which is also VN0610L hasn't failed. It works also in quite similar circuit.

-Paavo

[Artur D'Assumpção]

Hi Paavo,

I have some weird similar problems, with a fader that slowly goes down, and sometimes sticks... and then recovers. I will soon be in conditions to detect since tomorrow finally I receive my new extenders (i'll post some photos).

I think it's very weird, but there are several components that fail frequently. It's either temperature (it gets really hot in there) or those old SMD components weren't very reliable.

Have you noticed if the ones you fixed have failed again with the same problem?

Cheers,

Artur

[kilmister]

You have exact same problem. Solution is replacing TR6. Fortunately it is throught hole component.
I don't have any markings in my service diary that 2N7000 has fail yet.

"Sticking" happens because touch sense signal doesn't open TR6 so motor are still keeping torque against your finger. After you push/pull fader more amps are going in the motor so current sense hits in. Because current sense is in different circuit it still works and disables motor.

-Paavo

[Artur D'Assumpção]

This circuit is really over engineered.

One of the things I was told by Keith (recycled audio) is that one of the things he always does when servicing these faders is switching the 1 Ohm resistor, since this fails often.

I have one fader with your problem, other with a shaky complex (if you move it slaved to group, or if you automate) it starts shaking very quickly up and down. Looks like a feedback somewhere in the circuit.

I also have another that simple doesn't come down. I suspect this one is missing the negative voltage to the motor perhaps.

Do you have any suspicions why these circuits fail frequently?

Cheers,

Artur

[kilmister]

Haven't had too much another problems than TR6, just couple IC's has fried.

For "shaky" fader I would check IC chips for strange voltages, for last one you mentioned which won't move down I would check drive transistors.

-Paavo

[Artur D'Assumpção]

Paavo,

Thanks for the tips, I will definitely check that as soon as my extender cards arrive.

I will keep you posted of developments.

Cheers,

Artur

[Artur D'Assumpção]

By the way, I am curious,

Have you ever found yourself having to recap the electrolytic capacitors? Particularly the pair of capacitors that "feed" the touch sense circuit, which documentation say they have to be matched to the TS module.

Cheers,

Artur

[kilmister]

No I haven't because I have newer generation of touch sense circuit which has adjustable capacitor.
Older version uses capacitor which is matched with touch sense circuit. IIRC cap is ceramic type so it's very rare you have to change it.

-Paavo

[Artur D'Assumpção]

Paavo,

Do you know why P&G protected the TS circuits in molded "plastic"? I have some which are in this condition, other which aren't.

Cheers,

Artur

[kilmister]

I don't have a clue...
Maybe to protect from reverse engineering or emitting high frequencys, but in real I don't know.

-Paavo

[waltzingbear]

more likely to stabilize the stray capacitance's from changing over time. This thing operates at fairly high resistances and the capacitance required to change the operation is very small, a little contamination and it may not work correctly. Hence potting.

Alan

[Artur D'Assumpção]

It's weird, because old versions of the board have that gel, earlier revisions don't have it.

[kilmister]

It's not weird at all. During years some development has happend and P&G guys who orginally supplied touch sense circuits has noticed there is stray capacitances and did something to it.
Cheers for info Alan!

-Paavo