Problem with meter on Tascam m3500

[Kir_For_Pleasure]

Having problems with a peaking channel on the meterbridge
The peak comes when channel in is selected. have disconnected the current channelstrip and current inputs to the meterbridge but the peak still appears
Do you have any suggestions on how I should go ahead and solve the peak.
video that describes the problem
http://www.youtube.com/watch?v=OQI-a5uixJ8&list=UUlhLpluc7I-fDpATM276g_Q&index=1&feature=plcp

 

[sahib]

Any possibility of posting a schematic?

 

[Kir_For_Pleasure]

Here comes a schematics, is it possible U504 causes the problem ? TA7318P-1

 

[Kir_For_Pleasure]

 

[sahib]

Hi Thomas,

If the channel and the input in question are removed then there are only two possibilities.

Either there is a wiring problem which I suspect is not the case, or U104 (U504) which is a buffer. If you remove it, the input to the LED driver circuitry will be pulled to ground through R115 + R116 (R551 + R556) and it should definitely not light up. If it still does then you have to look for the problem in the LED driver module.

 

[Kir_For_Pleasure]

desoldred u504 and the meter went black, ordered a new one.

 

[Kir_For_Pleasure]

took out U704 and put in place of U504 with the same result as before
also tried to put the chip 504 in 704 and there seems not to be the chip that is faulty.
can it be a grounding problem ?
i did a starground on the console for a couple of years a ago and havent used it so much after that.
 

 

[sahib]

If you used it even for once after the star ground and it did not have this problem then the grounding is probably fine. However, we'll now have to consider my first possibility of wiring and the grounding.

If the power ground was disconnected then the LEDs would not turn on as they return to the power ground.

But if the signal ground was disconnected then the input will float and cause this problem.

If the power and signal grounds of that particular channel are somehow messed up at some point then you would again have this problem.

Check the wiring and let's see where we are.

 

[Kir_For_Pleasure]

i did the stargrounding listened after hum confirmed that all was ok, then i move to house so i had the console in store for a couple of years.
Should check up your recommendations !
Still think its strange when the channel is completly dissconected from the meter and i still got this peak ?

 

[Kir_For_Pleasure]

Here you can see what i have done with grounding around the psu
do you have any suggestion what i should do with the grounding here ?

 

[Kir_For_Pleasure]

Sahib what do you think about the grounding around the psu ?

 

[sahib]

Sorry Thomas, I drifted for a while.

Can you explain what you have done? The schematic looks original. Did you remove the ground from where you marked and connected to the star ground you created? I am not suggesting that this could be the problem but better to check.

However, we now also have to turn our attention to U504 again.

We removed it and the meter channel in question turned off. That means there is no problem on the signal line between the U504 output and the meter channel input.

When we put the U504 back on the meter channel turned on. That means U504 is now active and receiving signal in its input. Let's track this.

It's input is connected to U103 switch. If the switch is supposed to be off then something is turning it on.

You can also verify this by grounding the input of U504. Connect the node where R113 and 114 are joined to the ground. The meter channel should turn off as soon as you do that.

If that is the case then U103 is active which should not be.

 

[Kir_For_Pleasure]

grounded the input and as you said the channel turned off immediately. do you think something is wrong with U103 ?

 

[wkrbee]

The meter is showing a D.C offset. It's the LC4966 labled as U103 U203 etc.When those 4966 I.C.s fail,the control voltage,(usually +/-12), shorts to the input and output pins.The LC4966 is a high voltage version of the 4066 quad-bilateral CMOS switch.

 

[Kir_For_Pleasure]

Thanks for your help ! ordred a couple of new ones

 

[Kir_For_Pleasure]

What do you think of the starground around the psu ? i have marked the connection i have done, should the chassie ground be linked to the star ground also ?

 

[sahib]

Sorry again Thomas,

As wkrb also commented I think the last suspect is U103.

I don't think there is anything wrong with the grounding scheme as it looks on the paper. Let's get the meter running first .

 

[Kir_For_Pleasure]

changed U103 but still have the peaking channel. any suggestions how to proceed ?

 

[Kir_For_Pleasure]

can it be u201(1)

 

[sahib]

Hi Thomas,

You have to work backwards now. Remove the coupling capacitor on the input of U201.
This will tell us two things.

1). If the VU turns off then U201 is receiving signal which it should not. So you'll trace that back.

2). If the VU remains on then U201 is faulty.