[BUILD] 1176 Rev A - Back to the beginning...

[Echo North]

I dont think its the ratio pcb. Problem seems to be in the GR circuit. If I remove Q11, it zeros fine at every start. I put it in, nuthin. Could it be bad?

Are you able to calibrate the q-Bias?

WHat voltages are you seeing around Q11 including the gate?

http://mnats.net/files/1176REVA_125_VOLTAGES.pdf

 

[Chrome Heart]

I cant calibrate because the meter wont function. Q11 Gate voltage starts at -.5 and then drains to 0. I can get a reading from the pads if I pull Q11 from the circuit.

 

[Chrome Heart]

The strange thing is, the meter may get half way to zero before falling back down when I power it up. Sometimes it will actually get to zero, and when I try to get a voltage reading and put the probes on, on falls back down to -20. Bizarre. Ive built three of the other versions and Ive never seen anything like this.

 

[Echo North]

The strange thing is, the meter may get half way to zero before falling back down when I power it up. Sometimes it will actually get to zero, and when I try to get a voltage reading and put the probes on, on falls back down to -20. Bizarre. Ive built three of the other versions and Ive never seen anything like this.

That's really weird about the gate voltage.  That is your issue.  Is the Q1 gate declining in the same way?

If you swap Q1 and Q11 that would be away to find out if it's a dead transistor.  You gate voltage should be steady somewhere between -0.5 and -1.25 DC.

 

[Chrome Heart]

I will swap them this morning. I ordered from you so Im sure it wasnt DOA. I know you match them before you ship. I suspect Ill have the same problem. Its odd because I have gone through the whole board several times and I dont see anything visibly wrong. Could it be PSU related?

 

[Echo North]

I will swap them this morning. I ordered from you so Im sure it wasnt DOA. I know you match them before you ship. I suspect Ill have the same problem. Its odd because I have gone through the whole board several times and I dont see anything visibly wrong. Could it be PSU related?

If your voltages are stable in your two amp stages I doubt it.  If a transistor is going to fail, it'll fail in the first 24 hours most likely.  The 1 min they are in the test jig may not be enough to trigger at production failure.

 

[Chrome Heart]

Swapped 'em. Same issue. If I unplug the unit 4 times out of 10 it will zero. Although it struggles to get there sometimes. It also changes it position - it may go to -5 and Ill zero adjust it, it falls, I restart and it goes to +3. The one time it was stable I got these readings at Q11 G -1.5 (before a steady drop and VU fell to -20) S -1.027 D +11.22

 

[Chrome Heart]

Rail voltage is -9.68 and 29.58

 

[Echo North]

Rail voltage is -9.68 and 29.58

But the gate voltage on Q1 remains constant?  It's only Q11 that the gate voltages is wacky?

 

[Chrome Heart]

No. They both "bleed down" when I put the probe on the gate leg and the 0 ground. Simultaneously the VU drops.

 

[Echo North]

No. They both "bleed down" when I put the probe on the gate leg and the 0 ground. Simultaneously the VU drops.

Well those get fed from the -9 rail down by the bias pot.

Check your C8/C22 cap values.  Check the wiper of R59 and see if the voltage hold there.  That feeds the bias through your release pot.  Check your release pot wiring.

Mike

 

[Chrome Heart]

Ill check that. At the moment all the pots and ratio board a disconnected from the main PCB

 

[Chrome Heart]

The values are correct. Im using Orange Drops in C7 and C10 but on the pcb I notice there is a polarity stamped on the location. Did I miss something there?
They're SBE225P100V 105K

 

[Chrome Heart]

R56 wiper is -1.171

 

[Echo North]

The values are correct. Im using Orange Drops in C7 and C10 but on the pcb I notice there is a polarity stamped on the location. Did I miss something there?
They're SBE225P100V 105K

C7 and C9 should be polarized electrolytic caps..

 

[Chrome Heart]

I have a Nichicon lytic in C9 47uf 63v. I have the 1uf OD in C7. Ive seen other builds with the Orange Drop in C7, did that change? BOM shows a film cap there.

 

[Echo North]

I have a Nichicon lytic in C9 47uf 63v. I have the 1uf OD in C7. Ive seen other builds with the Orange Drop in C7, did that change? BOM shows a film cap there.

Film should be fine for C7.  Either way it would not mess with your Q11 gate bias.

If you disconnect the meter (X/Y on the main PCB) can you set the Qbias using this method:

If you have a version with the "off" switch on the attack pot, you can just switch that.  That will ground pad 22.  Feed a 1khz signal into the input at about 0dBu.  You can put your DMM across input xlr pin 2 and 3 and look for 0.775VAC which is about 0dbu.  Your DAW might say it's sending out a -16dBu FS signal or something.  Just do what ever you need to do to get 0.775 VAC across 2 and 3 at the input.

Now put your DMM across pin 2 and 3 on the output xlr and turn the qbias (in and out knob mid way).  You should see the output moving up or down.  Turn the bias until it gets to the max and doesn't get any higher. 

Then adjust the output knob so you see +11dbu (2.75VAC) at the output.  Now pull the qbias back until you get a 1dbu drop 10dbu (2.45VAC).  That will set your bias.

It would be nice to know if your amp/sidchain is functioning fine and if it's just something in your meter driver.

 

[Chrome Heart]

I have completely disconnected the pcb from any other components (VU included) and I still see the gate bleeding down on the meter when I attach the probe to ground. I understand the value of establishing .775V for testing purposes, but it seems to me that the issue here is to find out why the voltage is dropping at the gate of both fet's. Could it be one of the other transistors? I will however try to adjust  by way of the method you mentioned above.

 

[Echo North]

I have completely disconnected the pcb from any other components (VU included) and I still see the gate bleeding down on the meter when I attach the probe to ground. I understand the value of establishing .775V for testing purposes, but it seems to me that the issue here is to find out why the voltage is dropping at the gate of both fet's. Could it be one of the other transistors? I will however try to adjust  by way of the method you mentioned above.

Oh wait I thought just your meter FET was dropping V at the gate.  Yes if they are both dropping we need to sort that.

 

[Echo North]

The gate voltage is supplied by the -9VDC rail.  R35/R60 are a little voltage divider to feed that R59 QBias trimmer.  This wiper sends out the gate voltage.  You previously stated the -DC voltage was constant here correct?  If it is, we can move a long to release pot.  The voltages moves to the area on the release where the wiper and outer are bridged with a 270KΩ resistor where it meets up with some -DC from the ratio PCB.  Check your DCV on the other side of the release pot where the resistor is not connected...still steady?  Do all of this w/ GR off.

Check your C22 cap value and you R 7 and R65.