[BUILD] New FET/RACK Official Help Thread - Please read first post!

[mixmasta]

Examine your release pot. Is it soldered well? Are and of the three legs broken off the body?

Mike

Legs and solder pads of the release pot seem fine. However, the pot itself feels a bit scratchy compared to the other pots. Measuring the resistance between the bottom and middle leg (the way its mounted) gives me a resistance of 0R at both extremes, going rapidly up to around 180k to 230k around the middle of the turn range. This is with the unit powered off and its hard to read as the auto range of my DMM is jumping quite a bit.

It seems to be connected all right, but is there a way to check if it's operating right while its soldered in?

 

[Echo North]

With everything set correct on the front panel and  .775 V ac across the input ,  I am getting 4.8 VAC at tp 15.

I am going to go through the line amp and pre amp now.

I have to work on an A shortly...I confirm that should be closer to 2.75 VAC in a few.

 

[ccommon]

You should have about 2.5 VAC at 15 (using the enclosure as common) and 7.5VAC (20ish dB) between the output + and - if I recall. That's with no GR and the pots midway assuming you have the in/out max position aligned to "0" on the front panel and then set them to "24" for measurement.  0.775 AC V at the input is between + and - input...not the enclosure.

If all that is correct and you're still that hot then you have something off in the signal line or signal preamp. Probably the first one. Visually read and check all of the R values.  Check the schematic w/ voltages in those sections.

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


Voltatages are always checked using the enclosure as your reference (black probe) except when measuring the input/output AC level. That is always done between input/output +/- also known as pin 2 and pin 3.

Mike

All the votages look good  except for Q3  . where i should be getting 13.74V , I am getting 15.6V, and  where i should be seeing 6.13V , I am getting 6.9V.

does that sound like that could be the reason for the high output/ issue ?

-Chris

 

[Echo North]

Just to clarify. In GR OFF mode, your Rev A FET/RACK is basically a 45dB mic pre that looks like this:

Input Attenuator->input Transformer->Signal Preamp->Output Pot->Signal Line Amp->Output Transformer.

TP15 sits at the end of the signal preamp before the output pot and output +/- are at the end of the chain.

Here are my measurements:
With Q1 calibrated:
TP15: 3.2 VAC
Output +/-: 7.00 VAC

With Q1 out of circuit (removed or qbias more than -1.5 VDC):
TP15: 3.6 VAC
Output +/-: 8.00 VAC

All of that is normal. If you're getting 4.8 VAC at TP15 that is too hot. Like an extra 3.5dB and when that hits the next stage that hotness as going to get even hotter.

Really examine that first stage.

Mike

 

[Echo North]

All the votages look good  except for Q3  . where i should be getting 13.74V , I am getting 15.6V, and  where i should be seeing 6.13V , I am getting 6.9V.

does that sound like that could be the reason for the high output/ issue ?

-Chris

I don't think that's it. IIRC the value of R13 was change in the version which would account for the voltage difference. It would not be causing your issue. I'd say you could try difference FETs but it seems like you either have a gain issue or a measurement issue.  I'm kinda stumped.  Did you say this is your only unit?

 

[ccommon]

I don't think that's it. IIRC the value of R13 was change in the version which would account for the voltage difference. It would not be causing your issue. I'd say you could try difference FETs but it seems like you either have a gain issue or a measurement issue.  I'm kinda stumped.  Did you say this is your only unit?

I have two REV A , the first one tests normal and sounds great .  I have a good multi meter , and Im am positive that my 1k tone is at .775

I went through the  signal pre amp and  signal line amp and all the voltages look good , and everything is in the right spot . I am pretty stumped as well .

-c

 

[ccommon]

Legs and solder pads of the release pot seem fine. However, the pot itself feels a bit scratchy compared to the other pots. Measuring the resistance between the bottom and middle leg (the way its mounted) gives me a resistance of 0R at both extremes, going rapidly up to around 180k to 230k around the middle of the turn range. This is with the unit powered off and its hard to read as the auto range of my DMM is jumping quite a bit.

It seems to be connected all right, but is there a way to check if it's operating right while its soldered in?

One thing  , I am getting .775 at the Input test points, but if I adjust the input pot  it changes a bit ...then ,  if I un plug the XLR and test just the XLR , I get  .968 v. 

 

[Echo North]

One thing  , I am getting .775 at the Input test points, but if I adjust the input pot  it changes a bit ...then ,  if I un plug the XLR and test just the XLR , I get  .968 v.

The input pot rotation will change the input level at the XLR a little. In a perfect world, its load is constant, but in reality it's not. So the fluctuations in load change the input level, that's normal.

Are you saying you have .968 VAC at the input with no signal attached? That doesn't seem right.

Mike

 

[ccommon]

Just to clarify. In GR OFF mode, your Rev A FET/RACK is basically a 45dB mic pre that looks like this:

Input Attenuator->input Transformer->Signal Preamp->Output Pot->Signal Line Amp->Output Transformer.

TP15 sits at the end of the signal preamp before the output pot and output +/- are at the end of the chain.

Here are my measurements:
With Q1 calibrated:
TP15: 3.2 VAC
Output +/-: 7.00 VAC

With Q1 out of circuit (removed or qbias more than -1.5 VDC):
TP15: 3.6 VAC
Output +/-: 8.00 VAC

All of that is normal. If you're getting 4.8 VAC at TP15 that is too hot. Like an extra 3.5dB and when that hits the next stage that hotness as going to get even hotter.

Really examine that first stage.

Mike

Hmm... at TP15 im still getting  5 V, and a out put of 16 V..

Totally stumped

 

[ccommon]

The input pot rotation will change the input level at the XLR a little. In a perfect world, its load is constant, but in reality it's not. So the fluctuations in load change the input level, that's normal.

Are you saying you have .968 VAC at the input with no signal attached? That doesn't seem right.

Mike

No, my bad . im getting  .968 from the Signal source not plugged into the the Rev a .

 

[ccommon]

No, my bad . im getting  .968 from the Signal source not plugged into the the Rev a .

Also At TP 21 I am getting -5.8 VDC , but on my working Rev A its -0.756 VDC.

Not sure what happing there

 

[Echo North]

Also At TP 21 I am getting -5.8 VDC , but on my working Rev A its -0.756 VDC.

Not sure what happing there

Same ratio setting?  That - DC value is controlled by the ratio selected.

Mike

 

[ccommon]

Same ratio setting?  That - DC value is controlled by the ratio selected.

Mike

AHH,  yeah , the ratio was not the same .. Its totally normal there .  But Im still having that Gain issue..  ugh .

 

[Echo North]

Legs and solder pads of the release pot seem fine. However, the pot itself feels a bit scratchy compared to the other pots. Measuring the resistance between the bottom and middle leg (the way its mounted) gives me a resistance of 0R at both extremes, going rapidly up to around 180k to 230k around the middle of the turn range. This is with the unit powered off and its hard to read as the auto range of my DMM is jumping quite a bit.

It seems to be connected all right, but is there a way to check if it's operating right while its soldered in?

Let's start with why the bias is dying between the TP18 and the Q1 gate.

Set you TP18 voltage for the most negative. What is that value?

Then measure the pins for DC at CN10 list them here starting from closest to the front panel back:

1:
2:
3:
4:
5:
6:

Mike

 

[Echo North]

AHH,  yeah , the ratio was not the same .. Its totally normal there .  But Im still having that Gain issue..  ugh .

Ya I mean you could try different FETs. We'll send you a new pair, but I still think your unit is too hot, and that's what is causing the issue. I could be wrong. Email us if you want different FETs.

TP22 (under the big orange cap by the output) has low Ω to the enclosure when in bypass mode? Like less than an ohm?

Mike

 

[ccommon]

Ya I mean you could try different FETs. We'll send you a new pair, but I still think your unit is too hot, and that's what is causing the issue. I could be wrong. Email us if you want different FETs.

TP22 (under the big orange cap by the output) has low Ω to the enclosure when in bypass mode? Like less than an ohm?

Mike

Hey Thank man, I might email  you guys for  for Those  FETs  , I can not find anything that would be cause the super high out up , I have gone through it multiple time s, but nothing stands out .

and yes , TP22 read about 0.0 to 0.1  ohms.

 

[Echo North]

Hey Thank man, I might email  you guys for  for Those  FETs  , I can not find anything that would be cause the super high out up , I have gone through it multiple time s, but nothing stands out .

and yes , TP22 read about 0.0 to 0.1  ohms.

Ya thing is the Q1 FET cannot add gain, only take away.  That's why I don't think they are the issue.

We're happy to send some though. Of course you can always send it in for repair.

Mike

 

[Echo North]

Maybe replace Q2 and Q3 as well.

 

[mixmasta]

Let's start with why the bias is dying between the TP18 and the Q1 gate.

Set you TP18 voltage for the most negative. What is that value?

Then measure the pins for DC at CN10 list them here starting from closest to the front panel back:

I get -1.78VDC at TP18 with the q bias fully CW.

At CN10, I have:
1: -1.78 VDC
2: 0
3: 0
4: 0
5: 0
6: 0

This is with GR off. If I turn it on, I got some fluctuating voltages as 4 and 5.

 

[Echo North]

I get -1.78VDC at TP18 with the q bias fully CW.

At CN10, I have:
1: -1.78 VDC
2: 0
3: 0
4: 0
5: 0
6: 0

This is with GR off. If I turn it on, I got some fluctuating voltages as 4 and 5.

Ya you should have DC at 1, 2, and 3. (actually 4,5, 6...pin 6 is closest to face plate, I'm named them backwards in the first post).

What's your DC at PAD 21 in 20:1 ratio?

Mike