[BUILD] FET/500 Official Support Thread

[Echo North]

Sorry for the delayed response. Work has been brutal lately!

5.92V at C7 with the qbias rolled off completely ccw.

Wow that super hot.  Input set to mid?

 

[Echo North]

Hi,

I got a non functional fet500 A on the bench.
I could trace signal to the gate of q4, after that no  signal is coming through. Q4 seems to test ok, but i‘m not sure about that.
Does anybody have an idea?

Thanks, andi

Always use the + of D4 as your common reference.  With a 0dB signal and in and out mid way, what your VAC and C7?

Mike

 

[andia]

Always use the + of D4 as your common reference.  With a 0dB signal and in and out mid way, what your VAC and C7?

Mike

Hi mike,
thank you for your time.
I measure 4.59Vrms at C7.

Thank you again,
Andi

 

[Echo North]

Hi mike,
thank you for your time.
I measure 4.59Vrms at C7.

Thank you again,
Andi

What do you have at output tx brown wire?

 

[rachansky]

Wow that super hot.  Input set to mid?

Yes.  Everything as per your previous message.

 

[andia]

What do you have at output tx brown wire?

0.0Vrms  :-\

 

[Echo North]

Yes.  Everything as per your previous message.

Still nothing significant on output brown?

 

[Echo North]

0.0Vrms  :-\

Ya something is dead in that second amp stage that Q4 is in. 

 

[rachansky]

Still nothing significant on output brown?

Output brown vs. D4 as ground is reading -ve 13.55V DC

 

[rachansky]

Just to recap so you dont need to scroll back My output is super hot. I have good signal through the unit but when i get to the calibration of the meter bridge the ouput knob adjustment is so touchy I cant even dial it in to  .388V output. Additionally the output knob seems to max out within about 1/8 of its range ( call it 2 hours on a clock as a range reference ). I ordered a matched pair, the other unit is working perfectly as far as i can tell, components have been verified.

Thanks again for your help in this matter.

James

 

[Echo North]

Just to recap so you dont need to scroll back My output is super hot. I have good signal through the unit but when i get to the calibration of the meter bridge the ouput knob adjustment is so touchy I cant even dial it in to  .388V output. Additionally the output knob seems to max out within about 1/8 of its range ( call it 2 hours on a clock as a range reference ). I ordered a matched pair, the other unit is working perfectly as far as i can tell, components have been verified.

Thanks again for your help in this matter.

James

It's weird because your C7 voltage is high.  Makes me think what you're seeing at the output pot is just the result of your signal amp being too hot. Meaning the signal just before the pot is way too hot.

Let me look back...

 

[Echo North]

Input tx pad 1: I am not sure which one is pad one but on card connection side of the input transformer starting closest to the output tx I get 0V,  .072V, 0V and 0V

-ve side of C7 is 0.104V

output tx brown wire is varying in AC but in DC reads as 28.26V

Thanks very much for helping me out! I was almost ready to admit defeat and I hate that!

James

Looking back at this.  So I'm assuming the 0.072 VAC is the input secondary hot.  This voltage is perfect.

But you say you have 0.104 VAC at C7, so this has changed and is now like 5 VAC correct?

What was your actual AC measurement at output brown?

Mike

 

[rachansky]

Looking back at this.  So I'm assuming the 0.072 VAC is the input secondary hot.  This voltage is perfect.

But you say you have 0.104 VAC at C7, so this has changed and is now like 5 VAC correct?

What was your actual AC measurement at output brown?

Mike

My original measurement was made mid way through the calibration stage after I had adjusted the q bias and dialed back the output voltage to 2.45 VAC across pins 2 and 4.

Now with the q bias rolled all the way off my meter is ranging from zero to 19V AC or 31V DC between the brown and d4.  Between pins 2 and 4 on my test extender I am getting .025V AC.

 

[Echo North]

My original measurement was made mid way through the calibration stage after I had adjusted the q bias and dialed back the output voltage to 2.45 VAC across pins 2 and 4.

Now with the q bias rolled all the way off my meter is ranging from zero to 19V AC or 31V DC between the brown and d4.  Between pins 2 and 4 on my test extender I am getting .025V AC.

So that would mean you have no or very low output if you only have 0.025VAC at your output + and -.  I thought you said you were observing super high output?

 

[rachansky]

So that would mean you have no or very low output if you only have 0.025VAC at your output + and -.  I thought you said you were observing super high output?

Sorry  that last post should have read 0.25VAC.

Maybe higher output isnt the best description, the sensitivity of the output pot is all within maybe 1/8 of its range. So after the q bias  calibration when the output pot is used to cal the VU trim on the meter for example I am unable to come anywhere close to setting it to .388 VAC. If I simply touch that pot the output can go from 0 VAC to .7 VAC. The VU trim doesnt seem to have much of an effect on the signal either.

I hope this clears things up.  I'm thinking about mailing this thing off to you as we seem to just be going around in circles here.

 

[Echo North]

Sorry  that last post should have read 0.25VAC.

Maybe higher output isnt the best description, the sensitivity of the output pot is all within maybe 1/8 of its range. So after the q bias  calibration when the output pot is used to cal the VU trim on the meter for example I am unable to come anywhere close to setting it to .388 VAC. If I simply touch that pot the output can go from 0 VAC to .7 VAC. The VU trim doesnt seem to have much of an effect on the signal either.

I hope this clears things up.  I'm thinking about mailing this thing off to you as we seem to just be going around in circles here.

That's with the controls mid way and 0dB signal...GR off?

You should have like 6+ VAC on the output.

You can always email us if you want us to fix it.

Mike

 

[rachansky]

That's with the controls mid way and 0dB signal...GR off?

You should have like 6+ VAC on the output.

You can always email us if you want us to fix it.

Mike

Emailed.

 

[hasanova]

Hello,

I have built a matched set of Rev.F FET/500.
I'm in the process of calibration, but I have some issues.

Unit #1
STEP 1 Q bias went smoothly, but in  STEP 2 I cannot get 16dBu. Two LEDs stay lit (orange + red). VR1 has no effect. Pot meters are correct.
Same thing when I swap the meter PCBs.

Unit #2
STEP 1 I have too low output...

Where should I start looking?

---

BTW, I have succesfully built and calibrated a set of Sound Skulptur CP5176, a set of JLM LA500A, one  CAPI  FC526-XFMR and a set of DIYRE EQP5 passive EQ's.

 

[hasanova]

Next week when I return back, I will follow this guide and report back:

"Measure a few things in the unit.

Set your compressor controls:

Input: 12 O’Clock
Output: 12 O’Clock
Attack: Full CW
Release: Full CW
Ratio: 20:1
Meter: GR OFF
Signal: Sine 1kHz @ 0dBu (for this one voltage set your output using 0.775 VAC across input + and -)

With your DMM, measure AC at the following test points, using the anode (+) of D4 as your reference (black probe):

* Input transformer pad 1
* (-) neg side of C7
* Output transformer brown wire

Report back."

 

[hasanova]

I'm reporting back:

UNIT#1
* Input transformer pad 1
390mVAC

* (-) neg side of C7
0.5mVAC

* Output transformer brown wire
0.5mVAC


UNIT#2
* Input transformer pad 1
387mVAC

* (-) neg side of C7
0.5mVAC

* Output transformer brown wire
0.5mVAC

measured with a Fluke 179