[BUILD] FET/500 Official Support Thread

[Echo North]

Mike,
Yes the ICs are correct, 71 on the main PCB and two 72s on the meter board. Everything looks like it's oriented correctly and I'm not seeing damaged diodes. When I got them there was a lot of flux on the PCB (bought them already assembled), so I thoroughly cleaned them up when I noticed something wasn't right with the GR meter. What's weird is that before cleaning them up, only 1 unit of the 2 had the bar meter issue, after cleaning they now both have it.

So on the bar/dot jompers by the 16 pin ICs you have nothing installed there correct?  Just three empty holes?

Mike

 

[blueberryjam]

So on the bar/dot jompers by the 16 pin ICs you have nothing installed there correct?  Just three empty holes?

Mike

yes that's correct

 

[Echo North]

yes that's correct

Is there still a ton of goopy flux on it?  Sometimes that stuff can be conductive.  how clean where you able to get it?

 

[rachansky]

R23 output pot confirmed as 250K

 

[blueberryjam]

Is there still a ton of goopy flux on it?  Sometimes that stuff can be conductive.  how clean where you able to get it?

it's pretty pretty clean now, I don't think I could make it much cleaner.

 

[rachansky]

R23 output pot confirmed as 250K

Ok so next observation. When I rotate the input pot (bourns t-pad) my output only changes by about 0.3V.  I have reflowed the solder on the t-pad, input transformer and everything up until Q1 with no affect. My max qbias reading is still 17V+.

Any chance it might be the bourns pot? Are you able to send a replacement? 

 

[Echo North]

Ok so next observation. When I rotate the input pot (bourns t-pad) my output only changes by about 0.3V.  I have reflowed the solder on the t-pad, input transformer and everything up until Q1 with no affect. My max qbias reading is still 17V+.

Any chance it might be the bourns pot? Are you able to send a replacement?

Doubtful.  You have +17VAC on the output?  What are you using as your black probe reference?

Mike

 

[Echo North]

it's pretty pretty clean now, I don't think I could make it much cleaner.

Frustrating because I had a repair in like this, but with all the repairs we do I can't remember what it was.  That said your issue could be completely different.

I don't have a lot of reference voltages for those units.  The GR portion of that board is actually pretty small.  Just a few components.  I doubt this will help, but can you maybe post a top/bottom images of that board?

Mike

 

[rachansky]

Doubtful.  You have +17VAC on the output?  What are you using as your black probe reference?

Mike

Pins 8 and 10 from a soundskulptor 500 series extender.

 

[Echo North]

Pins 8 and 10 from a soundskulptor 500 series extender.

Measure a few things in the unit.  For every measurement I ask you moving forward, use the anode (+) of D4 as your black probe common reference.

Set your compressor controls:

Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR OFF
Release: Full CW
Ratio: 20:1
Meter: GR
Signal: Sine 1kHz @ 0dBu (OK i lied, 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 tx pad 1
- neg side of C7
-Output tx brown wire

Report back.

 

[rachansky]

Measure a few things in the unit.  For every measurement I ask you moving forward, use the anode (+) of D4 as your black probe common reference.

Set your compressor controls:

Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR OFF
Release: Full CW
Ratio: 20:1
Meter: GR
Signal: Sine 1kHz @ 0dBu (OK i lied, 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 tx pad 1
- neg side of C7
-Output tx brown wire

Report back.

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

 

[Echo North]

-ve side of C7 is 0.104V

AC?

That super low.  Something is messed up in the first signal preamp stage.

 

[blueberryjam]

Frustrating because I had a repair in like this, but with all the repairs we do I can't remember what it was.  That said your issue could be completely different.

I don't have a lot of reference voltages for those units.  The GR portion of that board is actually pretty small.  Just a few components.  I doubt this will help, but can you maybe post a top/bottom images of that board?

Mike

Thanks for the help Mike, here's the top  of one of the meter boards.

 

[blueberryjam]

Frustrating because I had a repair in like this, but with all the repairs we do I can't remember what it was.  That said your issue could be completely different.

I don't have a lot of reference voltages for those units.  The GR portion of that board is actually pretty small.  Just a few components.  I doubt this will help, but can you maybe post a top/bottom images of that board?

Mike

and the bottom

 

[rachansky]

AC?

That super low.  Something is messed up in the first signal preamp stage.

Yes AC. Can you please specify where the first signal preamp stage is on the mnats drawing that is linked through your site? I am not very well schooled in circuit analysis.

Thanks

 

[Echo North]

Yes AC. Can you please specify where the first signal preamp stage is on the mnats drawing that is linked through your site? I am not very well schooled in circuit analysis.

Thanks

First, do you have your QBias rotated fully counterclock wise?  Listen for the click.  Still getting the same 0.1 VAC at C7?

 

[Echo North]

and the bottom

Look good.

I'm super stumped.  You could send one in for repair and we can sort it and let you know what it is.  It's weird that it's two units.  Keep in mind that everything around the IC on the main PCB is part of that circuit. I know you already confirmed it's a TL071 (not 72) and oriented correctly.

If you look at the schematic, there is not a ton in that GR circuit. Basically just this stuff:

 

[Echo North]

And the meter board

 

[rachansky]

First, do you have your QBias rotated fully counterclock wise?  Listen for the click.  Still getting the same 0.1 VAC at C7?

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

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

 

[andia]

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