Hairball Audio

Re: [BUILD] FET/500 Official Support Thread
« Reply #460 on: October 08, 2019, 02:35:14 PM »
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
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Re: [BUILD] FET/500 Official Support Thread
« Reply #461 on: October 08, 2019, 09:39:49 PM »
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

Hairball Audio

Re: [BUILD] FET/500 Official Support Thread
« Reply #462 on: October 08, 2019, 09:42:12 PM »
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?
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Re: [BUILD] FET/500 Official Support Thread
« Reply #463 on: October 09, 2019, 12:25:48 AM »
R23 output pot confirmed as 250K

Re: [BUILD] FET/500 Official Support Thread
« Reply #464 on: October 09, 2019, 06:38:27 AM »
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.

Re: [BUILD] FET/500 Official Support Thread
« Reply #465 on: October 10, 2019, 02:28:04 AM »
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? 

Hairball Audio

Re: [BUILD] FET/500 Official Support Thread
« Reply #466 on: October 10, 2019, 03:06:29 PM »
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
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Re: [BUILD] FET/500 Official Support Thread
« Reply #467 on: October 10, 2019, 03:09:19 PM »
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
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Re: [BUILD] FET/500 Official Support Thread
« Reply #468 on: October 10, 2019, 05:16:00 PM »
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.

Hairball Audio

Re: [BUILD] FET/500 Official Support Thread
« Reply #469 on: October 10, 2019, 09:41:23 PM »
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.
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Re: [BUILD] FET/500 Official Support Thread
« Reply #470 on: October 11, 2019, 10:04:11 PM »
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
« Last Edit: October 11, 2019, 10:08:42 PM by rachansky »

Hairball Audio

Re: [BUILD] FET/500 Official Support Thread
« Reply #471 on: October 14, 2019, 09:41:16 PM »
-ve side of C7 is 0.104V

AC?

That super low.  Something is messed up in the first signal preamp stage.
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Re: [BUILD] FET/500 Official Support Thread
« Reply #472 on: October 15, 2019, 04:52:02 AM »
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.


Re: [BUILD] FET/500 Official Support Thread
« Reply #473 on: October 15, 2019, 04:52:52 AM »
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

Re: [BUILD] FET/500 Official Support Thread
« Reply #474 on: October 15, 2019, 06:11:07 AM »
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

Hairball Audio

Re: [BUILD] FET/500 Official Support Thread
« Reply #475 on: October 15, 2019, 02:18:46 PM »
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?
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Re: [BUILD] FET/500 Official Support Thread
« Reply #476 on: October 15, 2019, 02:27:13 PM »
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:



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Re: [BUILD] FET/500 Official Support Thread
« Reply #477 on: October 15, 2019, 02:27:45 PM »
And the meter board
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Re: [BUILD] FET/500 Official Support Thread
« Reply #478 on: October 18, 2019, 06:05:46 PM »
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

Re: [BUILD] FET/500 Official Support Thread
« Reply #479 on: October 21, 2019, 11:59:34 AM »
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