[BUILD] FET/500 Official Support Thread

[boneindian]

Try the signal generator.  That will tell us a lot.

Ok, I'm not positive I set this up properly (I pumped 1khz into a signal tracer, read 100mv across the two leads, clipped them together and touched the probe to TP1) , but that gave me 153mv at TP15.  Please let me know if there's a better way to run this test, I have an oscillator on my console and also in a DAW (obviously) that I've been running to the XLR of the 500 series rack.

I couldn't think of a better way to inject the signal and have another hand free to test the test point.

 

[Echo North]

Ok, I'm not positive I set this up properly (I pumped 1khz into a signal tracer, read 100mv across the two leads, clipped them together and touched the probe to TP1) , but that gave me 153mv at TP15.  Please let me know if there's a better way to run this test, I have an oscillator on my console and also in a DAW (obviously) that I've been running to the XLR of the 500 series rack.

I couldn't think of a better way to inject the signal and have another hand free to test the test point.

That's not the way you want to do that.  The signal generator probes are either one for signal and one for ground or two differential signals.  Either way, connecting them kills the signal.  Clip the signal lead to TP1 and the ground/or differential lead to the audio ground at D4.

Mike

 

[boneindian]

That's not the way you want to do that.  The signal generator probes are either one for signal and one for ground or two differential signals.  Either way, connecting them kills the signal.  Clip the signal lead to TP1 and the ground/or differential lead to the audio ground at D4.

Mike

Only getting 17.2.mVAC at TP15 injecting signal at T1 with the ground lead of signal generator at D4

 

[Echo North]

Only getting 17.2.mVAC at TP15 injecting signal at T1 with the ground lead of signal generator at D4

What are you getting at each side of R5?

 

[boneindian]

What are you getting at each side of R5?

Negligible AC voltage on either side of R5.  A just a couple mVAC, reads the same with signal applied to TP1 as without.

As a result of forgetting to change my meter settings I also know there is circuit continuity between D4 and R5. For whatever that's worth

 

[Echo North]

Negligible AC voltage on either side of R5.  A just a couple mVAC, reads the same with signal applied to TP1 as without.

As a result of forgetting to change my meter settings I also know there is circuit continuity between D4 and R5. For whatever that's worth

That's no good and would be your issue.

Can you remove the input tx safely?  Then test the resistance to AGND (D4) on either side of R5. No power needed.

 

[boneindian]

That's no good and would be your issue.

Can you remove the input tx safely?  Then test the resistance to AGND (D4) on either side of R5. No power needed.

I can break out the desoldering pump, obviously not my preferred course of action (not the highest end desoldering equipment) but I can give it a go. 

Just to clarify for my own understanding, the continuity is the problem? or the lack of voltage or both?

 

[Echo North]

I can break out the desoldering pump, obviously not my preferred course of action (not the highest end desoldering equipment) but I can give it a go. 

Just to clarify for my own understanding, the continuity is the problem? or the lack of voltage or both?

I'm not there, so I'm troubleshooting through you, which is never ideal.  But what you're describing is a dead short to ground at you're input. Which is why you have no signal.

I'd like so see if that's happening at the input transformer, which is highly unlikely, but we should test it.  Could also be that there is too much solder in the input TX pads and it's causing on of the pins to short to the can of the transformer, which is grounded. 

Unfortunately, removing the input TX is the next logical troubleshooting step.

Mike

 

[boneindian]

I'm not there, so I'm troubleshooting through you, which is never ideal.  But what you're describing is a dead short to ground at you're input. Which is why you have no signal.

I'd like so see if that's happening at the input transformer, which is highly unlikely, but we should test it.  Could also be that there is too much solder in the input TX pads and it's causing on of the pins to short to the can of the transformer, which is grounded. 

Unfortunately, removing the input TX is the next logical troubleshooting step.

Mike

Ok, thanks Mike that makes a lot of sense.  I myself have been wondering if there was a short at that stage just because not a lot else makes sense....I also considered that it might be too much solder on the T-pad or Input TX  causing a short even though I expressly tried to avoid that during assembly.

I'll take out the Input TX and report back

 

[Echo North]

Ok, thanks Mike that makes a lot of sense.  I myself have been wondering if there was a short at that stage just because not a lot else makes sense....I also considered that it might be too much solder on the T-pad or Input TX  causing a short even though I expressly tried to avoid that during assembly.

I'll take out the Input TX and report back

When you get it out, also test the resistance in ohms between the 2 outside pins on each side of the transformer, and the metal can.

 

[boneindian]

When you get it out, also test the resistance in ohms between the 2 outside pins on each side of the transformer, and the metal can.

There definitely seems to have been a short...R5 to D4 anode now reads 266ohms on one side and 305ohms on the other.

The outside pins on either side have no continuity with ground, they read open.

Between the outside pins on each side (not sure if you wanted this too) they are 52ohms and about 13ohms respectively.

 

[boneindian]

There definitely seems to have been a short...R5 to D4 anode now reads 266ohms on one side and 305ohms on the other.

The outside pins on either side have no continuity with ground, they read open.

Between the outside pins on each side (not sure if you wanted this too) they are 52ohms and about 13ohms respectively.

Do these readings on the Input TX and board look good to you Mike?  Upon removal, one of the pins definitely had a blob of solder suspiciously close to the can...the other transformer looked clean, but I might have removed the offending short while desoldering.

I'm tempted to try just throwing them back in and being extra conservative with my solder but I suppose I'll wait to hear your thoughts in case further testing is in order.

Thanks again for all your help!

 

[Echo North]

There definitely seems to have been a short...R5 to D4 anode now reads 266ohms on one side and 305ohms on the other.

The outside pins on either side have no continuity with ground, they read open.

Between the outside pins on each side (not sure if you wanted this too) they are 52ohms and about 13ohms respectively.

Ya everything looks good with your transformer and circuit.

I would re solder it making sure...

1) you leave a gap (spacer) between the transformer can and PCB.
2) lightly solder

Report back.

Mike

 

[boneindian]

Ya everything looks good with your transformer and circuit.

I would re solder it making sure...

1) you leave a gap (spacer) between the transformer can and PCB.
2) lightly solder

Report back.

Mike

Unfortunately I'm right back in the same place.  Soldered the first Input TX  in (being sure to solder as lightly as possible) and found that the pad of R5 closest to the T-pad drops to 13.2ohms to D4 ground.  Just to be sure I put an extra strip between the spacer that came with the kit and the pcb on the second board but, after soldering that TX in and being incredibly light with the solder I'm getting the same reading on it.

Tried measuring voltage at the input with a .775VAC 1k signal and am getting the exact same drop when the unit is taken out of full bypass.

One thing that's been strange this whole time is that, although there's a weird drop and I can't get up to .775VAC measured on the input...the signal doesn't disappear completely, and I can still crank the input and output and get a not insignificant level on the output.

 

[Echo North]

Unfortunately I'm right back in the same place.  Soldered the first Input TX  in (being sure to solder as lightly as possible) and found that the pad of R5 closest to the T-pad drops to 13.2ohms to D4 ground.  Just to be sure I put an extra strip between the spacer that came with the kit and the pcb on the second board but, after soldering that TX in and being incredibly light with the solder I'm getting the same reading on it.

Tried measuring voltage at the input with a .775VAC 1k signal and am getting the exact same drop when the unit is taken out of full bypass.

The DC resistance of the secondary is roughly  13Ω and it's in parallel with a 270Ω resistor.  So having 13Ω there seems normal.  Transformers really "work" of AC so I'm not sure any of this is problematic. 13Ω isn't a dead short.

Let me check a working unit quick.

Mike

 

[Echo North]

Ya I get tha same.  Makes sense, 13Ω in parallel with 270Ω is about 13Ω.

I'm totally stumped.  The transformer windings test fine.  You never tested TP15 with the transformer removed and the signal inserted at TP1?

Mike

 

[Echo North]

Ya I get tha same.  Makes sense, 13Ω in parallel with 270Ω is about 13Ω.

I'm totally stumped.  The transformer windings test fine.  You never tested TP15 with the transformer removed and the signal inserted at TP1?

Mike

If you didn't.  Maybe try inserting a 2 VAC sine wave @ TP15 and see what you get at output transformer wire brown.  Output pot mid way.

Mike

 

[Echo North]

One last thing before I pass out.  You have the meter PCB attached right?  The circuit will never work unless the meter board is installed. It's not just a meter, it provides the negative bias for the circuit.

Mike

 

[boneindian]

Yep, meter board has been attached for any and all testing. Also responded as expected during earlier Qbias adjustments.

I had to head home for the evening but I’ll jump back into testing in the morning. Should I take one of the Input TX back out and try injecting at TP1 and measuring at TP15?

Or go straight to measuring the brown Output TX wire with signal applied at TP15?

This has been so strange, might be worth noting again here that signal does make it through the unit, the levels just seem to drop far lower than expected somewhere along the input  stage (as far as I can tell)

 

[Echo North]

Yep, meter board has been attached for any and all testing. Also responded as expected during earlier Qbias adjustments.

I had to head home for the evening but I’ll jump back into testing in the morning. Should I take one of the Input TX back out and try injecting at TP1 and measuring at TP15?

Or go straight to measuring the brown Output TX wire with signal applied at TP15?

This has been so strange, might be worth noting again here that signal does make it through the unit, the levels just seem to drop far lower than expected somewhere along the input  stage (as far as I can tell)

Try inserting a signal at TP15 and testing the output at output tx brown first, before removing the input tx.  That'll bypass the input and test to see if the line amp and your test setup is working.