[BUILD] 1176 Rev A - Back to the beginning...

[Echo North]

Do you get the same DC reading at Q1 with compression engaged?

I might have to poke around one tonight and compare with you.

 

[steve_h]

Ok - just tested. Same reading as before at the gate of Q11. The needle doesn't always peg, but the reading is consistent. Not sure what's going there - I'm not connecting the probe to any other area simultaneously that I can see.

As for the gate of Q1 (again compression engaged, fast release, 20:1 ratio, input/output about 12:00 in GR mode), I get a reading of -0.9V DC. Input signal measured at 0.72V AC. Needle doesn't budge! I'm letting the unit warm up for at least an hour - until the needle settles at 0 in GR mode anyway.

 

[Echo North]

I'm a bit lost on the issue. 

So you are trying to set the GR meter tracking?  Are you seeing the meter needle move now but not seeing actual GR on the output?

Thanks,

Mike

 

[steve_h]

The needle zeros out fine in GR mode, and registers input in +4/+8 modes - the issue specifically is that no gain reduction is registering no matter how the input/output is set and regardless of how strong the input signal is. In other words, the signal is reflected on the meter when monitoring input, but not when monitoring GR. It stays at 0 and doesn't dip left as expected.

Are the voltages I reported not unusual? I'm confident the first two steps were done properly, so the gain reduction FET should be slightly into conduction as desired. No adjustment of the gain reduction meter tracking trimmer appears to have an effect on gain reduction being reflected on the meter. Are there any other readings I can take and report that might help?

 

[Echo North]

Is your unit compressing?  Specifically apply a 0dBu signal to the input.  with GR off, set the input and output to 12 o'clock (ratio 20).  Measure AC across output pin 2 and 3.  Now turn GR on.  Do you get a big AC drop?  If not, increase the input and see f you can get a drop. 

I'm not sure if it's an issue of the GR not showing in the meter, or the unit is not compressing at all.  You gate voltage seems reasonable. so you should be geting GR.

 

[steve_h]

Ok, set signal (sine wave @ 1k) to read exactly 0.775VAC at pins 2 and 3 of input jack with all other controls set as you describe. Release is at 7, as any other setting moves the needle up towards +1 in GR mode. Is that normal?

With compression disengaged, output jack pins 2 and 3 measure 1.096VAC. With compression engaged and moved up to 7, meter reads 1.092VAC. Voltage actually increases at output pins when input is increased. I don't hear compression upon monitoring output in other testing.

 

[Echo North]

What are the GR and Meter FET DC voltages under the same test w/ GR on and off?

 

[Echo North]

Release is at 7, as any other setting moves the needle up towards +1 in GR mode. Is that normal?

Yes the release will move the needle a little.

 

[steve_h]

Thanks Mike. The readings are as follows:

Compression disengaged:

Q11:

G: -0.774 VDC
S: -0.608 VDC
D: 3.72 VDC

Q1:

G: -0.9 VDC
S: 0
D: 0 (0.1 mV actually)


Compression engaged (attack & release at 7, 20:1 ratio):

Q11:

G: -0.774 VDC (basically the same as when compression disengaged - multimeter reading fluctuates between -0.772 and -0.776 VDC)
S: -0.608 VDC
D: 3.82 VDC

Q1:

G: -0.89 VDC
S: 0
D: 0

Bad FET? I posted pictures earlier of the attack/release wiring as well as the GR Meter Driver section. I don't recall anyone mentioning any issues that they saw there, but I went through it all again against Mako's wiring guide and it all checks out. I've touched up all solder joints in the same section (haven't tried around Q1 - not sure if I did Q11 either, or just all other components in that area) as well as the attack/release connections and ratio board. All resistor values checked out as well. Extensive continuity tests don't show any unwanted connections, but the desired connections seem happy. Have traced connections on the board itself without any unusual discoveries.

I was careful and meticulous in putting this together (checked all component values before installing and double checked that they were placed in the proper positions) and am handy with an iron, so I'd think these things can be ruled out, but anything's possible. Don't think I overheated a transistor, but who knows.

 

[Echo North]

When I get back home tonight I'll fire up my A and post back.  Hang tight.  We'll get it working!

 

[Echo North]

I think it's something in your side chain (aka GR driver).  Go to the mnats site if you have a chance.  On the rev d page there is a schematic with voltages.  Check your GR transistors against that.  Should be the same or close to your A.

Mike

 

[steve_h]

To be clear, you're referring to Q11 through 13? Test with no input signal? Will post back and report measured vs expected voltages, just wanted to be clear on these points.

 

[Echo North]

Sorry 7-10.  I meant the control amp.

 

[Echo North]

I keep forgetting on the Rev A, the GR FET source is connected directly to ground (no resistor in between like later revisions).  This makes measuring the G bias tricky I believe.

Poking around...

 

[Echo North]

With a 0dBu signal, input and output @ 12 and GR on.  What is your AC voltage at the anode (no line) of CR2 and CR3?

Mike

 

[steve_h]

Thanks again - I did some testing of those transistors, and was getting some odd readings so I touched up the connections, adding a little bit of new solder to the mix. All connections look solid - here are the readings I'm getting now - no input signal, same settings as before but compression off. Still not quite right, but hopefully helpful:

Q7:

B: 5.29 VDC
C: 16.9 VDC
E: 4.78 VDC


Q8:

B: 16.89 VDC
C: 29.9 VDC
E: 16.28 VDC

Q9:

B: 3.69 VDC
C: 14.94 VDC
E: 3.13 VDC

Q10:

B: 14.94 VDC
C: 29.9 VDC
E: 14.37 VDC

It seems that some voltages are missing from the Rev D schematic. Also of note is that my Q7-10 are 2N3708's as that's what Mouser had. Not expecting this to be important, but just for the sake of providing details. I do have 1 spare - they're not socketed unfortunately.

EDIT:

CR2 anode - 1.363VAC
CR3 anode - 1.359VAC

 

[Echo North]

All of that looks reasonable.  So you control amp looks ok.  Poking around...

Your meter doesn't show compression and your unit is not compressing.  Doubt you have two bad FETs.  Looking at the attack release network....

 

[steve_h]

I thought maybe a faulty attack pot, so I just tested it and it read 23.7k and measured as expected as the knob was turned/resistance increased. Supposed to be 25k of course, so that seems ok..

 

[Echo North]

Were those Anode reading with the 20 ratio enabled?  That seems a little low for 20 but you should still be getting compression.

 

[steve_h]

Yep - haven't deviated from 20:1 ratio in all of this testing, except to see if changing the ratio had a different effect, which has not been the case.