[BUILD] New FET/RACK Official Help Thread - Please read first post!

[Echo North]

Getting

0.032 VAC


Q1 is still removed (per your recommendation ) btw

Do you have all 3 L-Brackets installed on the back panel? Fully screwed in, tightened?

Mike

 

[Echo North]

If I had to guess based on this. Either your T-Pad is bad (very rare) or your input tx is bad (kinda rare).  Make sure all of the solder joints on those two parts look good. Look for any lifted pads or damaged traces on either of those parts. Make sure all 3 l-brackets are secured to the PCB and enclosure (one of those supplies common to the PCB).

If you're still having an issue, measure the input primary and secondary coils with your DMM Ω setting. You can do this in the PCB with no signal, unit powered off. MAKE SURE the input pot is mid way.

The pins are like this:

1          8
2          7
3          6
4          5

What is the Ω between:

1 and 4
5 and 8
1 and 6
4 and 6
5 and 6
8 and 6

Mike

 

[RR25]

Try this:

Input/output: Mid Way
release/attack: full CW
Ratio: 20
Meter: GR

Feed and confirm a 0.775VAC 1K signal measuring between input XLR + and -.

No measure these TPs using chassis as your common.

TP22 - Measure for AC for all ratios and list them.
TP21 - Measure for DC for all ratios and list them.

Mike

Hi Mike,

I have measured the DC voltage at TP21 and the AC voltage at TP22 for all ratios with a 0.775V AC signal (measured at input XLR + and -)

These are my results (settings as stated above and using chassis as common):

TP21
4:1 = -1.502V DC
8:1 = -2.333V DC
12:1 = -3.167V DC
20:1 = -5.825V DC

TP22
4:1 = 0.006V AC
8:1 = 0.013V AC
12:1 = 0.021V AC
20:1 = 0.039V AC

Thanks again for all the help.

 

[mixmasta]

Ok, I just finished the build of 2 Rev A kits paying a lot of attention to get everything right in the first place, but it seems both units have a/some problems in different spots:

The +30VDC and -10VDC from the power supply measure correctly on both units.

#1:
I fail at the first calibration step for the Q gain. There is signal coming through the unit, but it seems way too low in level. Sending in 1k@ 0dBu results in about -15dBu at the output XLR with input and output pots at 24,  GR off and 20:1 selected. If I crank the output pot all the way, I get just about 0dBu at the output and the only way to get it to +11dBu is to turn both input and output up all the way. I suspect I am losing 6 or 12dB of signal somewhere along the way. Where should I look, or what points to check along to way to find the problem area?

#2:
The first time I powered it on, the output level started to rise over the course of 10 to 20 seconds from about 0 to over 20dBu when I quickly switched it off again. Now the GR meter is just pegged from the beginning when I switch the unit on, even with no input signal going in and on 20:1 and with GR turned off. I noticed that touching/measuring the gate on Q11 brings the meter down but it goes back up all the way as soon as I let go.

I did notice that R17 in the kits was 4.7k but on the Rev A 1.2.5 Voltage sheet it is specified as 8.2k but that might not have anything to do with those issues.

Many thanks for your help!

 

[Echo North]

@RR25
Your TP22 is real low. What is your V AC at pad 15 under the same conditions?

@mixmasta
#1 Does adjusting the Qbias to either extreme change the output?

#2 Contact with the gate at Q1/11 just sends the unit into extreme gain reduction. Have you tried calibrating it yet? Make sure all of the L-Brackets are secured on both units. One of them ties the PCB ground to the chassis ground and is needed for proper operation.

R17 value is not the issue. The new value is correct.

Mike

 

[mixmasta]

@mixmasta
#1 Does adjusting the Qbias to either extreme change the output?

#2 Contact with the gate at Q1/11 just sends the unit into extreme gain reduction. Have you tried calibrating it yet? Make sure all of the L-Brackets are secured on both units. One of them ties the PCB ground to the chassis ground and is needed for proper operation.

R17 value is not the issue. The new value is correct.

Mike

Hi Mike,

Thank you for the quick replies!

#1: No, output stays around -15.6dBu over the whole range of the Q Bias trim

#2: Just tried calibrating it, and it seems fine! I must have been panicky about the +20dB output level in relation to what I got with the first unit.

 

[Echo North]

For the low output unit, I would start by going over all of your resistors and visually checking the values.

Looks for anything out of place in comparison to the working unit. Look for any suspect solder joints.

Mike

 

[mixmasta]

For the low output unit, I would start by going over all of your resistors and visually checking the values.

Looks for anything out of place in comparison to the working unit. Look for any suspect solder joints.

Mike

I compared all the resistors and the color codes are the same. Also had a look at the bottom side and it does look clean, no shorts or forgotten solder pads that I noticed. Did touch up a couple of solder joints but it did not make a difference in the levels coming through the unit. Also checked the transistors and small film caps to make sure I didn't mix up something there.

I got 0.775VAC at the input but don't know what the values should be later on as I haven't fully figured out the schematic yet. But I compared some points that I could identify being in the audio path between the 2 units, and levels seem to be down throughout the unit.

With input/output at 24, 20:1 and GR off, on the functioning unit #2 one I got:
TP15: 3.746 VAC
TP17: 0.6 VAC
at R8: 0.087 VAC
at R5: 0.086 VAC

The values for #1 were:
TP15: 0.5 VAC
TP17: 0.15 VAC
R8: 0.012 VAC
R5: 0.011 VAC

 

[CBarx]

Ok, so I hate to have to come here for this, but here we go.  I've got 6 Rev A v1.12s on the bench being built right now.  I just wrapped up two of them and began calibration.  Unit one started showing issues immediately.  Upon power up, the meter never moved from left position.  I attempted step one of calibration to find that I could not get the output to push beyond 1.04VAC. 

So being unable to progress beyond step one, I hooked the other completed unit up to the oscillator and immediately noticed a much hotter signal.  THAT unit calibrated just fine all the way through. 

So back to unit one.  After some reading and (most likely unnecessarily) reflowing some solder joints, I'm here with the same problem.  Strangely enough, even though I tested the power supply voltages of all PCBs before populating the rest of the components (All voltages checked out fine THEN), I'm now seeing that the left lead of CR9 is giving me a reading of -1.5mVDC.  The 30VDC test point check out fine.  I'm at a loss here.  I've gone over components and can't find any mistakes, and I populated all 6 boards at the same time, so if there was an issue with one, there'd be an issue with all of them.  Any help here would be greatly appreciated. 

CB

 

[mnats]

Strangely enough, even though I tested the power supply voltages of all PCBs before populating the rest of the components (All voltages checked out fine THEN), I'm now seeing that the left lead of CR9 is giving me a reading of -1.5mVDC.  The 30VDC test point check out fine.  I'm at a loss here.  I've gone over components and can't find any mistakes, and I populated all 6 boards at the same time, so if there was an issue with one, there'd be an issue with all of them.  Any help here would be greatly appreciated. 

If power supply voltages were correct before populating the rest of the board then something must be dragging the -10 v supply down. Set your multimeter to measure resistance and measure between the anode of CR9 and ground. Remove CN3 and try again to see if your ratio circuit is causing an issue.

If the resistance is still low after you have removed CN3 then start checking the GR Meter Driver circuit for shorts or incorrect component values. If the resistance jumps higher after removing CN3 then check your ratio circuit for same.

 

[Echo North]

I compared all the resistors and the color codes are the same. Also had a look at the bottom side and it does look clean, no shorts or forgotten solder pads that I noticed. Did touch up a couple of solder joints but it did not make a difference in the levels coming through the unit. Also checked the transistors and small film caps to make sure I didn't mix up something there.

I got 0.775VAC at the input but don't know what the values should be later on as I haven't fully figured out the schematic yet. But I compared some points that I could identify being in the audio path between the 2 units, and levels seem to be down throughout the unit.

With input/output at 24, 20:1 and GR off, on the functioning unit #2 one I got:
TP15: 3.746 VAC
TP17: 0.6 VAC
at R8: 0.087 VAC
at R5: 0.086 VAC

The values for #1 were:
TP15: 0.5 VAC
TP17: 0.15 VAC
R8: 0.012 VAC
R5: 0.011 VAC

The low voltage at TP15 indicates something is bad in the first gain section. Does removing the Q1 FET help get voltage at 15?

 

[mixmasta]

The low voltage at TP15 indicates something is bad in the first gain section. Does removing the Q1 FET help get voltage at 15?

Yes, when I take it out I get +3.9VAC at TP15 and +18dBu at the output. Does that indicate a bad Q1 or something else in that gain stage?

 

[RR25]

@RR25
Your TP22 is real low. What is your V AC at pad 15 under the same conditions?

Mike

Hi Mike, I have tested TP15 and I got a reading of 0.072V AC (0.775V AC at input test points, referencing the chassis, Input and output = mid, 20:1, meter = GR, attack and release = CW)

Thanks for the help Mike,

 

[Echo North]

Hi Mike, I have tested TP15 and I got a reading of 0.072V AC (0.775V AC at input test points, referencing the chassis, Input and output = mid, 20:1, meter = GR, attack and release = CW)

Thanks for the help Mike,

This is with GR bypassed? You have something wrong in your first stage labeled "signal preamp".

Mike

 

[Echo North]

Yes, when I take it out I get +3.9VAC at TP15 and +18dBu at the output. Does that indicate a bad Q1 or something else in that gain stage?

It indicates a bad FET or bad qbias circuit.

With the FET out, what is your DC at the GATE pad of Q1 as you rotate the qbias from end to end.

Mike

 

[mixmasta]

It indicates a bad FET or bad qbias circuit.

With the FET out, what is your DC at the GATE pad of Q1 as you rotate the qbias from end to end.

Mike

I have 0VDC at the gate of Q1 for the whole trim range, it must be the Q Bias circuit then.

 

[Echo North]

I have 0VDC at the gate of Q1 for the whole trim range, it must be the Q Bias circuit then.

Ya on the rev A that should have something (roughly) like -0.5 to -1.5 VDC.

What is your DC range at pad 18 rotating the pot?

Make sure R60, R83, and the trimmer R59 are all the correct value and properly soldered.

Mike

 

[mnats]

Hi Mike, I have tested TP15 and I got a reading of 0.072V AC (0.775V AC at input test points, referencing the chassis, Input and output = mid, 20:1, meter = GR, attack and release = CW)

Thanks for the help Mike,

This is with GR bypassed? You have something wrong in your first stage labeled "signal preamp".

Mike

As mentioned in my earlier reply regarding your issue, Mike's analysis agrees with mine. If you are building an "A" then your transistor voltages are wrong; check all components in that stage for value, soldering, shorts etc.

 

[ccommon]

Hey Mike , 

I Finished the Build on my second REV A , and things seem to go smooth untill i tried to CAL it.

The R59 Q Bias would not Drop the 1 db or down to 2.44V on the  ,  but the  meter on the REV A would drop all the way down as I adjusted. 

I check around and  looked for anyhing out of place or any cold solders , But all seemed fine .  The last thing I check was the sound of the output .  With a 1K tone @ 0 db Passing through  , the output of the unit was distorted even with the output pot all the way down. 

what could I have messed up this time haha .

let me know if you need more info

thanks for the support

-Chris

 

[Echo North]

Hey Mike , 

I Finished the Build on my second REV A , and things seem to go smooth untill i tried to CAL it.

The R59 Q Bias would not Drop the 1 db or down to 2.44V on the  ,  but the  meter on the REV A would drop all the way down as I adjusted. 

I check around and  looked for anyhing out of place or any cold solders , But all seemed fine .  The last thing I check was the sound of the output .  With a 1K tone @ 0 db Passing through  , the output of the unit was distorted even with the output pot all the way down. 

what could I have messed up this time haha .

let me know if you need more info

thanks for the support

-Chris

Calibration has to be done in order. If you can't get the qbias set, you need to stop there and figure that out before moving on.

What is your DC at PAD 18 at each extreme of the qbias pot?

Mike