[BUILD] 1176LN Rev D DIY

[jefepeters]

Can you increase your signal so you see about 0.775 VAC between pin 2 and 3 of the input XLR?

Mike

Yes.  When I do that I get the following:

TP1  : 0.078 VAC
TP15  : 1.538 VAC
TP17  : 0.179 VAC
OUTPUT: 3.302 VAC

I'm not sure what I was doing wrong before, but I can now pass step 1 (QBias ranges from -2.964 to -0.001 VAC).

 

[Echo North]

Yes.  When I do that I get the following:

TP1  : 0.078 VAC
TP15  : 1.538 VAC
TP17  : 0.179 VAC
OUTPUT: 3.302 VAC

I'm not sure what I was doing wrong before, but I can now pass step 1 (QBias ranges from -2.964 to -0.001 VAC).

That's good.

Your output is really low. Is that with GR OFF?

 

[jefepeters]

That's good.

Your output is really low. Is that with GR OFF?

Yes, it is.

 

[Echo North]

Yes, it is.

Is that out + referencing ground or your voltage between pin 2 and 3 of the output? What is your AC voltage between pin 2 and 3 of the output?

 

[jefepeters]

Is that out + referencing ground or your voltage between pin 2 and 3 of the output? What is your AC voltage between pin 2 and 3 of the output?

The voltage across output pins 2 and 3 is 3.167 VAC, just slightly less than across the brown transformer wire and chassis.

 

[Echo North]

The voltage across output pins 2 and 3 is 3.167 VAC, just slightly less than across the brown transformer wire and chassis.

You should check your DC voltages in the signal and line amp:

http://mnats.net/files/1176REVD_VOLTS.pdf

Mike

 

[jefepeters]

You should check your DC voltages in the signal and line amp:

http://mnats.net/files/1176REVD_VOLTS.pdf

Mike

Mike,

Q2-Q6 and Q14 all test within 10% of those specs.

For the remaining transistors, the collectors for Q12 and Q13 are  higher than %10,  testing at 16.98 and 16.97 VDC.

Thanks,
Justin

 

[Echo North]

Mike,

Q2-Q6 and Q14 all test within 10% of those specs.

For the remaining transistors, the collectors for Q12 and Q13 are  higher than %10,  testing at 16.98 and 16.97 VDC.

Thanks,
Justin

Those need to be calibrated.  How far did you get into calibration?

 

[jefepeters]

Those need to be calibrated.  How far did you get into calibration?

I never got beyond the Q-bias adjust, since I couldn't get the VU meter to drop 1dB

 

[Echo North]

I never got beyond the Q-bias adjust, since I couldn't get the VU meter to drop 1dB

What is your DC V at the Gate of Q1 as you rotate the bias?

Mike

 

[jefepeters]

What is your DC V at the Gate of Q1 as you rotate the bias?

Mike

OK, I am able get the 1dB drop now when adjusting R59. When R59 is fully CCW, I get -2.43 VDC on Q1. When  I reach the 1dB drop point, Q1 measures -0.97 VDC.

I was able to complete the alignment procedure and confirm that it is compressing, but only when the input it turned past 12 or so.  I re-did the test on the amplification stages and got the following:

TP1  : 0.67 VAC
TP15 : 1.40 VAC
TP17 : 0.16 VAC
BRN  : 2.98 VAC
XLR  : 2.97 VAC

 

[Echo North]

OK, I am able get the 1dB drop now when adjusting R59. When R59 is fully CCW, I get -2.43 VDC on Q1. When  I reach the 1dB drop point, Q1 measures -0.97 VDC.

I was able to complete the alignment procedure and confirm that it is compressing, but only when the input it turned past 12 or so.  I re-did the test on the amplification stages and got the following:

TP1  : 0.67 VAC
TP15 : 1.40 VAC
TP17 : 0.16 VAC
BRN  : 2.98 VAC
XLR  : 2.97 VAC

This is with GR OFF? 

When you have GR OFF. Is pad 22 shorted to ground?

Mike

 

[jefepeters]

This is with GR OFF? 

When you have GR OFF. Is pad 22 shorted to ground?

Mike

Yes, GR is off. When switched to off, pad 22 does short to ground.

 

[Echo North]

Yes, GR is off. When switched to off, pad 22 does short to ground.

Have you checked your TP21 DC V voltages?
With 0.775 AC V between pin 2 and 3 of the input, how high to you need to turn up the input to get 2.25 VAC at pad 15?  GR OFF.

Mike

 

[jefepeters]

Have you checked your TP21 DC V voltages?
With 0.775 AC V between pin 2 and 3 of the input, how high to you need to turn up the input to get 2.25 VAC at pad 15?  GR OFF.

Mike

DC voltages at TP21 are:

20:1  -6.66
12:1  -3.61
8:1  -2.44
4:1  -1.51

Starting with 0.775 VAC on input, set at 24 (12 o'clock), I need to turn it to about 19 to get 2.25 VAC at pad TP15

 

[Echo North]

DC voltages at TP21 are:

20:1  -6.66
12:1  -3.61
8:1  -2.44
4:1  -1.51

Starting with 0.775 VAC on input, set at 24 (12 o'clock), I need to turn it to about 19 to get 2.25 VAC at pad TP15

Ah so only one dot? That's good.  What is your output pin 2 and 3 reading in VAC at that point?

Mike

 

[jefepeters]

Ah so only one dot? That's good.  What is your output pin 2 and 3 reading in VAC at that point?

Mike

So after powering down and doing the measurement again, 2.25 VAC now happens at almost 3 o'clock (dot inbetween 18 and 12). At this point the output is 4.87 VAC

 

[Echo North]

So after powering down and doing the measurement again, 2.25 VAC now happens at almost 3 o'clock (dot inbetween 18 and 12). At this point the output is 4.87 VAC

How clean is the board? Are you confident in your soldering?  I ask because usually things like that don't change.

I'm pretty stumped. Your output seems really now.

Did we try carefully removing Q1 and seeing if that increased your output?

We also do flat rate repair if that interest you.

Mike

 

[jefepeters]

How clean is the board? Are you confident in your soldering?  I ask because usually things like that don't change.

I'm pretty stumped. Your output seems really now.

Did we try carefully removing Q1 and seeing if that increased your output?

We also do flat rate repair if that interest you.

Mike

I've retested the power supply and am now getting -8.9 VDC at CR6 instead of the the -9.9 VDC I measured both when I originally completed the power supply and also when I began troubleshooting a week or so ago. The meter in GR mode, previously aligned for 0dB, is now resting at -2dB.  Are there any likely culprits for this change in the power supply?

My soldering skills certainly aren't perfect, but in all the kits and repairs I've done over the years I haven't had problems. The board has been cleaned up with isopropyl alcohol.

-Justin

 

[jefepeters]

Did we try carefully removing Q1 and seeing if that increased your output?

I just tried this and it made no difference; still measuring 0.07 VDC at TP1.

But... R4 is measuring 12.5Ω instead of 270Ω. The color coding is the same as R41, so I don't think I swapped resistors with another one somewhere in the circuit. Guess I missed it when originally checking the resistor values.