1176 best low noise build

[Spencerleehorton]

I’ve opted to use some 24v bulbs I had I didn’t know how much current to give them but experimented and ended up liking 27ma at around 18v seemed to light them up nicely.
I wanted to use 3 of them, 2 for both VU meters and one for a front panel jewel light.
So using the 180 ohm 10W resistor with 37v off the diode bridge gives me what I want.
I have some 22k switched pots for the attack pot but the switch is disconnected in the CCW position, can’t seem to figure it out with the switch this way??

 

[Spencerleehorton]

Channel 1 isn’t playing ball, for some reason signal is only getting to q1 drain and no further? I’ve put in a brand new 2n5457 and checked the pin out?
Will try another fet but I’ve also changed q2,q3 and q4 and used scope but no signal through?
I’ve checked all resistors and checked all continuity through the first and second stage.
Also checked all voltages and all voltages are fine?

 

[Spencerleehorton]

Also one of the vu meters is sticky?
And since putting in the bulbs ch2 has got noisy so I’m thinking of changing the vu meters.View attachment IMG_0732.jpeg

 

[Spencerleehorton]

Also now thinking of going with a separate psu to feed both channels due to the noise?

 

[Spencerleehorton]

Have changed the Q10 to 2n5458 in channel 1 and now get small signal to q4 and see very low signal on output.
So then changed Q6 and Q7, no change.
Will now change Q8 and Q9 and see if anything changes.

 

[Spencerleehorton]

I’ve built a LTspice model of the 1176 now so this should help me track down the problem.

 

[Spencerleehorton]

Have changed Q8 and Q9 but still no change?
Also changed the NE5532.
so signal still not getting past Q2 collector?
Have tried to input signal further in circuit to see if line amp works but also no signal gets through?
Hope to make a break through today!!

 

[Spencerleehorton]

what i have noticed both on my LTspice model of the 1176 and on my pcbs, if i change r17 from 6.8k to 3k i get much better voltages which align with the G1176 schematic, setting both input and output to middle positions i get clean waveform, 12v positive side and 12v negative side.
there maybe some difference as ive made the meter circuit using the transistors rather than the TL071 IC.
im going to check today.
this is all refernce for me for the future as within the other threads i cant seem to find any answers.

 

[Spencerleehorton]

Well what a frustrating day!
Have got ch1 now to a stage where it is passing audio and signs of compression but the level is quite low and GR has a problem where when I select GR it cuts about 5-6v of voltage on the 30v rail?
It seems to be the meter circuit at fault and I’ve tracked it down to the R58 100k resistor which in circuit measures either 45k or 25k depending on which way you toggle the tracking trimmer?
On channel 2 the tracking trimmer does not change the resistance of R58, it just stays at 100k!!!
I’ve been searching all day for what is causing this and not found it!!
Will continue tomorrow, all voltages are now within what they should be apart from channel 1 meter section.
Pin 2 voltage and pin 6 voltage on the TL071 are off, I don’t have the IC in for this problem to happen?