L5 Preamp Compressor Circuit Issue

[jfurgason]

Hi all,

So I've been wrestling with the L5 preamp for a while and I have finally got it about 90% working, but the compressor is behaving super strangely.  It sounds super fuzzed out when it engages and is highly sensitive.  It will also sometimes trigger even when the switch is off.  I've checked all the voltages and only found two components out of spec with expected values.

IC11
1 - 0V
2 - 0V
3 - 0V
4 - -15.1V
5 - -14.9V
6 - -0.06V (should be 0.23V)
7 - 14.74V
8 - 0V

Q1
D - 14.97V (expected)
S - 0.05V (should be 0.02V)
G - -0.06V (should be -0.02)

I originally had bad 3080 chips in IC10 and IC11, but did replace them with chips from SmallBear a while ago. IC10 gives all the correct voltages so I don't think the problem is in the 3080 but I'm not sure where to look at this point. I've also tried switching out Q1, 2 and 3 (currently using a surface-mount 5457 with an adapter on Q1). I appreciate any insight anyone can offer on this one

Thanks!

[mauman]

Drew just built one, maybe ask over in this thread?  https://www.madbeanpedals.com/forum/index.php?topic=33813.0

[Drew Hallenbeck]

It's such a complex circuit and there are any number of things that could throw it's performance out of whack....
Did you follow the calibration instructions as far as injecting a 1kHz signal at the appropriate voltage and setting the distortion trimmer first?
Also, make sure you have the master volume all the way up and the compression at about 2/3 before adjusting the compression trimmer.

[jfurgason]

Yep, I did the distortion calibration and was seeing the correct AC voltage pin 6 on IC10. I can't seem to get the compressor trim to get the right value, at its maximum and minimum ends I'm seeing between 0.244 and 0.29 VAC.  I've reflowed what I can reach, rewired everything,  and rechecked all resistor values and remeasured all chips and transistors. Power supply is still good, getting the proper +/-15V at the test points. Here are any readings not in spec with what voltage is expected per Aion's voltage doc.

All DC voltages
IC 4 (I'm using OPA2134 instead of 4558)
1 = 0V (should be -0.02V)
2 = 0
3 = 0
4 = -15.17
5 = 0
6 = 0
7 = 0.009
8 = 14.96

IC 5 (OPA2134)
1 = 0 (should be 0.01)
2 = 0 (should be 0.01)
3 = 0 (should be 0.01)
4 = -15.17
5 = 0 (should be 0.01)
6 = -0.9 (should be 0)
7 = 0
8 = 14.96

IC6 (OPA2134)
1 = 0 (should be -0.05)
2 = 0
3 = 0
4 = -15.17
5 = 0
6 = 0
7 = 0 (should be -0.06)
8 = 14.96

IC7 (OPA2134)
1 = 0 (should be -0.04)
2 = 0
3 = 0
4 = -15.17
5 = 0
6 = 0
7 = 0 (should be -0.05)
8 = 14.96

IC8 (OPA2134)
1 = 0 (should be -0.03)
2 = 0
3 = 0
4 = -15.17
5 = 0
6 = 0
7 = 0 (should be -0.02)
8 = 14.96

IC11 (CA3080)
1 = 0
2 = 0
3 = 0
4 = -15.17
5 = -14.62
6 = -0.09 (should be 0.23)
7 = 14.96
8 = 0

IC12 (OPA2134)
1 = -0.003 (should be 0)
2 = 0
3 = 0
4 = -15.17
5 = 0
6 = 0
7 = 0.004 (should be 0)
8 = 14.96

IC13 (LM741)
1 = -15.16
2 = 0
3 = 0
4 = -15.16
5 = -15.16
6 = 0.008 (should be 0.02)
7 = 14.96
8 = 0

All other ICs are showing what is expected

Transistors
Q1 (MMBF5457)
D = 14.96
S = -0.08 (should be 0.02)
G = -0.1 (should be -0.02)

Q2 (2N3906)
1 = 0
2 = -0.006 (should be 0)
3 = 14.58

Q3 (MPSA13) these values all look correct
1 = -15.17
2 = -14.43
3 = -0.369

QX1
1 = 0.291 (should be 0.4)
2 = 0.792 (should be 0.9)
3 = 14.91

QX2
1 = -0.332 (should be -0.22)
2 = 0.29 (should be 0.4)
3 = 13.48

Tested on a 9VAC supply with the distortion trimmer properly calibrated. These were the values that jumped out to me, some of the discrepancies are probably within acceptable variance, but I'm including them just to be through. Maybe this helps point to a problem area? I might try subbing in 4558 chips for my 2134 chips and see if that does anything. I may also try new Q1-3 as well. Appreciate any help or insight on this one
Cheers

[jfurgason]

Just popping back in to the thread to say that I've resolved the issue and can finally take this one off the troubleshooting bench. I ended up replacing all my 2134 chips for 4558s as well as new Q1-3 and new 3080s. I wasn't very scientific and just replaced everything at once but it now works like a charm!

[gordo]

Doesn't matter how you get it there, as long as it works