GSSL HELP THREAD!!!

[Pat Maki]

@Script - TL072 orientation is OK, but I wonder if it may have been fried when i originally fried the 2 5534's in the VCAs (due to installing them in reverse....they were immediately replaced and installed correctly). Regarding the DC testing...with a sine wave going though the GSSL, I put the black DMM probe on ground (where transformer connects to main PCB) and the red DMM probe on the - (negative) pad that the threshold attaches to. That reading is 11.25VDC....doesn't change when the threshold knob is moved. On the + threshold pad, the reading is -12VDC and doesn't change with knob movement either. I then put the red DMM probe on the 3 wiper prongs on the ratio switch that are in the circuit. Those changed slightly when moving between the three ratio positions. The are (prong 1: -5.7V, -9.3V, 0V), (prong 2: -2V, 0, -2V), (prong 3: 0V, -116mV, -106mV). Thoughts? Thank-you very much!

 

[Script]

You sure you have connectivity between point "C" and pole of ATTack rotary when flipping bypass switch ?

And does DC change on THR pot wiper when turning ? (I've seen pots die when applying too much heat and/or solder.)

 

[Pat Maki]

You sure you have connectivity between point "C" and pole of ATTack rotary when flipping bypass switch ?

And does DC change on THR pot wiper when turning ? (I've seen pots die when applying too much heat and/or solder.)

Yes, there is continuity between point "C" and Pole (A) of Attack switch when bypass is disengaged. When bypass is engaged, continuity is broken (as expected I suppose).
Yes, DC changes on THR pot wiper when turning. Although it behaves strange. When pot turned all the way to the left (counterclockwise), DCV is -12.25, as i rotate the pot clockwise, DCV increases gradually. When the pot is rotated nearly all the way to the right, DCV goes from -8VDC to 0 very quickly....not gradual. Maybe this is just this type of pot or maybe there's an issue with it. Either way, seems like functions generally ok?

 

[Script]

In schematics should have a CW or CCW written next to the pot for how to wire it. Also schematic tells you whether you need a Lin or a Log pot. I don't remember now but it says somewhere.

If all good, while turning THR knob, could then also measure DC following the signal through the sidechain (to TL072 IN and OUT, then to SC VCA (pin 3 Ec+, I think). All good ? Or fried 072 ? Finally could measure AC on VCA output (pin 8) when sending music through the unit. Does AC reading change on pin 8 when turning THR knob ?

 

[Pat Maki]

In schematics should have a CW or CCW written next to the pot for how to wire it. Also schematic tells you whether you need a Lin or a Log pot. I don't remember now but it says somewhere.

If all good, while turning THR knob, could then also measure DC following the signal through the sidechain (to TL072 IN and OUT, then to SC VCA (pin 3 Ec+, I think). All good ? Or fried 072 ? Finally could measure AC on VCA output (pin 8) when sending music through the unit. Does AC reading change on pin 8 when turning THR knob ?

The Pot is a 50k lin per the schemo, so I think it's ok. On the 072, I get -12VDC on ground (pin 4) and +12VDC on Vcc (pin 5), but no readings on ins and outs except for pin 2 (in 1 -), it goes from 3.1VDC to 2VDC when turning THR. Will check the VCA later today. Thank-you!

 

[Pat Maki]

In schematics should have a CW or CCW written next to the pot for how to wire it. Also schematic tells you whether you need a Lin or a Log pot. I don't remember now but it says somewhere.

If all good, while turning THR knob, could then also measure DC following the signal through the sidechain (to TL072 IN and OUT, then to SC VCA (pin 3 Ec+, I think). All good ? Or fried 072 ? Finally could measure AC on VCA output (pin 8) when sending music through the unit. Does AC reading change on pin 8 when turning THR knob ?

Well I'm not getting VAC on pin 8 of the side chain VCA. Would do you suppose that indicates? Thx!

 

[Script]

You have AC on VCA IN ?

Any chance you had the VCA installed with wrong orientation and then powered ? If so, likely fried.

Checked for shorted solder joints around VCA ? Connectivity ?

 

[Pat Maki]

I want to be sure we're talking about the same component. Is the VCA in the pic what you're referring to? Orientation is correct. 0V AC on pin 8. The reading one pin 1 dances a little with music playing through it. Also ran a sine wave through it and got a reading of .55V AC when THR turned CCW and .68V AC when THR rotated CW.

 

[Script]

Replacing TL072 ?

 

[Bernardo]

Hello, I have a very strange failure, in the ratio number position, and when my unit compresses the most is in ratio 2, in ratio 10 at least. the lorlin connector is well placed, where it says A and pin 1. the 1.2M, 1.8M and 3.9M resistors are well placed.thank you

 

[Davey78]

Hey guys, built my first unit and just checked the dry Voltages without ICs/VCAs. Everything is as expected except that I have 3.5-3.8 V on the TL074 pins 1/2/13/14. Solderings around it seem ok, so is this normal? Thank you!

 

[Pat Maki]

Replacing TL072 ?

Your comment was "Finally could measure AC on VCA output (pin 8) when sending music through the unit. Does AC reading change on pin 8 when turning THR knob ?" Can you please confirm which component are you referring to when you say VCA? one of the THAT 2181s? Thanks!

 

[Script]

Everything is as expected except [...]

You are in the rectifier section (five diodes in total). Looking at the schematics, you see those two op amps with the diodes in the negative feedback loop?

When ICs are inserted, audio comes in thru 2x47K (on the left in schematics) into the Sidechain VCA, which you can look at as a fancy volume control driven by a DC control voltage into pin 3. The attenuated audio signal (AC) leaves the SC VCA thru pin 8, goes thru an inverting buffer op amp followed by an isolation cap (which BTW with resistor also forms a HP filter), and is then turned into a DC control voltage (CV) via the diodes in the op amp rectifier section. There, the signal is mixed with DC coming from the ratio network, which itself is connected to +12/-12, to change the amplitude of the audio-derived DC signal (i.e. set the RATIO).

That's why you get AC and/or DC readings on some of the pins of the TL074 when all ICs are in circuit and when feeding audio thu the unit. When ICs are not inserted, there is no audio and hence no audio-derived DC, but there should be DC from the ratio network. You see it ? That DC reading should change when flipping though the ratio knob positions. How high DC I don't know from memory, and less when ICs are not inserted.

If in doubt, check connectivity around the ratio and rectifier sections again. IN PARTICULAR, check for solder blobs and that there is no connection between things that should not connect, so no wrong connections.

Hope that helps.

P.S.: Welcome to the forum !

 

[Pat Maki]

Replacing TL072 ?

Replaced the TL072, but no compression still. Ordered replacement 2181's and TL074 and will pop those in and report back. Thx!

 

[Davey78]

Thank you so much for the welcome and the detailed explanation Script! Makes totally sense and I very much appreciate that you took the time to explain it so well! Learned a lot and this brings me so much further! Have a great day

 

[timtom]

One of my GSSL that has a strange behavior. One out of ten times when I turn it on, the meter stops in the middle and you can hear the transformer buzzing and only noise at the audio output. I switch it off and on again and all is fine....
Any tips?

 

[gyraf]

timtom - have you read through this thread? your symptom sounds like power latch-up like we've seen many times earlier

/Jakob E.

 

[rapidcore]

Hello. Gssl on That 2181. I have problem with debugging channel loudness difference.
Left channel is louder about 6db than right channel, and has 'groundloop' brum (about 50hz ,150hz etc.)
Swapped opamps and VCAs (didn't swap channel problem) . didn't find problems in 'soldering area'.
Any tips for dubugging?

 

[timtom]

timtom - have you read through this thread? your symptom sounds like power latch-up like we've seen many times earlier

/Jakob E.

Thanks so much your help and for the tip. I'll check that. I just wasn't sure what to look for...

 

[gyraf]

RapidCore - 6dB points towards a balancing problem:
- Check that your in- and output signals (including e.g. cables) are balanced (or both NOT).
- Check level after input debalancing circuit.
- Check that both output driver opamps delivers signal.

/Jakob E.