GSSL HELP THREAD!!!

[Bluzzi]

I'm wiring up the bypass switch that connects to the switch pcb.

There is a Common, On and Off for the bypass itself and there is 2 connections for the bypass of the Makeup gain.

I have what seems to be 2 conflicting drawings. The one showing the rotary switch one seems to show the Makeup gain shorting when the Bypass switch is engaged.

The other is a drawing (I forget where I picked it up) showing 2 toggle switches being used. However they are drawn wrong from the standpoint of where poles are normally located. Nevertheless the circuit is the same except for the Makeup hain bypass, which is opposite of above.

I am trusting the first one as its a GSSL drawing showing the rotary switch.

If my description is a bit confusing I can post the 2 pictures.

Thanks

jim

 

[OlympicPhil]

Hi Gyraf et al...

just finished putting together one of these units, with a 5-way sidechain filter section.

Passing audio fine, gain control seems to work, although not as much swing as I was expecting, and in the top third it starts doing a strange distort-and-gate combo effect to the signal... (which is actually quite a nice usable effect (for dance music especially) if that's what I was trying to do, but for a straight gain control it's obviously not right )

Is there a "test-point cheat-sheet" anywhere with the expected voltages/readings from various key test points around the board?
(Other than the regular voltage rails, which seem fine on mine - +/- 15V are spot on - although only after warming up... the first time I switch it on, there's 80V AC(!) between the + and - 15V DC rails, but if I disconnect the power, wait a few seconds and power up again the rails are fine and the AC has vanished :-S).

The next problem is that the ratio, threshold, attack and release don't seem to be doing anything (only ear-tested so far, so it could just be the threshold and ratio - attack and release could be working but at a 1:1 ratio at -0dB threshold for example - like I said, only ear-tested).

I read the first 20 pages of this thread so far, and am gradually making my way through the whole thread, looking for similar problems.

Thanks in advance,

Phil

 

[OlympicPhil]

This is it, by the way... (before I added the side-chain filter)

 

[Bluzzi]

Phil, this is just a wild hunch but can you check the way the VCAs are installed in the sidechain part of board? I think I see the notch correctly positioned on the top 2 in the picture but not in the bottom one (sidechain). That one is oriented opposite of the other 2. I know I installed mine last night and almost made that mistake.

Could that be it?

Like I said this is a wild hunch. Sorry if it takes you off course.

jim

ps: I don't know if you are getting any hum induced but you are running your AC secondary right over the PCB. that can't be good. Can you put the transformer on the right side so the wires do not pass over the PCB? If you can't then at least pass them from higher up around the PCB.

 

[OlympicPhil]

Hi Bluzzi, you're right, I did have it the s/c vca installed backwards when this photo was taken, but I had spotted it since and turned it back the right way round.

Haven't noticed any hum, but I'll keep that in mind if should hear any once the unit is fully functional. Thanks for that.

 

[OlympicPhil]

Ah - the AC I was talking about - it doesn't disappear with unplugging/replugging the unit - it disappears when I plug audio into/out of it. :-(

Clearly a grounding problem, this one... presumably this would be sorted if I attached the 0V rail to the chassis?

 

[OlympicPhil]

[quote author="gyraf"]
- At point "C" (going to the control board) you should have a DC voltage that is zero for no input signal, and going gradually more and more negative with higher input levels. When the unit is working properly, the threshold pot will also vary the negative voltage here (when there is input signal present)[/quote]

I have the reverse of this... 0 with no signal, +0.2V at high input level with threshold at lowest pont, 0 with threshold at highest point.

edit: my mistake... I'd just been showing someone the AC on my board and hadn't switched teh meter back to DC

 

[OlympicPhil]

Here's my readings for DC voltages on my 072:

Pin 1 - -2.4V -> +3V dep. on threshold (min -> max thresh values resp.)
Pin 2 - 0v
Pin 3 - 0v
Pin 4 - -12v
Pin 5 - 0v
Pin 6 - 0v
Pin 7 - +0.6V -> -0.25V dep. on thresh (min -> max vals resp.)
Pin 8 - +12v

+ve terminal of the power LED reads as +1.6V DC.

For a 1V sin wave input at min threshold and min gain it's just above unity. As the gain control is raised, the output raises through a peak of 4V, then back down to 300mV at max gain setting.

Are these any help?

 

[OlympicPhil]

Frequency sweep - 1V sin wave.

Nice and flat up to 5kHz, then steep drop off in level.

Any use?


edit: that's only the LHS... the RHS looks the same but the dropoff occurs after 10kHz instead of 5kHz.

 

[OlympicPhil]

[quote author="gyraf"]
Check that you have the needed +/-12V voltages present. [Check]

Check that you have signal coming to your sidechain, TL074 pin7. [check]

Check that you have DC varying with signal at the output of the sidechain, TL074 pin8
[-1.6VDC@ 40kHz, -1.0VDC@ 11.5kHz, -0.5VDC@ 4.5kHz, -0.2VDC@ 1.7kHz, -0.1VDC@ 1kHz, 0VDC@ 50Hz]

Check that pin 1 [noisey 0.1V amplitude offset-square-wave @<0.3V input signal, increasing signal level eventually morphs the output to a noisey replication of the input signal @around +2.5V input level, though no DC offset] and 7 [same as pin 1]on the TL074 has DC varying with signal level as well as a superimposed DC signal from threshold (TL072 pin1) [-2.6VDC - +2.6VDC min-to-max thresh] and makeup (TL072 pin7) [makeup pot has no effect on this pin - 0VDC regardless of pot position] . These two are your VCA control voltages.

Jakob E.[/quote]

Hi - still making my way through the thread, comparing other people's readings with my own... this lack of VCA CV on TL072 pin 7 seems to be a clue, I think?

 

[OlympicPhil]

Gah, my makeup pot does nothing now!!! Not even the distortion thing it was doing before.

Tried swapping the 072 and 074 for new ones but no difference.

 

[ipagel]

I had the distortion thing when i built mine. it was that i had not put the link wire on the control board right next to where the the boards link together.


as for the other bits i had similar probs with all the controls dying out. it was a little solder blob near the tl074 and the 20k resistors there.

 

[OlympicPhil]

[quote author="ipagel"]I had the distortion thing when i built mine. it was that i had not put the link wire on the control board right next to where the the boards link together.[/quote]

Thanks for the reply, it's not that though... my link's on there.

 

[OlympicPhil]

Ah, scratch that... found the problem... I'd wired the +15V rail and corresponding capped ground backwards on the supply for the sidechain filter op-amp.

Sounds great now :grin:

Gain pot still distorts at the uppermost extremes, but I can handle that I think

edit: yeah, it's a nice distortion, not the weird gating-distortion I was getting before.

 

[OlympicPhil]

OK, interesting...

This is the graph of audio input (in Volts) to output (in Volts) of the 3 ratio settings.

Tone used was a 1kHz sin wave from 0.5V up to 7.75V.

It seems that the ratio control isn't changing the ratio at all, only the threshold.

The ratio appears to be roughly 2.5:1 regardless of switch position.

Any ideas why this might be, anyone?

 

[OlympicPhil]

I've also noticed a couple of my audio output caps (100u electrolits) are bulging. I probed them and there's a -3V DC offset on each of them with the makeup gain set to 0, falling to a -14V DC offset with the makeup gain on maximum.

NB with a 1V 1kHz sin wave, I can see the signal present on the DC offset when the makeup gain is at zero, but as I approach the maximum gain setting, the signal is clipped on the negative side of the waveform at the -14V offset mark, and at absolute max makeup gain, the signal waveform has gone completely, leaving only the DC.

Pics to follow....

 

[OlympicPhil]

1kHz 1V sin wave input... probing pin 7 of either 5532 audio output op-amps... Oscilloscope Y-axis is set to 0.5V per division

Gain on minimum - notice that the waveform has a -3V DC offset...

Gain a bit higher - notice that the amplitude of the waveform is greater, but so is the DC offset...

Gain higher still - notice that the waveform is clipping as the offset hits the -14V maximum...

Gain at maximum - notice that the waveform has completely gone, leaving only the 14V DC offset...

Dunno what it is, but my output caps aren't going to last very long like this.

 

[vertiges]

I've finished my first GSSL !!! At the beginning everything was perfect !!!

But I've been obliged to disconnect the molex connectors between the main PCB and the control board a few times (and between the main PCB and the XLR inputs, outputs) when I've moved everything in the chassis.

Since everything is in the chassis : nothing work anymore, I mean no audio at all. So I spent hours and hours reading this whole thread and I ran thousand tests. (and I've learnt so much !!!)

I finaly discovered that there were bad contacts "under" the molex male sockets of the inputs and the ouputs. I guess I'm discovering how fragile the PCB are :wink: ! (but :evil: these molex are so hard to disconnect)

After reading this thread and the advices from chrissugar :

http://www.groupdiy.com/index.php?topic=6338&postdays=0&postorder=asc&start=15&sid=db0cd97904b9d57f2eb6357b67228b08

I decided to take my time and to go step by step: I took off all the IC's (VCAs as well) except the 5534 and the 5532 of the inputs, outputs. The goal here is to check if the audio will pass the opamps normaly.

I want to fix the main pcb first (so I leave the control board disconnected)

First of all, I fixed all the bad contacts under the molex of the inputs and outputs.
Secondly, I checked the voltages everywhere. Everything was normal (+/- 0.2 volts)
Finally, I connected the xlr in-out to my computer and I play a *.wav.
But I realized that I forgot to bypass the VCA's (by linking pin 1 to pin 8) and the fact is I hear perfectly the audio undistorted and clear on both channels. Is that normal ??? I thought the VCAs had to be bypassed for passing the outputs of the balanced inputs receivers to the inputs of the rebalancing output circuit. Am I missing something here :?:

Thank you !!! I have a lot of fun making this project !!! :thumb:

 

[vertiges]

Finally I decided to take a break !

When I came back to this project a few days later I immediately found many problems :

1) I did solder all the +s of the inputs and the outputs on the 1s of the XLR and all the ground to the 2s of the XLR.

2) I found 3 broken traces under the Molex.

3) And the worse : I haven't been able to connect correctly the wires between the MAIN PCB and the control board !!!

This is a good example of what happens when you are exhausted !!!
I'll never try to troubleshooting electronic projects late in the night again ! :wink:

Now everything works except a little hum on both channels.
I've got to check the release in AUTO MODE too. It seems to be very slow. Maybe it's normal.

10000 x thanks to everybody ! This thread is a golden mine of informations ! 10000 x thanks to Jakob to make this project possible ! I've learnt so much ! :thumb:

I'll post pictures, comments, parts list in the following weeks !
(And I've got to start the second unit)

eD)))

 

[Bluzzi]

vertiges,

Sorry I can't help too much yet. I'm about finished my first of 2, I'm almost there! I'm not soldering the switches directly to the switchboard so it was a pain.

Sidechain, XLRs, power transformer/fuse/plug left.

Wish there were more than 24 hours to a day!

This is exactly the kind of thing the Montreal group would be good for. We could bring the problem unit and everyone gets a chance to help out!

Sounds like you are doing fine by yourself.
jim