Possible cause of TOO MUCH distortion on 1176 Rev D?

[Dubka]

Built 2 and one unit has much more distortion than the other.

Distortion trimmer works great on one unit.  Doesn't seem to do anything on the other.

Built the same day with identical components.

Any ideas where to start looking?

 

[abechap024]

That wouldn't be the one by chance that you ran vocals through for sound cloud is it?

I don't have the schematic off hand, but I would start looking around the distortion trim pot, maybe the pot isn't working or maybe there is a dry solder joint. Since you have a working unit you could use the working unit as a voltage reference, and make sure voltages around transistors are comparable.

Happy hunting!

Abe

 

[Dubka]

Yeah, exactly!  That's what got me thinking about it... never noticed it on bass or drums.

Any advice on HOW to test the voltages would be greatly appreciated.

 

[Biasrocks]

Any advice on HOW to test the voltages would be greatly appreciated.

There's voltage charts posted in the appropriate build threads, which is the best spot to post questions.

First guess, check/recal the FET biasing.

I found that the distortion trimmer to have very little effect.

Mark

 

[Dubka]

Cheers Mark, will do.

Dumb question but am I right putting probes between ground and each leg of the transistors to measure this?

It's just, when I did - one of them pegged the meter...?

 

[Dubka]

Okay, I feel like I'm getting a BIT closer...

At the very end of the output pots travel (6 - 0) I'm getting weird distortion/static and pops even with no signal applied.

When I change the ratio, the pops become more frequent with progressively higher ratios... when the output is set to full, the quality of distortion changes with each different ratio...

Any ideas?!!!  :-[

 

[abechap024]

Yes, you check the voltage by having the Black of you Multimeter on ground and red at your test point. If your meter gets pegged, change your setting to a higher DC test value.
I would check your biasing of your FET like Mark said....something is a little screwy....

I would first check your Calibration.

Then if you have an old pair of headphones wire them so ground goes to ground on your project and then wire the positive to a 10uf or higher capacitor and wire that to a probe or whatever. Then play something were you can obviously hear it distorting and  you look at the schematic and or  start poking around to hear your signal in various parts of the circuit. Probably the FET causing the distortion, but you never know, could be a make up stage or something.

Also did you clean the flux off your board? sometimes not doing that will create little gremlins...

Or check your switches. Double check visually for solder blobs.


I'm just giving you a run down of what my common mistakes are....your mileage may vary.

 

[Biasrocks]

If your having problems with noise when cranking the input/output, most likely it's oscillating at higher gains.

Rearrange signal wires around and/or move your input/output transformers around to see if the problem goes away or changes. Also check your grounds.

Mark

 

[Dubka]

Cheers Abe.
I meant the COMPRESSOR meter is getting pegged   :-\
I was measuring correctly but I found that a little bit alarming so I stopped...

Okay, I should have mentioned I re-calibrated the unit just prior to my last post..  All seemed normal.
Checked for solder blobs but haven't cleaned the flux yet.
Why might the FET be causing the distortion?  Faulty?  Mismatched?

Thanks Mark - I'll have a go at that.  The thing that's puzzling me is the other (identical) compressor is fine...

 

[Biasrocks]

Could be a lot of things, bad FET, bad transistor, cold solder joint, ya da, ya da...

Recheck your voltages, compare them against the voltage charts. Disconnect the meter if it bothers you.

Mark

 

[Dubka]

Okay, I finally tracked the strange noises down to a couple of 2n3391's in the signal preamp..
Trouble is, now the noise has gone, the level seems to have dropped massively and the unit is no longer compressing!
Any ideas?  This thing is driving me mad....

 

[Dubka]

Anyone?
Possible cause of no compression?

 

[gemini86]

Did you ever compare your voltages to the chart?

Check voltages EVERYWHERE... that will tell you where you have a problem.