Problem with 1176 A build

[walding]

Ok long story short, a friend of mine is building me an 1176 A clone and as hit the wall with problem shooting the unit.

He has built them before but this one just has him stumped, so I thought I would ask the brains trust here for any clues or if someone has built one to take a look at it, of course I would pay them.

In a nutshell, the problem is the unit goes into oscillation around 12-3 o'clock, it varies in intensity depending on what compression ratio is selected it also can happen with the compressor turned off.

He says the the gain reduction circuit is stuffed.

Any help advice is greatly appreciated.

Based in the north of Melbourne.

Cheers.

[Alistair]

FET way out of spec/faulty? Also, there is a distortion trimpot in the MNATS builds (l'm assuming that's what model it is). That could be way out of spec, but l wouldnt expect oscillation.

Does he have a scope?

[walding]

FET way out of spec/faulty? Also, there is a distortion trimpot in the MNATS builds (l'm assuming that's what model it is). That could be way out of spec, but l wouldnt expect oscillation.

Does he have a scope?

Thanks for your reply.

Re fet/s yes he has tried different ones.

Yep he has a scope.

[Chinagraf]

I've heard of something like this happening in rares cases on blackface rev F ureis. I have one but it doesn't do it. David Kulka talks about it here:
http://www.gearslutz.com/board/geekslut ... itter.html
It may have nothing to do with your problem but might help? At least it suggests checking all the shielded cables have a good connection to ground.

[walding]

Thanks Andy.

The oscillation in the unit he is building for me is very different, its very much in the audio range and not unlike the sound you get from the fuzz factory pedal.

There is some DC issues as well.

I will pass this on though.

Cheers.

[Futureman]

I had oscillation in one of my rev 'D's (very similar to A).

Same deal as yours.. Although I only got it when using an unbalanced output.. (from memory)

Check your grounding.. Then check it again.. Move some of the grounding wires (NOT the mains!!) while it's on and oscillating.

Best of luck

[Alistair]

Has he used the premade case or his own case? Transformer placement could be causing oscillation at high levels.

l'm not sure if it applies to the A build, but l know there was an oversight on the Rev D design and a resistor value was halved, leading to excess gain in the circuit. lf it's the same in the Rev A, it wouldn't be the cause but it wouldn't help.

[rob]

grounding, power supply voltages all correct at different sub circuit points, transistors in the right way round.
Pull the GR FET completely out of circuit. Do you have a good clean amp with 40dB of gain with input and output controls up full?
You have two amplifier sections in there joined via the output level pot. Lift the wiper wire of the output pot, inject signal in here, does the output amp work ok.

[astrovic]

Hi guys

I'm the builder. I think I'm going mad.

I have had a number of issues with this build, which I have gradually knocked on the head, but still have a great big DC oscillation in the gain reduction circuit. Basically unit goes into oscillation upon turning on, irrespective of whether a signal is passing. There's no noise (any more, I had a grounding issue I sorted), just a constant "thump thump thump" like a kick drum almost, which is the DC voltage spiking constantly. Obviously I can't get a meaningful read on the voltages on the transistors as they veer all over the place.

Pull a transistor out of the gain reduction circuit and it's fine. The amplifier section works beautifully. The power supply seems fine and the voltage rails nice and solid.

I've tried different transistors (and types of transistors), checked hfe's to ensure they're high enough, checked all trimpot make sure they're not the culprit, gone over my wiring a hundred times and improved/replaced any wiring that is even slightly dodgy, moved wiring around, checked the colour code of every single resistor in the GR circuit, triple=checked the heatsink on the voltage reg to make sure it's not a thermal issue, gone around the GR circuit and the meter circuit with a MM and scope to see if I can track anything down that might point to something being amiss - short of completely pulling the GR circuit and starting it again, I think I've done everything in the troubleshooting manual. My guess is an incorrect or faulty resistor somewhere, which I won't be able to track down without pulling them all.

Any thoughts would be greatly appreciated.

[rob]

have you tried replacing the FET? Is the FET pinout correct for the pcb layout?

[Sorr]

Back in the 1990's I had an 1176LN in for service that also oscillated. Sometime ago now so my memory is a bit vague but my brief note on the schematic is that it was caused by C14. Which is Q6's emitter bypass cap. Increasing its value fixed the problem.

[rob]

I think you need to clarify wether the unit is oscillating ... ie making a continuous tone or if the DC control voltage from the sidechain is making a "thump" when the unit goes into or comes out of gain reduction.

If it is a thump have you adjusted the bias? If so, does the gain of the unit respond in a typical way .. ie from counterclockwise nothing happens as you rotate clockwise then somewhere around 1/3 of the way the gain begins to decrease and continues to decrease very rapidly for small further tweaks clockwise of the trimpot. This is fair test if the FET is doing it's job and working correctly

[astrovic]

Thanks guys.

Ross, your comments about C14 sound interesting - I will have a poke around that section and see if the problem is C14 or somewhere around there. I initially had problems with R32 letting out smoke (faulty Q6 caused it) and possibly I damaged C14 when fixing that.

Rob, the FET is something I haven't focussed on yet but will definitely do so. From what I've been able to tell, the bias trimpot works and bias is adjustable, and the FET works and is installed correctly, but definitely worth looking closer at that area.

Cheers, gives me something to go on with!

[walding]

Firstly a big thanks to all who chipped in with advice, secondly chris found the problem and I have a very nice 1176 clone sitting in my rack sounding very nice indeed.

Um err the problem was............... chris fixed a few problems to do with wiring but had left the pcb board NOT screwed down, so when the other problems had been sorted the pcb caused all the problems we described, simply by screwing it to the chassis fixed the problem.


Big thanks to chris for building this for me and nearly losing his hair in the process.