All things G1176 - the new "repost" thread.

[gswan]

What voltage do you have on the gate of the FET? It sounds like you have it fully saturated.

 

[3nity]

Gates of Q1 i have 549mV.???

 

[gswan]

Is that +549mV or -549mV?

Measure Vgs directly on the FET and see if your Qbias point matches where you expect it to be on the device characteristic curve.

I found the spread of characteristics to be quite wide. -0.5V in some devices was fully saturated and in others is almost off. This is why Qbias needs to be done very carefully and with a reasonable degree of accuracy.

 

[3nity]

Wow i just measured again and i have -5.49V???

I think it works now!

Vu meter works in VU mode...i'm gonna try to calibrate it!

By the way everything works now and all scope shots match once again!!

thanks for the patience!!

Jorge

 

[3nity]

hey guys i'm very happy it works! :grin:

My problem number one was: Fets pins in sockets weren't making good contact.
number two: Pad # 18 wasnt wired!!! argh!!!

I'll let you know how the calibration works!

Up till now i calibrated but its not accurate and FETS are not matched yet!

Thanks to Geoff (Gswan) and Kenric!

Of course to Gyraf and Gustav Too!!! :green:

Jorge.

 

[adoucette]

Hey Gswan, I got lucky, it was the fuse that blew! I already ordered another transformer, but i guess ill just use it on another project

gonna fire it up and calibrate tonight hopefully

 

[adoucette]

What a let down.. in my attempt to calibrate Im not having much luck at all.

Following instructions to calibrate the Q bias, I put a pure tone of 1khz at 0db into the input, follow the directions, and nothing happens on my meter, in VU mode it rests at around -18db, and in GR mode it rests at about -15db, bypass seems to work fine. But the point being that when I put a loud pure tone in the input, it doesnt do anything and therefore I cannot make any adjustments.

As an experiment to see if the tone was going through the unit anyways and perhaps was just bypassing the meter, I ran the output back into protools and armed a track to my great disappointment, here not a tone, but this..

www.solidmanagement.ca/andredoucette/1176noise.wav

yuck. what a horrendous sound.. and somewhat embarrassing.. does anybody have any ideas to what could cause such a sound?

I checked all the values of the components so there shouldn't be anything wrong with them.. no solder blobs or crosses,, no wires crossing,, im stumped.

Any help would be really appreciated

Thanks
Andre

 

[gswan]

Check the DC bias conditions first. This will help in identifying any problems with component values and devce orientation.

Then we need to check each of the functional circuit blocks to see that they are operating correctly before attempting Qbias calibration.

 

[adoucette]

Thanks Gswan (its Geoff right?)

I feel bad asking all these questions because my knowledge is so little, I tried to look up some information first before bothering you, but what is invovled in checking the DC bias? Is there somewhere you could recommend I read to learn about this?

Thanks again

 

[gswan]

You need to measure the voltages at each of the transistor legs with no input signal (ie DC steady state conditions).

Here's a table of values. Yours should be very close to these. Any that are way off indicate a fault or component value problem.

http://www.axtsystems.com/index.php?view=article&catid=34%3A1176ln&id=57%3A1176lnproblems&option=com_content&Itemid=62

Once you confirm these you are well on your way.

- Geoff

 

[adoucette]

I checked a few of the transistors and the values are way off.. with a needed reading of say 28.3, id get 2.5.. I double checked my points at -10 and +30 and to my surprise found that the +30 mark is now reading 2.8. This is with my multimeter set at 200V with decimal places at "00.0" So this is probably a big cause of the problem right? Would this mean a resistor is of the wrong value?

 

[kenrinc]

Verify that your meter is set to DC volts and not AC volts. This happens a lot. Right down all of the measurements on a sheet of paper for each transistor leg and then compare them to the gswan site. If some are way off then you've more than likely got a few wrong resistor values.

ken-

 

[adoucette]

Multimeter is set to DC voltage. should I not solve the +30v issue first? If that voltage is weak then would that not effect the readings on the legs of all the transistors?

 

[gswan]

Yes, I thought that you had already verified your PSU was working OK. Check this first. Disconnect the load and check it and then re-connect the load and check it.

 

[adoucette]

The voltage coming on on the mains after the transformer is 26 volts on each side.. a little much since it asks for 24volts, dont know if that makes a whole lot of difference..?

The readings at the points on the power supply is -10V at the -10V point, but only 2.8V at the +30V point.. So a really low voltage there.

I checked the legs on the 7824 voltage regulator as well, it reads

1 * - 30V
2 * - .8V
3 * - 2.8V

These measurements seem odd to me.. what do you think?

Thanks

 

[adoucette]

sorry on the voltage regulator measurements, those are not - (negative) signs, all those voltages read + positive

 

[gswan]

Is the regulator getting hot? If so then you have an overload on the output rail. If not then check the resistors around the regulator to make sure they are the right value, particularly R87.

 

[adoucette]

ya it gets really hot, hot enough to burn you if you touch it..

 

[gswan]

Then you have probably overloaded the power supply.

Did you build and check the power supply before loading all the other components (as explained in the construction notes) or did you just load everything, hoping that it would work?

You need to search for wrongly inserted transistors (particularly any that are getting hot), solder shorts or wiring errors.

 

[adoucette]

I was stupid and loaded the board, should have done more reading before I did so then I would have found that you should check your power supply first. I underestimated the complexity of these sorts of projects and rushed it. A lesson im learning from..

My multimeter is acting up, so I need to return it hopefully tomorrow, only prob is im in the middle of tracking a really important record so time in the mornings is usually spent running around prepping the studio.. maybe if i get up earlier.....

once i get a new MM ill get back into it and start checking. Looked over the PCB and there arent any solder blobs or crosses, also I dont feel any transistors getting hot, only the voltage regulator. I guess the MM will show me whats up

Thanks for all the help so far, you really are providing knowledge that is incredibly helpful :thumb: