Neve 1290 build completed!!

[haima]

OK, so i tried as you said,
once again you're right but the resistor doesn't changes anything...
in fact the transformer noise appear at the 2 last gain step and disappear if i turn my output trimmer down..

well, you've got oscillation in the input amp stage of the preamp. this is probably caused by a little bit of the LOUD output leaking into the quiet input stage, creating "positive feedback". try moving the input transformer away from the output transformer... try moving the fader pot around, try moving anything around, while listening - see if anything changes.

check the all those shielded cables are terminated (at only one end) to 0v - perhaps something isn't shielded?

have you tried using the preamp with a mic yet? try it on the lower gain settings - does it sound ok?

btw: your components are sitting a little bit sloppy on the board - you should pull the leads down through the board and angle the legs apart before soldering to keep the component flush to the PCB. keeping things neat will give you more reliable builds, and in some extreme cases you could possibly get oscillation from components sitting too close if they are all sitting high on the board like some of them are...

hopefully your preamp will be working 100% soon - i think you're almost there! 

 

[adrian]

thank you for your help,
i gonna try this!

 

[adrian]

some news about my unit!!
so i checked my shield connection and there was a bad contact on the wire that goes to the power-supply case..
so now my output transformer continue to do some noise at high gain level but the signal is ok… (in fact i can ear what i feed in the preamp…but i saw in the assembly guide(bias part) that it seemed to be normal)

i checked the audio output and it's wonderfull! i compared it between others good preamps and it sound really great with a  very nice midrange!!!
thank you madriannse!!!!!

i just have a things that  ask me… when i try calibrate the bias i calibrate the underpart-sin to be equally clipped as the top part…  but then when i diminue the signal the under part is clipped again but not the top of it… (see photo)
and when i push the signal the 2 part are not the same.. is that normal?

             

 

[desol]

That slope to the square part of the wave doesn't look normal. They should be straight lines...symmetrical i guess. If you shorted the output at all...the 470/35 caps could have blown...leaking dc into the output section. That's what happened to me. All was fine after i replaced those cap's. Just a suggestion.

Scope the amps and output to see where the wave becomes distorted, sloped.
Also, make sure the probe is calibrated too, to the square wave generator on the frontpanel of the tek.

 

[adrian]

i tryed with an other unit(i builded 4) and it have the same problem...
i check my probe scope it's ok,
where should i scope the board to compare?
also i found something bizard... i don't have the same level on XLR pin 2 and 3 at the output... (about 23v ac pin 3 (trasformer point 5 and about 5,5v pin 2 (transformer point 8 )...)

 

[adrian]

i checked transistor voltage :
TR1  C 2.3      B 0.9    E 0.35
TR2  C 23.4    B 2.3    E 1.7
TR3  C 23.4    B 1.4     E 0.8
TR4  C 3.5      B 2      E 1.5
TR5  C 12.2    B 3.5    E 2.9
TR6  C 20.5    B 12.2    E 11.6

and they should be :

TR1  C 3.6      B 0.9    E 0.4
TR2  C 22.6    B 3.6    E 3.1
TR3  C 22.6    B 3.1    E 2.5
TR4  C 3.8      B 2.4    E 1.8
TR5  C 12.4    B 3.8    E 3.2
TR6  C 20.6    B 12.4    E 11.8

TR3 seemed to be completly wrong!?

 

[desol]

(about 23v ac pin 2 and about 5,5v pin 3...)

That's very odd. The coils are in series. I have 23.19 at 2 and 23.48 at 3.

 

[adrian]

yes, so i tried to check the signal on board during the bias operation...
sine is good before C1 (point L)
then after TR1 it begin to be clipped but i think it is ok?
TR1 B : 

TR1  E  : 


TR2 C : 2 picture because i can't see all the sinusoid on the screen...     

 

[desol]

At a lower input level(signal gen in), you should have a perfect sine all the way. If the signal goes off the scale, just lower the voltage division on the scope.

 

[adrian]

yes i have the signal good all way long if i lower the input,
in fact i don't know what i have to check first...

 

[desol]

That should be good then...i don't understand why your square wave was sloped/weird edges(first pictures)...or why you have large differences in your voltage at the primary coil on lo1166... Also, there shouldn't be such a curvature to the square wave...in the second set of pictures...off scope ones....upper part of square wave has large curve..

hmm

Maybe put it back to just barely clipping...with amp at full gain...and scoping again. Voltages should be same at primary(lo1166)...more less.

 

[adrian]

let's take everything from the beginning..
here are picts i took:

output transformer pin 8 :

output transformer pin 5 :


there is a difference about 20 volt between hot and cold...

then when i try to bias the preamp (output transformer pin 8 ) :
   

do you have an idea?
what should i see on pin 3 of the output transformer? (because i see the signal on pin 1 but nothing on pin3...)

 

[desol]

Do you have any up close pictures of the board adrian? You sure there are no backwards/wrong value capacitors?

Darrick

 

[adrian]

i checked capacitor value (but i can't mesure it for now)
they are good labeled,
i do something wrong but where...
i think it's on the output stage... TR3 doesn't have good voltages..

here some others pics...
 

 

[desol]

Do you have shield and 1 connected together at the output transformer wiring?

If so, that is BAD. Dead short.

Do you have a fuse hooked up for testing?

 

[adrian]

i've about 2,7k between output transformer pin 1 and 0v on board...

 

[desol]

It looks like in the picture you have a link across 1 and shield on the wiring? Blue and red heatshrink wires...

 

[adrian]

no it's the angle of shooting but there isn't in reality...

 

[ilfungo]

Sorry for a stupid question....
If I use JLM Go Between kit
I must still add the two resistors 6k81?
Thanks

 

[wolfgang]

Do you mean on the input XLR Connector?

If so: no!

They are phantom power. You have them on the goBetween already!
Take a look on the JLM website, there should be a schematic.


regards,
Wolfgang