Neve 1290 questions

Hi guys,

Been a while since I’ve built something and had a couple of pcbs I got off eBay from ages ago.
They are blue pcbs and marked BA189 and BA283am.
I’ve populated both pcbs and my first question is about the two gain stages.
I’m only using gain stage 1 so far and I’m a bit confused with the labelling of the connections as the guy who made these pcbs has changed all the labels and I’m working from either the ez1290 docs or the original 1073 schematic.
Gain stage 2 looks like it should be used for the eq of the 1073 but can I use it for the preamp?

Also I get a drop in level when at position 8 of the pot?
I’ve checked the resistors everything seems fine?
Is there a chart for what ohms I should have on the switched gain pot?

I’ll try and put together a drawing of how I’ve wired up the pot and see if it’s correct, if I’m honest I don’t really fully understand the schematics of the pot.

And I suppose what I meant about the gain stage 2 is, as I’m using an sm7b which is low output I need a bit more gain to get a nice output, could I use the gain stage two?

This is the link to the pcb

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F122905205391

I’m using an OEP 1:2 txf on input and have wound my own  LO1166 which spec wise has come out great.
Might wind a 1:4 input txf and see if bottom of waveform improves as bottom of sine looks a bit off.
Using a 24 position switch but have stopped after 12, as per ez1290.

Just managed to sort it, stage 2 gain feeds stage 1 gain!!
Not the way I thought it would work but hey ho!!!

Hopefully sort the dodgy waveform bottom section tomorrow with new input txf!!

Then build Psu and put in case!!

Basically you have a signal amp on the 283 & an output stage. 

On the 284 you have 3 signal amps (2 used for eq).  When you operate the gain switch it only uses the  BA283 until you get to 55db gain when it switches one of the amps from the BA284 in as well to give more gain.  So below 55dB gain you only need the BA283.

Ah that makes sense now!!

What about this waveform, using the OEP 1:2 isn’t ideal but could this be why the waveform is a bit messed up on the bottom half?

I’ll try and post a pic tomorrow, I will be winding a 10468 anyways, I don’t wanna take apart any of my others!!

For the waveform, there should be a bias variable resistor on the output line amp that may need to be adjusted.

Yeah I’ve adjusted that to 22.66v at no gain as per the ez1290 notes, is this correct?

I’m assuming the waveform is distorted as the OEP doesn’t have the inductance?

Spencerleehorton said:

Yeah I’ve adjusted that to 22.66v at no gain as per the ez1290 notes, is this correct?

I’m assuming the waveform is distorted as the OEP doesn’t have the inductance?

Click to expand...

What level signal are you applying ?  Is it mic level ?

0.775v I’m applying

It’s part of the bottom peak that is chopped off, if I lower the output of the frequency generator I can get a nice waveform, doesn’t seem to change at different frequencies?

What could it be?

Is mic level 0.5v ac? Rather than 0db at 0.775v ac

You need to adjust the BA283 trimmer to that the clipping is asymentrical

Yeah I do adjust the trim but on 10th click of gain switch with 0.050v ac into it I get a bit of a messed up waveform before it goes square wave?
My inductance is 5H on secondary so I’m gonna try winding another and try and get 9.5H or there about’s like the original and see if I get a cleaner waveform throughout.
The waveform is better on my wound txf compared to the OEP 1:2.
I’ve done mine 1:4 and it’s moved the chopped section of the waveform on the upper peak now.

Spencerleehorton said:

Yeah I do adjust the trim but on 10th click of gain switch with 0.050v ac into it I get a bit of a messed up waveform before it goes square wave?
My inductance is 5H on secondary so I’m gonna try winding another and try and get 9.5H or there about’s like the original and see if I get a cleaner waveform throughout.
The waveform is better on my wound txf compared to the OEP 1:2.
I’ve done mine 1:4 and it’s moved the chopped section of the waveform on the upper peak now.

Click to expand...

WHat happens if you turn the output level down on your sig gen ?

If I turn the volume down I get a nice waveform

Spencerleehorton said:

If I turn the volume down I get a nice waveform

Click to expand...

It's working correctly.    If you apply too big a signal to any preamp it's going to clip & distort eventually.

Ah ok cool, just finished case so will start putting together tomorrow.

Hi, Spence,

I've built a full 1073 with these boards. I did a post here also includes info on the BA boards you have:

https://groupdiy.com/index.php?topic=72019.msg912567#msg912567

Let me know if you have any questions and I'll try to answer.

Thanks!

Paul

Hi Paul,

thanks for that, so far i think i'm nearly there, just the PSU i will make tomorrow, i've had trouble with the PSU before so will revisit what i've done and replicate.
I've also had a bit of a revelation with the input txf 10468 and want to try out another way of making a hum cancelling transformer and then put in a mumetal case and then in a steel case, this should hopefully remove any possible problems from the input txf.
If all goes well i should be ok to use the same idea for making some BV8 transformers as well.

I did read you used screened leads on all the connection form the PCB, did you also use screened leads for the Gain pot?
The documentation i have is a little vague of exactly what connection are what, so to start with i was quite confused, i think i understand it now though.