Api-312 500 series style build

[Spencerleehorton]

Ah ok I'll go for the 47uf/35v then, it's just in my access 312 pcbs it has 470uf in those positions.
Perhaps I should take them out and replace!!

 

[Spencerleehorton]

i've put in a 47uf/35v and a 68uf/63v as this is what i have on hand, will order some more and replace along with some 12v zeners, only have 11v and 16v zeners

 

[Spencerleehorton]

cant seem to find a schematic for the 25k rev log stepped gainswitch?

could anyone post please? as i want to make a couple form these lorlins

this is what ive guessed at:

1. 15k
2. 4.7k
3. 2k
4. 1.2k
5. 750R
6. 510R
7. 360R
8. 240R
9. 180R
10. 120R
11. 82R

does that seem about right?

Edit: these values are pretty close and should work

 

[boji]

Perhaps I should take them out and replace

Should be fine as is. Compare to be sure.

Some helpful advice and links on stepped attenuators:
https://groupdiy.com/index.php?topic=44800.0

 

[Spencerleehorton]

Just waiting for 120pf and 220pf styros, have put in some temporary txf that should get signal through until I come up with a winding plan for the output txf.
Building the psu today and hopefully testing with some power as I can just pop in some poly 120pf and 220pf to get it working.
Will post some pics soon.

 

[Spencerleehorton]

well it seems there are many errors in that schematic so im following the api version now as it all seems correct.
1st channel i've powered up and had a slight fault on the instrument input which was just a dodgy connection to gnd.
i'm running at 16v at the moment, whats the advantages of running at 18v? just curious.
working on the 2nd channel now and the case, need to drill a few holes etc and mount the pcbs in there.
also going to wire the polarity but i notice the api VP312 version has the polarity before the output txf as with the access 312 versions they have the polarity before the input txf? does this really matter? is there a preference?
will wire in the 48v and pad switch as per the VP312.
i have noticed on the VP312 shematic R3 is 10k, where i have used 5.1k here so will change and see if anything changes?
stepped switched work great with lorlins.

 

[Spencerleehorton]

A few pics

 

[Spencerleehorton]

Insides

 

[Spencerleehorton]

And yes I get that it's not 500 series yet, but I havent managed to get into my shed to do metal work.
Just managed to get hold of some new brass to make some u47/fet47 headbaskets, so got a couple of weeks next month were I'll hopefully knock out a few mics and do the metal work.
At the moment to get them working using this 2U enclosure which I was going to put 2 x neve 1290 in.

 

[Harpo]

i'm running at 16v at the moment, whats the advantages of running at 18v? just curious.

marginal higher headroom, if needed at all, or some smoke if your type of DOA isn't designed to take this amount of +/-18VDC (=36VDC differential between rails) supply voltage.

also going to wire the polarity but i notice the api VP312 version has the polarity before the output txf as with the access 312 versions they have the polarity before the input txf? does this really matter? is there a preference?

with polarity flip before the output transformer, you could change polarity of the line input as well.

i have noticed on the VP312 shematic R3 is 10k, where i have used 5.1k here so will change and see if anything changes?

already told you, this resistor is part of the zobel network and its parts value is depending on type of transformer and if you hook up the primary windings in series or in parallel.

stepped switched work great with lorlins.

nice. My take on this subject would be based on some calculation for a linear dB increase, like this excel sheet. Just overwrite the parts values or number of step positions in the yellow cells to match your plan.
best regards, Harpo

 

[Spencerleehorton]

after a few mishaps with volkers PSU, i blew a few LM317, a 240R got very hot and a 100uf cap exploded!!
found my errors, 50k pot rather than 10k, changed the other 10k pots to make sure and replaced all the other bits, works like a charm!!

Both channels now working, just need to wind another input txf for channel 1 as its not looking pretty!!!

Now onto the 48v, Pad and the polarity, nearly done but i do keep smelling something getting hot?
need to keep an eye on these 2520 opamps!!!????

 

[boji]

need to keep an eye on these 2520 opamps!!!?

Not if you added something like 10ohm 'protection' resistors to P-in.
BJT's 139 and 140 should get warm but not blazing hot.

 

[Spencerleehorton]

I've gotta laser heat thermometer and will go over the circuit and see where the smell is coming from!!

 

[Spencerleehorton]

channel 2, BD139 and BD140 are getting very hot!! ??? im using Gar2520s.

will have a poke about and see what gives!

 

[ruffrecords]

channel 2, BD139 and BD140 are getting very hot!! ??? im using Gar2520s.

will have a poke about and see what gives!

Either an assembly error or the bias is set too high.

Cheers

Ian

 

[mjrippe]

also going to wire the polarity but i notice the api VP312 version has the polarity before the output txf as with the access 312 versions they have the polarity before the input txf? does this really matter? is there a preference?

Switching at the input can cause loud pops.  I always put polarity reverse at the output.

 

[Spencerleehorton]

I haven't had a chance to poke around yet on ch2 but I'm only feeding 16v in at the moment and it takes a few minutes for the bd139/40 to get hot.

Ian, you say bias could be set too high?
Do you mean on the 2520 or elsewhere?
I dont have a bias pot or anything?

Yes, I'm going with polarity before output.

Will wind input txf this morning, change it over then check ch2 and compare voltages to ch1, hopefully will find a little mistake!

 

[ruffrecords]

I haven't had a chance to poke around yet on ch2 but I'm only feeding 16v in at the moment and it takes a few minutes for the bd139/40 to get hot.

Ian, you say bias could be set too high?
Do you mean on the 2520 or elsewhere?

I assume the BD139/140 are on the 2520? So if they are BOTH getting hot then they are passing  too much dc current. Did you build the 2520 yourself? Maybe that is where the error is.

Cheers

IAn

 

[boji]

if they are BOTH getting hot then they are passing  too much dc current.

Both channels now working

What does idle output or 1kHz look like on scope? You mentioned blowing a few 317's. Might have affected 337 side.

 

[Spencerleehorton]

On first look I have a 68uf/35v and a 47uf/35v in c1 and c2, this might be my problem!!
Will change both for 47uf/35v

16v is good on psu on +/- and 48v is good, so no problems there