Api-312 500 series style build

[Spencerleehorton]

Sorry, my ch1 has 100uf/63v for the first caps after the diode for power coming into the 2520.
I will change ch2 to the same and see if still get the same problem.

 

[ruffrecords]

Hi Spencer. I just clicked on the link to your website and got an error message. It says "There has been a critical error on your website." followed by a link to 'Learn more about debugging in Wordpress'

Cheers

Ian

 

[Spencerleehorton]

Yeah my website is a bit up the spout, if you reload it, it does come up?

Need to rebuild it really

Both channels bd139, bd140 are both getting scolding hot after about 3-4 minutes with 100uf in?

Just wound another input txf, tested it, checks out fine, no shorts, dcr good, inductance great.
Put in channel 1 and I get cut off waveform and when I adjust output level, all the way up I see waveform cut off top, all the way down I see top part of waveform but no bottom!!!?

 

[ruffrecords]

Yeah my website is a bit up the spout, if you reload it, it does come up?

Yes, that does it. No idea why but it does seem to work.

Cheers

Ian

 

[Spencerleehorton]

just realised i still have in 5.1k and meant to change to 10k for the R3 resistor, could that be why its drawing too much?

 

[Spencerleehorton]

Found a couple of errors and changed, now the bd139/40 just seem to get warm after about 10 minutes, can still hold my fingers on them and not get burnt like before!!
D5 wasnt down to gnd and the 10k/220pf were the wrong way round.

Ch2 looks good and I think I can continue and put 48v in and polarity and pad switches and finish this channel.

Ch1 waveform is not looking good, I've used the same spec output txf as ch2 but wound my own input txf.
Normally my input txf come out fine, but will swap with ch2 input txf and see if waveform changes, if it doesnt the problem lies somewhere else?

 

[Spencerleehorton]

I will check where the waveform gets weird as I suspect its after the 2520.

 

[Spencerleehorton]

Changed over 2520 for another I had and waveform is good now on both channels.
Will finish off other switches then give these channels some use!!

 

[Spencerleehorton]

Well it seems ive hit a weird one again!!

both channels work fine through instrument DI input, get nice level and with gain can boost and pad works.

On mic input i get very low level at output with gain on full?

ive wound 1:1:3.5 ratio txf and have Primaries parallelled so i get 1:7 step up and just dont get the output?
Ive tried different output txf but the level doesnt seem to be coming out of the 2520?

am i missing something here?

I have C1 = 220pf and R3 = 10k with R2 = 150k, R8/R10 = 680R and R9 = 150R
R4=20k with C2 = 20pf, R11=200R and C8 = 220uf.
I have left out R12(1R)
I have both C6 and C5 = 1000pf

48v is working fine (R5=200R with R6/R7 = 6.8k, R1=12k with also R13= 10R/5W to chassis.

something must be loading it down but so far i havent spotted it!!????

 

[Spencerleehorton]

a couple of errors to fix, the jack socket was putting the 0v to chassis so ive put in some isolated jack sockets and C2 (100uf/63v neg leg was down to V+!! so have moved it to GND!)
have taken off the zobel but will put back on 5.1k and 220pf and see what the rings like!

 

[Spencerleehorton]

I've wound some output txf for these, they are on slightly bigger bobbins and lams.
250 turns of 0.25mm x4 and I end up with 11 ohms and 130mH inductance per winding, I even managed to twist the four winds!!!
This tests very close to my fab 2503 versions which are 10R and 104mH.

Input txf wise I've wound a 1:8 to the same spec as the 2622.

Most of my problems have been due to the 2520 I've used and out of 4 I had left over they all seem to have problems do I've ordered two more which should hopefully fix issues.

Zobel ive ended up with 10k / 220pf and 10k to gnd.
Gain switch I have 20k / 100pf and 22k stepped with 220uf cap to gnd.

C1/C2 is 47uf/35v with 1n4007 diodes.

Only issues now is the 1k log attenuator seems to adjust output on pin1 but not on pin 3? And when flip phase I get different levels on pin 2 and 3?
I think once I change 2520 it will all be fine.

 

[mahoule]

A 10dB pad is hard to do whilst maintaining the right loads for both the microphone and the transformer. 15dB is about the minimum you can do and meet these criteria, 20dB is more normal.

Cheers

Ian

Hi,

Just thought I'd document my api-312 500 series build.
Single sided pcb which you can make through the iron on method using laser printer, winding my own txfs, 48v, -10db pad, polarity switch, di input, gnd lift, output attenuator pot.
Gonna use a couple of 2520 op amps I had left over.

Have design and printed pcbs and going to put them in ferric chloride then drill.
Will detail parts etc once I get to that stage.

Hello to you
I would like to add a VU meter on my 312 api on which output transformer tap or 2520 out and which opamp for vu meter Thank you and good day
Marc