Mitsos API 312 / 990 Build Thread

[mrclunk]

thanks mitsos.
I'm doing some trouble shooting today.
Hoping its not the opamps...
Anyway here's some pics.
I shall report back
edit--
I'm getting about 23dB gain from the (parallel prim.) input xform. 0 to 6dB max gain from the DOA.
The DOA is getting 15.3V +/- . Should it be higher?

 

[mitsos]

Is that the group buy trafo (3402 I think)?  IIRC those were 1:8 or so, which should be closer to 18dB gain.  23 is a bit high, but it depends on your measurement methods I guess. 

My first thought would be do triple check the wiring to/from the toggles.

Your DC voltages should be fine (as long as they're not reversed ), 2520s should work fine with 15V, but if you can't test them in a known working preamp you are sort of shooting in the dark.  You can often tell a faulty opamp from a higher than normal current draw, so if you can test that, it might give you a clue.  They should measure pretty low, between 10-20mA normally, going up to 50mA or so going into 600R.

I have make an adapter PCB for an IC opamp on vero board. You can use solid core wire or leads from a TO220 device for the pins, they should fit fine in the opamp sockets.  Or, just rig up a DIP8 socket to some solid core wire and plug it directly into the DOA sockets, doesn't have to be pretty, but it will allow you to use an IC opamp rule out the DOAs.

Another thing you could do is wire a DI jack to the DI input (use a 100k or 1M load R between tip and ring), put a small cap in series (maybe in place of the 1K series R), and take the output from after the output cap. If you get nothing, the opamp is not working.

What are those two wimas near the input trafo? Are they part of a zobel network or something else? They're wired to the CZ pads?

output resistor (R21) = ?

I hope I gave you some things to check, let me know how it goes.  Otherwise, the build is way cool, I'd love to have a little 2 channel box like that!

Good luck, and post back with news/questions.

 

[mrclunk]

Fixed!
I'd put a 220r resistor in the feedback loop (R17) instead of a 22K. Hidden under the Opamp that i'd soldered in. USE SOCKETS!
Swear i'd been through all the resistors twice...

Thanks for the DIP8 opamp suggestion. I built it and hence ruled out any opamp issue which lead me to the problem.
I've just got to build a little DC meter buffer and i'm done.
thanks
paul

 

[mitsos]

Awesome!  glad the opamp idea helped!

 

[mitsos]

Hi, I don't know the spacing or order of the pushbuttons on the access312.  It might work, it might not, check the faceplate and compare the pad/phantom/phase switch spacing. I don't think the access has an output pad though, so you'd have to leave that out, IIRC you might need a couple of jumpers.

 

[Rocinante]

Thank you.    I'm not interested in leaving anything out so it looks like I'll have to check elsewhere.
Cheers

 

[Rocinante]

Okay so I am having a hell of a time finding 4pdt illuminated switches for this.  Five fish only has 2 pole.  Hairball has the same deal.  Mouser is out of stock.  And Grove Audio's contact info leads to a 'failed to send' email address.  That was after of course I had spent hours collecting stuff into my cart(s).
Any ideas where else I can turn to?
What did others use?
I wouldn't mind going the more like the 3124 route.  But those illuminated switches are nice (I have a five fish x-12).

 

[mitsos]

We did a group buy for most parts back then for the hard to get parts.  PCBs, transformers, and switches.  Ask Niels if he has any left from the group buy, his forum name is Nielsk.  I traded emails with John from Grove Audio a few days ago about some transistors.  He did take a few days to get back to me but I've bought from him a few times in the past and never had an issue.  He has really good prices on a lot of things. 
Try this address:  jburdick [at] groveaudio  com

 

[Rocinante]

Hey Mitso thank you.  I will try both and keep my fingers crossed.  Yeah I was like a kid in a candy shop at the Grove Audio site (despite several pages that were "not found 404") and had left my shopping cart full on an open window on my laptop in hopes that someone got back to me or its starts working correctly.
Lot's of great affordable goodies there.
Thanks for the always quick response.

 

[Rocinante]

Ahh I think I found some of the illuminated 4pdt I believe at Digi-key, are these compatible?  They look awfully similar.

http://www.digikey.com/product-detail/en/WBL4UEEGBQR05CLR/EG5010-ND/3522182

They don't have many of colors to choose from ATM (not that many left either) but I can get the dpdt in a variety of colors so at least they're not all the same. 

 

[toneboner]

Been scouring threads about hybrid 312 and having a hard time distilling the bare facts -- Could someone please summarize any changes to the basic 312 circuit when using a (diy)990 vs. 2520  ?  My specific purpose is to use a Jlm 1:4 (or 2 paralleled together for 1:2) and a diy990 on an access 312 board.  THANKS

 

[mitsos]

Find the M1 schematic from john hardy and compare it to the 312 schematic.  Besides the circuit changes (mainly input bias and DC servo)  the M1 also uses better transformers and better opamp.

 

[mitsos]

I just realized the docs for this build were in the main 312 thread so I added the links directly in the second post of this thread, and here they are once more:

Schematic
BOM for 312 with 1646 output
BOM for 312 with trafo output
BOM for 990
Component Placement

 

[Rocinante]

Hey does anyone have some cut out dimensions I can use for Front Panel Express?  Or a Front Panel Express file I can manipulate?  Honestly I really only need the dimensions from the gain pot using Capi's Bourn's and the next two push button switches over since the daughter board is obviously identical to the other two switches.  This way at the very least I know I'm on target.  Thanks.       

 

[mitsos]

unfortunately, I can't find the right file for these, it's been that long.  The distance between the switches is about 19.5mm, and I think about 15.5 between the pot shaft and the middle of the phantom switch.  If you have calipers, measure them out and let me know if it's close to that.

good luck!

 

[Rocinante]

19.5 looks about right to me. I decided to go for it and made a 1u front panel in my shop that surprisingly is relatively on.  Having the daughter/mother board thing going on definitely gave me some room to play with.  I'm working on the silk screen now using the same set up my band had used for patches and t-shirts. The ink supposedly will work on metal but I have only done the emulsion so we'll see.
  Sooooo I finally finished wiring them up and after endless hours of triple checking everything (and in a state of 3am delirium) decided to fire them up with fingers crossed and all.
  Success!
Well mostly/sort of.
  One channel works beautifully. Punchy, strong, with a lot of gain.  Awesome!
The second one.... Wellll..... I got a signal but it's crackling intermittently with the occasional pop. I cansee a signal in my daw but its as intermittent and reacts with a time lag. I'm pretty sure its the op amp since its one of the first I'd ever assembled and I felt like a bull in a China shop trying to be as accurate as I needed.  The other (working one) has a sl2520 red dot in it that I obviously didn't assemble which leads me to believe its the self fumbled/assembled gar 2520.  I'm gonna also rewire the problem board just to be sure since i don't feel a hundred percent confident of how i wired it. Worth mentioning is that the working 312 has the optional caps in it (c18, c19) that I may remove or install in the other.  I'll report back after I've switched op amps and rewire it.  So far though I'm pretty excited that it didn't burst into flames.  Also that I've learned sooooo much.  Forever grateful for the awesome project. Thanks Mitso. Thanks Dan Deurloo. Thanks Neilsk. 
Na zdrovyeh!!!!

 

[mitsos]

Did you check DC before plugging in the opamp? Check it now, if all is OK, you can swap opamps to see if the problem follows the opamp.  Sounds like an opamp issue, or a bad/intermittent solder joint, or maybe a bad/wrong component maybe.  Probably nothing to do with C18/C19 though.

You can also use a signal probe to probe the PCB for signal.  If you don't have a signal probe, build one NOW, it will be one of your most used tools.  Mark Burnley(?) had posted nice instructions for a DIY version, but I don't think it's up any more. Very similar to this:
http://www.diystompboxes.com/pedals/debug.html

Use whatever connector type you need to plug into your interface or amp (1/4" TS is what I used, so I can plug into an interface or guitar amp). 

good luck!

 

[ding]

I got those images.

 

[ding]

...and...
probe - aligator to 0v - input jack to amp.

 

[Rocinante]

Thank you!  I will start working on one tonight.