DR-412 Mic Pre build - need help with XFMRs

[Toobme]

Update: Finished the psu, now just wiring pots.

Hey all - I’m nearly done this one, but having trouble wiring up both the psu transformer and the output transformer.
I do not have schematics or a layout (save for the board), so am getting list and don’t want to fry anything doing the trial and error type testing I’m capable of!

It’s an API style build with EA2503 output transformer (need help with wiring to this board).
The PSU toroid is dual 115v, with two whites and two blacks, and dual 25v secondaries with two reds and two blues. No part number, but it looks Slovenian.
I want to wire for 120v (North American), so would i just use one black and one white, and then just one blue and one red?
On the psu board, it just labels the pins as 1, 2, 3, 4, 5, 6, G, G.

Wish I had the schematics, but if you have a clue, I’d appreciate it!

Has anyone put this mic pre together and know these two answers?

Here is a shot of the transformer and board - no label on the pins of the transformer header.

Thanks for any nudging/advice!



And here’s my attempt (minus the grounds yet) for the psu.
Totally confused, but I’m hoping/assuming that brown lead is pin 1.
Again, without the layout and schematic, I’m flying blind (for my ability, I’m sure many of you out there can do this asleep!).

 

[soulsonic]

No schematics? Was there a bill of materials? How did you know what components to buy? Where did this all come from? Was it a kit?

 

[Toobme]

Not a kit, but the boards came with the transformer.

Last night I finished the psu and dialled in the +/- 16V and the 48V phantom.
Now to wire the gain switches and volume pots.

The boards and case are from DIY-RACKED.COM, Tat Perushka.
There was a BOM, bad sadly he can’t find the schematic.

Now I’m just trying to figure out the gain setup.
6 pins on the output header, presumably going to 4 pins on the volume pot (Hairball Audio’s), and 2 to ground.
But it’s a bit of a guessing game without a schematic, as to the hooking up or wiring.

Nice boards, though, I built a pair of La2a’s from his cases. Nice.

When I finally get this together, I’ll make a PDF of the finished wiring and post here.

Thanks for looking/helping!

 

[Toobme]

I decided to try going with a hopefully logical assumption and wired the output transformers in the order the wires exist in. Brown=1, etc.

Where I’m stumped now is the output level header. It has six pins. The Gain header has 2, which I assume go to the 2 pins on the gain switch board.
But the output level header must go to the volume pot board, which has 4 pins. I’m guessing the other two need to go to ground, but that might be only on one end.
Scratching and hmmm-ing here.

 

[soulsonic]

The CAPI website should show what the colors mean on the transformer. Then I would trace the board to see where which pin was connecting to what and then wire accordingly.
If you look at the schematic for a 312 clone using the EA2503 here: https://capi-gear.com/catalog/images/gallery/VP312/VP312-VPR-Rev-B-Schematic.pdf
you will see that the red is the + input, brown is the - input. The blue wire is the + output, orange is the - output, and green and yellow are tied together. Gray and violet aren't used (but possibly could be depending on how your board is wired - which is why you need to trace it).

The hairball board is an output attenuator that goes in series with the balanced out of the board. Somewhere on the board should be an output pair that should be labeled "+ -" like on the hairball board. They go to that board and then the "+ -" out from that board goes directly to the + - of your output jack. The ground of the output jack goes to the chassis ground and isn't connected to the hairball board. You can see the same kind of output attenuator in the CAPI schematic linked above. If you can't figure out that output transformer header, you could always follow the colors above and attach the blue wire directly to the + input, orange directly to the - input and tie green and yellow together, and it should work as intended. Though you will still need to trace the board to figure out where the inputs connect.

If I were in this situation, of getting a board with no schematic, the first thing I would have done is trace out the board and drawn a schematic from that. I've done that with a couple things I bought recently (both recording-related) where they didn't see fit to provide a schematic. From all my years doing DIY guitar pedals, I've never seen boards without schematics until I started getting into these DIY recording gear boards. Not sure why these board designers are allergic to schematics. It's a mandatory thing. Sorry, I can't help but rant here because this is honestly a disturbing thing to see. You can't be expected to sanely troubleshoot your build without a schematic. Trace trace trace, that's what you're going to have to do.

 

[Toobme]

Thanks @soulsonic - I’ll start that process.
Was afraid of that, but it’ll be a challenge.


After a re-read of your last post, @soulsonic, I've got a much better idea of what's going on.
I really appreciate the time you took and I'll post back when I've got something done!
I'd buy you a bev!!!

 

[Toobme]

TLDR; vendor did not have a schematic or layout, but a nice BOM.
The board is labled well to match the BOM, but the control headers' pinouts and XFR pinouts are stumbling me big time.

Ok, I'm feeling pretty down now. I went through the board and found where everything is making contact via a dmm.

The problem is that without trial and error (or a bigger brain), I'm not going to be able to bring this board to life and I'm a bit leary of injecting power into it at this point with the unknowns I have.

The PSU is put together now and working great - I was able to set the levels to +16V, -16, 48V to a tenth of a volt. It stays stable, but I've not yet loaded it with the 4 channel board.

Without figuring out which pads on the main board go to which leads on the output transformer, I'm hooped.
I have been staring at a shot of the board and how I'VE wired in the EA2503.
Can anyone tell from looking at the pics below what the correct order would be for the output?

I labelled the four relays what I THINK they are, but again, not used to this kind of layout and am just guessing.

Then if that gets sorted, I'll have better clues for the last part of the puzzle - the Control board. It has 4 SPDT toggles and an LED (pic higher up in the thread). That little board has a 7 pin header, which I assume connects to the 7 pin header on the main board called CTRL.

The unknowns for me are the pin assignments for these interfaces.



I've assembled about 20 DIY projects (all audio stuff) and so I thought this looked doable. With a schematic, I'm fine - it's like having a map. But I've been drifting for a few weeks now and starting to hear the groans from my better half about my dropping 2k on parts for this thing and it starting seriously to lurk on my bench.

I won't let it be a doorstop!
Anyone built this thing out there?
Somebody surely has.

Anyway, thanks for looking - hopefully I get it all done and posted for anyone that ends up with this in the future!

 

[TillM]

So if I understand right, you need to know, which wires from the output transformer have to go to the 2503 transformer ?

 

[Toobme]

Hi @TillM - thanks for responding!!

I'm trying to figure:

1. Which wires from the 2503 go to the which pins of the 8 pin header. I wired them in their natural order just to try for now, but am not sure if I should power it up yet.

2. I'm also confused about wiring the Hairball volume pot to the board. There are 4 pins on the pot's little board (+-in and+-out), but there are 6 pins on the header labelled Output Level.

3. Also there are 7 pins on the CTRL header going to the 7 pins on the switches board (4 toggles = phase, phantom, pad, and mute).

4. Lastly, there are 2 pins on the gain switch's board, and there are two pins on the main board labelled "Gain."

2, 3, and 4 seem interrelated for sure, but I'm not fathoming this.

Thanks so much for your question.

 

[TillM]

@Toobme
PIN 7/8 is more likely for an AUX.
I’m sure the 6 pin header is for a 2523 output transformer.
So the most important are the first six wires on the 2503 output transformer.
The wiring is from Output Trafo tithe hairball tpad pcb and then from tpad pcb to your headers.
Please search for the ML12 pcb, I’m sure diyracked took a lot from Mikes build.
I’m also sure; that also a Capi schematic will help you. I’m on my mobile phone, so it’s hard to help you with the search, will take a closer look in the next days.

In general your output trafo should not get damaged while firing up, even if the wires a mismatched.

 

[TillM]

Orange and blue should go to the input of your tpad and then from output to your header.

Just by looking at your picture I think the wiring should look like this.
1. brown
2. red
3. blue
4. green
5. yellow
6. orange
7. grey
8. violet

Maybe 1 and 2 are in other direction and maybe also 3 and 6.

 

[Toobme]

Wow, @TillM thank you for the detail. I'm at work right now, but will try this out late tonight.
You and @soulsonic have given me hope. That is amazing, so thank you!!