RCA BC 100, What to use for input transformer?

[substitute]

Hi, I'm finally pulling some RCA BC100 modules off the shelf and getting them racked up.  There's plenty of info about pinouts and what not in the tech docs section but it's unclear to me what the input transformer should be.  There's some docs for the mic preamp that show a UTC A-24 feeding the opamp, however I don't have the mic pre, I'm just looking to line into the EQ module.  The docs show a transformer feeding this stage but don't specify what.  Any insights?
Thanks!

 

[emrr]

Any repeat coil should work in that case, or something like a THAT receiver IC.  I have speculated the original may have been a UTC A-20. 

 

[substitute]

Thanks Doug!  You're the best.

 

[mjrippe]

Do you have the RCA BE100 EQs or the Melcors?

 

[substitute]

They're the BE 100 modules.  I've been experimenting with an A-24 and an a A-21 I had sitting around.  It seems like either works fine.  The  eq's are really noisy right now, seems like a recap is in order. I'm a little fuzzy on how the grounding should be set up for these guys.

 

[mjrippe]

I'd be interested to see how you progress.  I have one BE100 and I wired it up unbalanced to test it.  Sounded very nice but the presence band was not working.  Ordered a replacement 739 opamp but have yet to install it. 

I also have a few RCA opamp cards with the RCA labeled Melcor 1731.  Might try those in the API 312s I'm building.

 

[mjrippe]

So I took the time today to install the new 739 and the BE100 is fully working.  I LOVE the low frequency band.  The highs are nice but not exactly silky.  The presence band has an odd curve to the boost control where it goes from very subtle to too much in a short space.  Overall it is decent and useful.  Maybe I'll like it better with transformers. 

What are your grounding questions?

 

[substitute]

I've attached a pic of my channels strips and the pertinent tech docs to make the conversation easier. 

 

[substitute]

And the schemematic

 

[substitute]

I'm confused about pin 5 or point 13, it's labeled power supply common, which is simple enough.  On my unit though this point connects to a pin on the fader where a few shields are tied together.  If I alligator clip this over to the power supply common, the noise clears up a lot but the signal level all but disappears as well.  I think my next step will be to test the EQ and line amp modules individually.

MJ, how do you have your EQ modules set up?  Does the output feed a line amp?  On my modules the output of the EQ goes to the fader, then the line amp card, then the various buss switches.

 

[mjrippe]

I don't have the entire channel, just a few of the amplifier cards with opamps and a single BE100 EQ.  So for my testing purposes I wired it as follows:

Pin 1 - Sleeve of unbalanced 1/4" input jack
Pin 2 - Tip of unbalanced 1/4" input jack
Pin 3 - Tip of unbalanced 1/4" output plug
Pin 4 - Negative 15 volts from bench power supply
Pin 5 - Ground from bench power supply, Sleeve of unbalanced 1/4" output plug
Pin 6 - Positive 15 volts from bench power supply

Plugged my ancient ipod into the input, plugged output cable into a crappy Behringer mixer, everything worked fine.

In your setup it would make sense that power supply common should be ground.  Not sure why you lose level when making that connection.  Check to make sure your voltages are not getting dragged down when you connect it that way.  Could something be shorting a rail to ground?

 

[substitute]

Thanks so much for the detailed hook up info.  I think testing the various modules individually will clear up a lot, good call on confirming the voltages under load.  I'll post results.

 

[mjrippe]

Definitely try pulling out the line amp card while testing the EQ and vice-versa until you are sure that they work on their own. 

Other sort-of-related questions - What impedance is the fader?  Does the channel have an output transformer?  Wondering if the EQ can drive a 600:600 output without a line amp stage.  If so, I might try to build a 312 style preamp with the RCA opamp card and an input transformer, then add an output transformer after the EQ for a full blown mic pre/eq channel strip sort of vibe.

 

[substitute]

Long time no post, this project has been (still is) death by 1000 cuts.

First some answers, the fader measures 1.2k.  I haven't seen any mention of an OT in any of the docs.  I'm guessing the buss assign switches directly fed summing networks, maybe the OT's were just on the output after summing.

I've confirmed one of the EQ modules works all by itself, meaning no transformers, no line amp.  However one of the EQ modules, (the one I was initially testing that made me think I wasn't hooking it up right) doesn't work.  So, that's one issue.

I just tried to add the line amp to the working module, L902, L901 and the Opamp are getting uncomfortably hot and I have no signal coming out.  Of course I think I've got the connections right but let's make sure...
+V pin 3
-V pin 1
GND pin 16

Input pin 10
Output pin 17

 

[emrr]

use pin 2 for power common and be sure all those pins that look grounded are truly connected to one another. 

 

[substitute]

I have pin 16 physically connected to the power supply ground, I verified continuity between 16, 2, and 6.  I can't make out what the other GND pin is, maybe 11?  Or H?  On the un-messed with module there's no wiring on pin 2, pin 16 connects to power supply common.  Seems that pin 2 is connected internally on the card.  I believe I have the rewired module set up just like the original module, obviously I'm missing something though.

 

[substitute]

Ah Ha, I just swapped line amp cards between the two modules, it's working fine.  So I have one good EQ module and one good line amp card.

 

[mjrippe]

Ah Ha, I just swapped line amp cards between the two modules, it's working fine.  So I have one good EQ module and one good line amp card.

That's a good start!  Now you can compare the good and bad ones 

 

[substitute]

Haven't found much yet.  R903 isn't reading on the bad board, it reads about 2k in the good board.  The other resistors, diodes, and inductors check out ok.  I'm worried the opamp is bad, but we'll get to that.

The line amp adds quite a bit of gain to the output, I don't have the fader wired up between the two at the moment.  I'll measure the gain this weekend.

 

[substitute]

Well, replacing that resistor didn't get me anywhere.  I suspect the opamp is dead on that card.  I'm shifting my focus to getting both  EQ modules functional and sounding good.  The bad module is extremely noisy, until I turn the HF pot all the way up then a clean wave form shows up on my scope.  The wave form is little distorted since the HF is so cranked, but it matches the out put of the good module (when it's cranked).  Did I explain that in a way that makes sense?  I tested the resistance of the HF pot on the bad module, it seems fine, shows a normal taper of resistance across the travel of the pot. 

Since the IC's are socketed, I just switched them between the two modules, no change.