Help identifying mystery RCA board with Melcor opamps, UTC A24 and T4A Cell!

[pucho812]

all this reminds me of the universal operational amplifier boards found in the rca console I service for a producer.  IF you look into the tech doc section under RCA, you will find my rca uaob schematic and data sheets.

 

[orangechili]

I will definitely check those docs out, thanks! I reached out to Ken Mckim from Retrospec audio for some insight and here's what he said:

"My vote is RCA. They had a huge R&D and manufacturing arm and they made all kinds of interesting devices. One of them, in the form of a limiter, crossed my bench for repair. It was very much akin to what I see in your pictures, with some differences. The one I repaired was an early-on (1960s) 500 series module, which is to say it was originally designed to slide into a slot on a 1600 series API. It had threshold and gain pots, and a meter on the front panel. The reason, in particular, that I think it's an RCA is that big honkin' UTC transformer, that if one were to reverse engineer, would find that it was being used to "amplify" the voltage used to drive the electroluminecent strip in the T4B can.

Unfortunately, I didn't have time to reverse engineer it myself, so, no documents. But looking at the trace side in your picture, I see what appears to be pins for ground, plus and minus power, input and output. Missing are any access to off-board controls like threshold and gain. So, this must be for a fixed application where those parameters are set by the pots on the component side of the circuit board and that's pretty much that.

That .0047uF cap at the output of the 1731 is curious. That's a blocking cap intended to block any DC offset from the 1731 amp into the transistor buffer. But if you run the numbers, in that circuit a .0047uF cap will also block any signal below 400Hz. So, if what is represented on the schematic is in fact the signal flow, maybe the cap had been replaced at some point with the wrong value, or at very least, these cards were indeed very use-specific - like, some kind of frequency limited compression. For that cap, 2.2uF (25volts) gets you down to 10Hz, and 10 uF would not be a crazy value for full range.

I remember them sounding great. Let me know if I can be of any further help."

 

[orangechili]

I'm waiting on the proper transformers to come but I do have this compressor hooked up now and it does indeed sound great, the .0047uf cap was cutting low frequencies like crazy so I placed a 10uf 400v cap in its place like Ken suggested and the sound opened up immediately. It sounds and reacts very much like an LA-3A. I only have some Edcor Wsm600:600 right now for the input and output iron cause thats all i had lying around. If someone has opamps lying around it is certainly a simplified way to build an opto type compressor unit.

 

[PRR]

It is same-as an LA-2 except made with opamps.

Signal comes in via 68K and 2.7K to photo-resistor.

Signal path has gain of 10 to what is probably a Darlington emitter-follower (strange values though).

Sidechain skips the 2.7K (for stronger action in high GR), also has gain of 10, feeds a transformer to step-up to glow-panel voltages.

The emitter follower should not be essential for driving the output transformer. The other thought is that the strange values add-up to a 2nd-order active low-cut filter. I'd be tempted to take signal output from the upper opamp for initial testing.

If the zener diodes are really 15V, then the input supply is logically higher than 15V. But probably less than 25V or the 300r would run hot. +/-18V seems a good start.

 

[orangechili]

Thanks! I have a feeling you're correct about the power supply being higher and the darlington pair doing further band-limiting(that would account for strange values). I'm gonna test the signal out of the upper opamp into the output iron. This was likely a custom made device not for studio use which may be why most don't recognize it. It's been a fun distraction from regular work for me though

 

[pucho812]

O.k. I spent time this week with the RCA console I spoke of earlier.  For starters the UOAB cards which make up the console does have  mecors on them, but there are also UOAB cards in it with RCA branded doa's

I am wondering  how much this card resembles the uoab cards. I posted the data for the uoab in the rca tech documents. It has schematics for it...

The uoab cards run off +/-28VDC but the transistors and subsequent circuitry  is used as voltage regulation to provide +/-15VDC to the melcors, discrete opamps.

 

[orangechili]

Nice! The 'other' board pic I posted earlier from the same seller had +25/-25v melcor opamps on it. That board was only one part number higher than mine.

PRR,
I tried connecting out of the top opamp right into the OT like you suggested and the actual gain response increased considerably of the limiter. Its hard to say if anything changed frequency wise but it definitely sounds more 'open' and less 'restricted' if that makes sense. It seems like the compression doesn't come on as heavy in the beginning of the pot range which is nice. I'm gonna do some more critical listing tonight. This thing is a little beast!