Melcor opamp and RCA mixer cards

[pucho812]

thanks Pucho. So it was made that way, for any unknown reason. No trouble to run them at 28V, right?

Well I am sure there was a reason, but there is no documentation as to why and nothing on the board uses it as +/-28VDC

 

[Deepdark]

You're right. The only thing i have is the schematic and little info about the mixer. I could cut the traces and run it straight 15v into the opamp or feed it 28 and let the transistors works as intended, right?

 

[mjrippe]

Or desolder the opamps and buy some 312 cards ;-)

 

[emrr]

or remove components to not have to cut traces. 

 

[Deepdark]

Yup, i will desolder stuff haha. Implementing api 312 element us pretty easy here, no need for a card

I was only wondering about feeding at 28v. It's made to be feed with bipolar 28v, so i could just be lazy and feed it as intended haha, unless someone warn me to not doing it lol. For the trim and other un-needed element, i will desolder everything and be happy with that. I think it's fairly easy plan.

 

[pucho812]

The op amp is what your seeking here, not the UOAB card.  The card has it's purpose in the RCA desk. I  have yet to see it or hear of it used elsewhere. 

 

[Deepdark]

You're right. I could simply get an API312 card over here and put the opamp on it, instead of using the card as an 312 card.

 

[pucho812]

if you really just want the opamps, PM me. I could always use another spare  uoab card for the rca desk I service and swap you a couple  doa's for your troubles. 

 

[emrr]

This; what pucho812 says.

 

[Deepdark]

haha. The cards was brought to me by a friend of mine who wanted to use them as mic pre. This is why I thought about the API 312. With not much modifications, we can use it as an 312 mic pre card As I stated, the only thing I was wondering was the 28v supply. If one transistor came bad, I guess I would just have the wrong voltage at the opamp, but not the full input voltage (28v). Right?

 

[sonidos]

Hi guys, just wanna chime in since I also have like 12 of these cards, plus transfos and other parts of the original custom board.
DeepDark, since you're form Montreal I would guess your friend bought from the same lot as me. In that lot were parts of mixing boards from both RCA Studios of Mtl & Toronto.
They were built custom, with Melcor/Neumann/Gotham parts mainly, by RCA staff around 1968 (Mtl).
(+ Hammond transfos, + Daven stepped attenuator, + PEC pots, etc.
Eq's were germanium based, don't have any of these though.)

I've been researching the history of this console for a while, and I feel a bit less like a useless archive digger now that I found some like-mindeds 

Actually there was a bunch of 4PDT relays, lamps inside switches, and some reaaaalllly old school looking phase scope (with a kind of CRT-powered display). I can't see any voltage rating on the relays, but 28V is one of the standards for that kind of relay.

So yeah, I would guess that all the other stuff to power is the reason for the 28V.
One of my teacher looked at the card and said that there was a delayed-power-up scheme around the transistors (like the +15V firing a bit before the -15, in order to minimise "pops" on power up, or something like that). He might be mistaken though.

 

[Deepdark]

Hi

I think my friend got at the same as you, yes. I really don't much about it's history. It came with hammond 813 transformer input. It's feed at 28v and passing through a transistor network to finished at 15v bipolar. All informations i got were in the document found on the forum. I plan using them to make an summing mixer for my friend

 

[lotus11]

thanks Pucho. So it was made that way, for any unknown reason. No trouble to run them at 28V, right?

Old thread. I have 4 of these boards, any ideas on what they are worth if I were to sell them?

 

[emrr]

Old thread. I have 4 of these boards, any ideas on what they are worth if I were to sell them?

less than the value of the op amp if still on the board, market value for the op amps if removed and tested.

 

[lotus11]

less than the value of the op amp if still on the board, market value for the op amps if removed and tested.

Thanks, Opamps are still on boards, no way to test but, worked when console was de commissioned many moons ago.Came from the RCA console at Institute of Audio Research.

 

[reverbperv]

I was told many years ago that this was a swinging op amp design with the transistors. I know they regulate the DC and serve to decouple the card from the main PSU bus. There's also resistors serving as fuses that were a thoughtful inclusion. Basically they didn't want these boards coming back. Check out this link, pages 31-32.

https://worldradiohistory.com/ARCHIVE-RCA/RCA-Engineer/1970-06-07.pdf
Not as detailed as the hand drawn manual RCA published for these boards. I'll try to upload them if anyone still needs them. Old thread but still active within the past year. Hope everyone is well.

 

[mjrippe]

I was told many years ago that this was a swinging op amp design with the transistors. I know they regulate the DC and serve to decouple the card from the main PSU bus. There's also resistors serving as fuses that were a thoughtful inclusion. Basically they didn't want these boards coming back. Check out this link, pages 31-32.

https://worldradiohistory.com/ARCHIVE-RCA/RCA-Engineer/1970-06-07.pdf
Not as detailed as the hand drawn manual RCA published for these boards. I'll try to upload them if anyone still needs them. Old thread but still active within the past year. Hope everyone is well.

Thanks for sharing, I had not seen that article. I have four of these boards in working condition, but no opamps, if anyone wants them.