RCA BC5B Partial Rebuild Story

[FPALB]

I've been working on this unit for a while now and thought I should begin a thread to document the rebuild. With the budget available the criteria was simple - Get the main mono out working (with front panel volume control) as well as provide a direct out from each of its preamp cards (also with their front panel volume controls)- Amplifier and booster cards for monitor path are not necessary. Id like to have shut off switches for each channel post fader as well - easy to do with so many unused switches. The unit is in disrepair with many wires cut, all the tubes missing, and the power supply is completely stripped of components except for the mains transformer, and 6 can caps installed by a different tech. All the cards and transformers are present.

Game plan: Rebuild the power supply, the preamp cards, and the program amplifier card. Remove all wiring and keep any shielded wiring that is still good. Rewire just the circuits im using and send direct outs to new jack on the rear of the unit.

The guys over on this thread did a ton of heavy lifting for me , so thank you to them.

Where I currently am:

PSU: Fully rebuilt the power supply (replaced 4CR1 with diodes- still need to add a resistor to drop the voltage down slightly).
Kept the same can caps that the last tech put in as well as added a couple other stand alone they are as follows:
4C1(A,B,C): 40uF, 40uF, 20uF
4C2(A,B,C): 40uF, 40uF, 40uF
4C3: .47uF film
4C4(A,B,C): 40uF, 50uF, 50uF
4C5(A,B,C): 50uF, 50uF, 20uF
4C6(A,B,C): 50uF, 50uF, 50uF
4C7: 2200uF


Cards: cleaned and rebuilt all of the cards that I will be using.

Next step: remove the riser plate from the chassis and use it to create a test platform on my bench to test the power supply loaded with all of the cards.

Issues im still pondering -
- Im not going to be using the monitor section at all, and because the PSU is a non regulated unit im assuming ill have to put loading resistors in place to keep the PSU happy - is this correct thinking? Im a bit of a novice when it comes to unregulated supplies like this.
-Direct outs: Can an unbalanced direct out post 1C5 be sufficient for a direct out into a converter? How does one calculate this? Do I need a transformer here or a buffered circuit?

Thank you for all of your shared knowledge in advance.

 

[My3gger]

You will need some voltage dropping resistors when using ss diodes. 1uF without 220k resistor at the output should be fine using new film caps. I recall EMRR saying they used two because old caps had tendency to leak DC. Thread you linked to has some of this answered on first page, post #55, etc. It is always nice to see such projects, good luck.

 

[emrr]

Yes, you'll have to monitor voltage rise with fewer modules, on all B+ lines. It's almost easier to keep the monitor amp in play for the significant current draw, which sets the baseline for the rest.

 

[FPALB]

Yes, you'll have to monitor voltage rise with fewer modules, on all B+ lines. It's almost easier to keep the monitor amp in play for the significant current draw, which sets the baseline for the rest.

I had the exact same thought...I was going to keep it out to keep costs down (and time for new wiring), but it may not be worth the headache. I had already prepped the modules I wanted to use, but....whats two more I guess.

Thank you.

 

[FPALB]

You will need some voltage dropping resistors when using ss diodes. 1uF without 220k resistor at the output should be fine using new film caps. I recall EMRR saying they used two because old caps had tendency to leak DC. Thread you linked to has some of this answered on first page, post #55, etc. It is always nice to see such projects, good luck.

I will check for DC. Accept for 2 of them, all of the 1uF caps actually measured a low ESR so I kept them as is.

 

[FPALB]

Another wonderful builder informed me of the tone differences that I will encounter at a direct out due to the high impedance output of the 12AXX vs a transformer that will closer match the standard 10k of a converter input...He recommended the 27101-G from Cinemag with a Zobel network to knock down resonances. I am new to Zobel networks, but I understand the concept - what a cool thing. Its a cost issue, so Id like to buy one and compare it to a post 12AXX + Cap direct out. Any thoughts on this before I try it.

How does one know what the output impedance of say a 12aXX in a circuit is? Pulled up a spec for a 12AU7 and found some curves that may give me the answer - is the attached image what im looking for? Would like to get more confident at glancing at amplifiers like this and understanding impedances.

 

[My3gger]

Two 27101-G i have are gapped iron core transformers meant for single ended stages with DC into output transformer, see RCA BA-2 schematic for example. Gapped transformer in a cap coupled stage as yours can degrade the sound, add additional costs. Don't know what is the problem driving 10-15k inputs directly from BC5B's preamp outputs with cathode followers made of 12AY7, see with others if transformer with higher step down like 7:1 is necessary.
I would listen to what EMRR said, there are other reasons for leaving monitor amp; it might serve as useful 6V6 PP amp and you will preserve part of the technical legacy of your country. Most old caps should be fine, trying how something like polyester film Orange Drops sounds is cheap.

 

[Disco Volante]

Such a beautiful machine.
Good luck!

 

[FPALB]

Two 27101-G i have are gapped iron core transformers meant for single ended stages with DC into output transformer, see RCA BA-2 schematic for example. Gapped transformer in a cap coupled stage as yours can degrade the sound, add additional costs. Don't know what is the problem driving 10-15k inputs directly from BC5B's preamp outputs with cathode followers made of 12AY7, see with others if transformer with higher step down like 7:1 is necessary.
I would listen to what EMRR said, there are other reasons for leaving monitor amp; it might serve as useful 6V6 PP amp and you will preserve part of the technical legacy of your country. Most old caps should be fine, trying how something like polyester film Orange Drops sounds is cheap.

I was actually impressed by how many of the caps were still good. Such an old unit. All those that were bad were replaced with modern film.

 

[My3gger]

I was actually impressed by how many of the caps were still good. Such an old unit. All those that were bad were replaced with modern film.

I'm impressed by everything in this mixer, build quality, electrical design. Manual is great too, transformer info is lacking but they explain function of components like 1R12/1C6/1R13 we were not sure about, maintenance, functions, etc. In my experience most old film caps perform fine in good quality European gear like WSW, Siemens, TAB and some East German tech. Why RCA paralleled PP wired ECC82 is a bit of mystery to me, maybe it was done to avoid tube matching, or for additional drive. PP stages are very effective at transferring power, will check other manuals from this series of mixers for info.

 

[FPALB]

Still slowly working on this project! Thank you all for your help so far!

 

[emrr]

I Why RCA paralleled PP wired ECC82 is a bit of mystery to me, maybe it was done to avoid tube matching, or for additional drive.

both. Replaces PP 6F6 or 6V6 in the ranges those were run, more bottles averaging. I forget if the wiring of the bottles is cross-coupled, I'd expect the arrangement that delivers the best auto-balance odds.

 

[FPALB]

Bit of an update:

Got 5 of the 6 preamp style cards back in (the monitor amplifier card is located elsewhere and a lot more annoying to wire back up) and wired the audio side that goes to the front panel input/output switches...many a cramps occurred. My precision tweezer hand got a workout.

Channel 4 is identical to the others, but in the original design it is wired to the transformer for a different impedance (Input was designed for Network Program? Any classic folks know what this is?). I rewired it to match the others at 150 ohm input impedance. Ch4 also goes through a pad before hitting the input transformer and I just removed that as well. Its now identical to the others.

Took me a while to figure out how the hell to read the front panel switch pinouts, but I finally figured it out.

One half of the output select and input select front panel switches go to what I believe are the relays that control studio lighting for on air, which is pretty cool. I wont using be any of these relays in the new design..hoping it doesn't mess with the PSU load to badly.

 

[emrr]

Channel 4 is identical to the others, but in the original design it is wired to the transformer for a different impedance (Input was designed for Network Program? Any classic folks know what this is?).

Incoming network feed from a phone line. NBC New York for example.