R*C*A BA-43 Program Amps

[1954U1]

I've got a BA-43 too some days ago [thanks emmr!].

Mine has slight different layout, bigger C1 than the one in the pics, 3800uF for C18.
I've checked all resistors and caps on-circuit, and DC voltages points are approx as per specs of schematic.
Only done an adjustment of R32 PS trim.
So, no recap this turn..  ;D

This thing sounds 60/70'ish great to me, and way less noisy than I was expecting.
Lots of lows and highs, amazing freq response [due to irons?].
Its mandatory, for to have really low noise, to run it at nearly maximum "gain" [=> minimum R8 attenuation, pad after the output].

 

[emrr]

I went through a group of five BA-33B's recently, none had been power up in decades. 

After initial trials, I pulled all the power transistors, cleaned the pads and insulators, and re-greased them.  Most were completely dry.  The output pair give off a lot of heat, and that made a big difference, solved some problems.  Socketed transistors are kinda like tubes too, contact resistance after 55 years might not be great unless you exercise the joint. 

The filter cap job is a pain.  You can do it in place, or you can pull one of the four can caps and those small square Radio Shack project boards will fit perfectly in the space, and hold all 4 caps.  Getting the ground wires in correct layout is crucial to hum. 

Biggest flash:  The front end is 2N2270's, was in the BA-31B/BA-71B also.  The BA-72 update moved to 2N3391A, and a lot of BA-31/71 have been updated in the field.  I've never seen a BA-33B with a 2N3391A, and I've never seen a service note about it.  I tried one; noise improved 12dB below 500Hz, and about 6dB 1-16kHz.  Tried the rest, measuring, and same result.  All had a grainy hash and shot noise before, totally gone.  Well worth it. 

 

[rackmonkey]

That’s great info! I’ve been staring at two sets of BA-43 transformers (with UTC outputs) and wondering how to put em to use for a couple of years. These have those big, round inputs (much larger than the rectangular UTCs on the 33) that don’t look like anything from UTC (could they be Kenyons?) I’ve gone back to your old posts on the 3x, 4x and 7x family tree several times, and I keep coming back to maybe something that uses the x1 circuit (lower gain that the 82dB in the original 43, as I think you said the additional gain doesn’t do anything useful). But I like the idea of using these big a$$ inputs and being able to hit them pretty hard without the input attenuators. Whatever I do, this is a new wrinkle that I think needs to be a part of it.

 

[emrr]

I have a pair of 43’s to deal with sometime also.  I’ve never had to do much to those, need to refresh on that circuit.  All around lower noise parts, I recall. 

 

[salem]

I have a bunch of these, along with some compressors that match and a pair of BA-31's from WNBC-TV in NYC. There are matching rack shelves that hold 3 units. Some of mine were modded by the tech shop at NBC for phantom power.

I let them sit for about 10 years, because they were solid state. I recently tried a pair of them and was really pleased with the sound.

I also got a bunch of the xformmers from the same source, a surplus house that was stripping them for scrap!


Peace,
DrRick

Hi DrRick,
I’m in need of the audio IO pin out for the rca ba-43 program amp. Thought I’d ask around. The fuzzy schematic I found is a bit confusing to me. Much thanks.
—Salem

 

[salem]

I have a pair of 43’s to deal with sometime also. I’ve never had to do much to those, need to refresh on that circuit. All around lower noise parts, I recall.

Hi Doug,
Rich here in AZ. Thanks again for servicing my Collins/Gates gear. I need the audio pin outs for the rca ba-43 program amp. Having trouble understanding the fuzzy schematic posted here.