the real RCA ba71?

[aaquilato]

EMRR, here are the notes from the tech who has the unit on his bench. As I mentioned, this is beyond my ability.

Upon investigation, approximately 60% of all electrolytic capacitors were observed to be physically leaking electrolytic solution. One even has a significant crack and bulge in top surface. Approximately 6 resistors are observed to be hyper-sensitive to temperature and vibration (tested w/ cold spray and soldering iron). Input transistor (Q1) is observed to be intermittent and noisy. Excessive leakage was found upon testing.

You were right on the money here.

His response...

Full recap of all electrolytic capacitors on the PCB. Replacement of all noisy resistors. Replacement of Q1 with a suitable low-noise, germanium device.

I've asked him to repopulate the entire pcb. To basically rebuild it. Is there anything particular NOT to change on the pcb?
Attached is a pic of the full pcb.

 

[emrr]

Gain is low enough here that resistor noise usually. isn't a problem, having said that the front end resistors contribute the most noise since they get the most gain afterward. I have rarely had to replace a resistor on one of these. Never say never. There are a few that are already precision types. He should note from the schematic the output caps are 4, but they really only need to be 2. Finding a good Q1 is definitely key.

 

[aaquilato]

Gain is low enough here that resistor noise usually. isn't a problem, having said that the front end resistors contribute the most noise since they get the most gain afterward. I have rarely had to replace a resistor on one of these. Never say never. There are a few that are already precision types. He should note from the schematic the output caps are 4, but they really only need to be 2. Finding a good Q1 is definitely key.

OK.
Any recommendations on Q1 replacement? I'm reading another blog on the Antique Radios Forum. The 2N1010 is not easily replaced it seems.

And earlier in this thread (#28) you stated... My personal units are all 71's with the modifications to meet the 72 spec (the 71A's having the larger output cap for flatter lows), and all have the UTC's.

Can you go into that as these will be ready for a mod.
Does that mean you increased the low end with your mod?

There's also notes about this from (#26)... The 6 transistor 'A' versions have less low end (smaller output caps) and a slightly less linear sound than the 4 transistor versions. They are a little smearier/browner. They sound a little more in the direction of lower feedback tube circuits, though that's a ridiculously general comparison. I keep some around (with larger output caps) for some guitar tracking, as the difference is worthwhile. I use the 4 transistor version for drums all the time, and bass pretty frequently.

Before this unit went on the bench it did work, noise and all. A U47 plugged into it sounded fantastic once I got the input up over the noise floor. I liked the sound and didn't feel it needed more low end.
I won't be tracking drums with this as I only have one. I certainly would try it for bass gtr but it would be a different flavor option for vocal and acoustic instrument tracking.

Thoughts?

 

[MidnightArrakis]

OK.
Any recommendations on Q1 replacement? I'm reading another blog on the Antique Radios Forum. The 2N1010 is not easily replaced it seems.

And earlier in this thread (#28) you stated... My personal units are all 71's with the modifications to meet the 72 spec (the 71A's having the larger output cap for flatter lows), and all have the UTC's.

Can you go into that as these will be ready for a mod.
Does that mean you increased the low end with your mod?

There's also notes about this from (#26)... The 6 transistor 'A' versions have less low end (smaller output caps) and a slightly less linear sound than the 4 transistor versions. They are a little smearier/browner. They sound a little more in the direction of lower feedback tube circuits, though that's a ridiculously general comparison. I keep some around (with larger output caps) for some guitar tracking, as the difference is worthwhile. I use the 4 transistor version for drums all the time, and bass pretty frequently.

Before this unit went on the bench it did work, noise and all. A U47 plugged into it sounded fantastic once I got the input up over the noise floor. I liked the sound and didn't feel it needed more low end.
I won't be tracking drums with this as I only have one. I certainly would try it for bass gtr but it would be a different flavor option for vocal and acoustic instrument tracking.

Thoughts?

Here's your -- RCA 2N1010 -- Germanium transistor replacement:

>>> From Digi-Key:



Hope this helps!!!

/

 

[aaquilato]

Thank you!

 

[aaquilato]

The board have been completely rebuilt with a Mullard AC107 transistor in the 2nd position and 2N2430s for the rest.
NOS Allen Bradley CC resistors and audio grade electrolytic capacitors.
My tech noted that the 2N2430 has better current handling and runs cooler...
Doing listening tests now.

 

[galensjp01]

Hi, sorry I was not clear - I have the full BA-72 manual as well They are nearly identical to the BA-71 but with some higher value caps and sometimes Kenyon transformers.

Mike

Hey! Just joined the forum as I’ve got a BA-72A I’ve been working on. Would you be able to PM me the manual/schematic you referenced in this post? Thank you!!

 

[galensjp01]

The front end is 2N2270's in the BA-31B/71B. The BA-72 update moved to 2N3391A, and a lot of BA-31/71 have been updated in the field. I took measurements of a 2N2270 BA-31B that had just been recapped, then again with a 2N3391A as Q1; noise improved 12dB below 500Hz, and about 6dB 1-16kHz. This after same result in a group of five BA-33B's. All had a grainy hash and shot noise before, totally gone. Well worth it. Not as big a deal with the lower gain of the 31/71, but an improvement.

Measured response on these again for the first time in years, looks like a straight line until you zoom in and go wide. Looking at 192kHz I get -1dB points of 10Hz and 32.4kHz. 1/2dB points of 12 and 21.2kHz.

Is there any benefit to replacing Q2-Q4 with 2N3391A as well? I have that in Q1 of my BA-72A but the rest are

Q2 - 2N404
Q3/Q4 - 2N2270

Reason I ask is that with drums, I’m still getting what feels like a “grainy” transient. Like there’s something missing in the low end/attack overall.

May swap to the 2200uf output cap and see if that helps too