RCA BA-73A Rack

[dmp]

I'm racking up a pair of RCA BA-73A amplifiers.

I completely re-capped both of them and had to replace some bad transistors. The 2N526 were bad in both. Once those were replaced, one of them was very noisy. I found all four of the 2N220s were noisy on the one unit by replacing with BC327 silicon transistors, one-by-one, starting at Q1. So the unit was then working well with 4 silicon transistors. The other unit was working with all original Germanium 2N220s.
Strangely, the BA73A with the 2n220 has some 120 Hz noise. The one with the silicon transistors does not and just shows uniform noise. This is testing with the units in the same position in the rack.
Then I got some NOS 2n220 and replaced the silicon transistors in the one unit, testing the noise after replacing each transistor. When I replace Q2 with the 2n220, the unit starts showing the 120 Hz noise.
I tried a different 2N220 in Q2 and switched back to the silicon and confirmed the noise shows whenever a 2N220 is in the Q2 spot.
Any ideas why this would be?

I'm using a toroid transformer with a typical 2 rail voltage rectifier, to get -30v (setting +15 to ground and the -15 to -30v). Then hooking up pin4 (-30v) for the BA73s and pin6 as ground. Pin 18 goes to the chassis ground. The PSU ground is going to the chassis ground. All the grounds in the BA73s test OK (cable shields, transformer cans)
When I recapped it I used 100uF for C4, C7, and C20 so I don't think it could be noise coming from the PSU.

 

[dmp]

Pics

 

[dmp]

The noise plots were measured with the BA73 set for about 60 dB gain.

 

[dmp]

After diving into this again, I'm not measuring the 120 Hz noise anymore. Not sure what resolved it. I re-set up the test cabling. But moving forward.
My power supply is currently getting pretty hot, supplying 600 mA to both units. Need a bigger heat sink.

I'm currently running these with a 5k pot and a bypass switch for the middle gain stage (Q3,Q4,Q5). This middle gain stage is about 30 dB, so in 'low' gain mode the preamp is 0-60dB. In high gain, 0-90dB. The measured noise is the same between modes, so switching out the middle stage doesn't improve noise performance at all. The 1st stage is responsible for the noise floor.

May build a stepped switch for gain, will try out it for awhile before making final decisions.

 

[dmp]

I removed the hi/low switch that bypassed the middle gain stage, since it didn't improve the noise performance, and put an -20 dB output pad switch in it's place, using 1k/220r/1k. The noise performance of these isn't a problem at all, measuring at -105 dB EIN.
The SMPS 48v phantom module is nice - first time using one. Cheap. I put a RCR on the board with it into a cap on the switch for soft start.
All that's left is to paint and label the front panel.

 

[mylesgm]

Interesting. Did you need to do anything to substitute the 2n220 with the BC327 or was it a straight drop in?

 

[dmp]

Interesting. Did you need to do anything to substitute the 2n220 with the BC327 or was it a straight drop in?

I subbed them to troubleshoot and they worked fine. No mods. I didn't even check the specs but I doubt they'd be overpowered.
I am curious why RCA used the 2n526 instead of all 2n270. I subbed 2n441 (I think?) for them and it worked fine as well.

 

[mylesgm]

Good to know! I have a mic preamp here with a single 2n220 stage and two of the channels have gone noisy but I’ve found it hard to get replacements.

 

[dmp]

I paid about $6 each for nos on eBay. Depends on if you want the 'germanium' sound.

 

[mylesgm]

I paid about $6 each for nos on eBay. Depends on if you want the 'germanium' sound.

Really? I’ll keep looking then. I really like the sound of this awa broadcast mixer so keeping original would be good.

 

[MidnightArrakis]

Good to know! I have a mic preamp here with a single 2n220 stage and two of the channels have gone noisy but I’ve found it hard to get replacements.

You might try this for a 2N220 direct replacement source:

https://www.nteinc.com/quickcross/


The datasheet is here:

https://www.nteinc.com/specs/100to199/pdf/nte102a.pdf
"Germanium Sound" is here:

Inventory:




I am also your "friendly" GroupDIY source for all of your mechanical chassis designs and PCB-layouts for your projects!!! YAY!!!

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[MidnightArrakis]

To those whom may be interested.....I just happened to find out that in a later version of this "RCA - BA73 Program Amplifier" (called the "BA-73D"), that the transistors used have been changed to be as the following:

Q1 & Q2 = 2N4250

Q3, Q4 & Q5 = 2N1189

Q6 = 2N526

Q7 = 2N1193

Q101 & Q102 = SK3014

Perhaps this information may help you in some way.

Attached is the "BA-73D" Program Amplifier Manual for your DIY enjoyment!!!

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[emrr]

To those whom may be interested.....I just happened to find out that in a later version of this "RCA - BA73 Program Amplifier" (called the "BA-73E"), that the transistors used have been changed to be as the following:

Q1 & Q2 = 2N4250

Q3, Q4 & Q5 = 2N1189

Q6 = 2N526

Q7 = 2N1193

Q101 & Q102 = SK3014

Perhaps this information may help you in some way.

Attached is the "BA-73E" Program Amplifier Manual for your DIY enjoyment!!!

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Addition of compensation caps around Q1-2 to consider as well. Might be more, that's what jumps out at a glance.

I have only rarely encountered 73D or 73E, I suspect they are a small minority. I wonder if there was an update addendum issued for the earlier models.

 

[MidnightArrakis]

Addition of compensation caps around Q1-2 to consider as well. Might be more, that's what jumps out at a glance.

I have only rarely encountered 73D or 73E, I suspect they are a small minority. I wonder if there was an update addendum issued for the earlier models.

[Addition of compensation caps around Q1-2 to consider as well] -- This would be -- in addition -- to whatever other circuitry and component value changes that RCA had done itself? If so, then why wouldn't RCA have also included "compensation caps" when they revised the earlier versions? I'm -- not -- a "Circuit Designer", so I wouldn't know about these types of things. Just asking.

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[emrr]

They are new items on the schematic you provided.

 

[MidnightArrakis]

Pics

DMP: Could you please be so kind and provide the following detailed information on your BA-73A Printed Circuit Board?.....

A) PCB board dimensions (Height & Width).

B) Mounting Hole location dimensions.

C) Mounting Hole diameter dimension.

Kind of like this:



THANKS!!!

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[MidnightArrakis]

They are new items on the schematic you provided.

GOT IT!!! It was around 4:00 AM or so when I came across the later version of the schematic, so I hadn't gotten around to checking what other possible differences there may have been. I'll check them both out possibly sometime later this week. THANKS!!!

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[emrr]

put an -20 dB output pad switch in it's place, using 1k/220r/1k.

Output or input? Output would confuse me since it's only rated for +24dBm into 600.

 

[dmp]

Addition of compensation caps around Q1-2 to consider as well. Might be more, that's what jumps out at a glance.

Interesting, I hadn't noticed that. Q1&Q2 subs are silicon, while Q3 remains Ge. I wonder if the subs needed compensation vs the 2n220.
Interesting they still have that 2n526 in Q6. I don't see what aspect of the design requires that one there.

DMP: Could you please be so kind and provide the following detailed information on your BA-73A Printed Circuit Board?.....

A. Board is 170mm x 82.5mm
B. holes are 160mm x 72.5mm (5mm in from each edge)
C. bolts are 4-40

 

[dmp]

Output or input? Output would confuse me since it's only rated for +24dBm into 600.

Currently I have a -20dB input and a -20 output pad. The output pad is because I already drilled a hole for the hi/low gain switch, and still wanted a way to control the high gain somehow. I'm still not 100% happy with it. Maybe I'll build a stepped gain switch to sub for the 5k pot.
I want to spend a little time using/listening and particularly experiment with it pushed into distortion.