Question for RCA BA-31A experts

[substitute]

Grounding - I'm assuming you're going to put this in some sort of metal chassis.

• Connect the ground pin of your IEC or power cord to the chassis
• Connect pin on H on the power header to chassis - I think the other pins shown as ground are connected internally but you should buzz them out to confirm.
• For the I/O I can't remember - looks like pin A on the audio header gets grounded (to chassis)

Gain strap - pins 4 -6 aren't on the transformers, there are two little wire terminals on the audio pcb around R4 and R7, you just connect them with a bit of wire

Regarding pads and what not - whatever floats your boat. When I had a pair of my own I did the variable gain mod and like emrr didn't use it much. I worked on a rack of four for someone recently, they asked for the output attenuator - it's cheap and easy and works well, couldn't say if they use it or not.

I can't remember feeling like any particular mics really came alive with the BA31, but they do a midrange thing on guitars I liked a lot.

 

[mjrippe]

Sorry for this noob question, just trying to learn and confirm before taking actions... I see two numbers on the schematic for capacitors. For example C11 reads 35 / 50V. Would that be read 3.5mfd and 50V, and would I be looking for a radial capacitor?

That would be 35mfd, not 3.5, and 50V. Those tall black cylinders are the caps, so a modern radial cap will fit nicely. As for the better low end question, increasing the value of caps in the audio path can help with this. Mainly the output caps (C11, C12, C13, C14) could be 150mfd or even 220mfd. Some of the original values like the 70mfd will not be available these days anyhow. For those, go up to the next common value (100mfd). The exceptions would be that 50mfd can be replaced with 47mfd, and 25 with 22. Voltage ratings should be the same or higher.

Here is a list of the cap values I would use. They are all commonly available and I increased the output and PSU cap sizes.

RCA BA-31A or BA-71A Cap List (6 transistor germanium boards)

C1, C3, C4: 100/16V
C2, C8: 47/16V
C7: 150/6V
C9: 10/25V
C10: 47/25V
C11, C12, C13, C14: 150 or 220/25V (increased values)
C15, C16: 100/16V

Power Supply (BA-31A only, increased values)
C201: 470/50V
C202, C203, C204: 47/50V

 

[emrr]

Those 4 output caps are really only 2, doubled up due to size constraints at the time. No need to stuff 4 caps.

31B value was 100mfd. 72A value, 1500mfd. Translating to 31A that's 750mfd x 2. Go 470 or 680 at least.
Same output transformer relationship.

 

[madshamanstudios]

Grounding - I'm assuming you're going to put this in some sort of metal chassis.

• Connect the ground pin of your IEC or power cord to the chassis
• Connect pin on H on the power header to chassis - I think the other pins shown as ground are connected internally but you should buzz them out to confirm.
• For the I/O I can't remember - looks like pin A on the audio header gets grounded (to chassis)

Gain strap - pins 4 -6 aren't on the transformers, there are two little wire terminals on the audio pcb around R4 and R7, you just connect them with a bit of wire

Regarding pads and what not - whatever floats your boat. When I had a pair of my own I did the variable gain mod and like emrr didn't use it much. I worked on a rack of four for someone recently, they asked for the output attenuator - it's cheap and easy and works well, couldn't say if they use it or not.

I can't remember feeling like any particular mics really came alive with the BA31, but they do a midrange thing on guitars I liked a lot.

Thank you so much for this info!

Yes, curious to see if I get more of a ribbon type result w my condenser, but anyway I do a lot of guitar.

Hmm.. Possible to leave these out of chassis and solder I/O breakout cables and IEC but still have proper grounding? I've seen these around with just breakout cables.

 

[madshamanstudios]

That would be 35mfd, not 3.5, and 50V. Those tall black cylinders are the caps, so a modern radial cap will fit nicely. As for the better low end question, increasing the value of caps in the audio path can help with this. Mainly the output caps (C11, C12, C13, C14) could be 150mfd or even 220mfd. Some of the original values like the 70mfd will not be available these days anyhow. For those, go up to the next common value (100mfd). The exceptions would be that 50mfd can be replaced with 47mfd, and 25 with 22. Voltage ratings should be the same or higher.

Here is a list of the cap values I would use. They are all commonly available and I increased the output and PSU cap sizes.

RCA BA-31A or BA-71A Cap List (6 transistor germanium boards)

C1, C3, C4: 100/16V
C2, C8: 47/16V
C7: 150/6V
C9: 10/25V
C10: 47/25V
C11, C12, C13, C14: 150 or 220/25V (increased values)
C15, C16: 100/16V

Power Supply (BA-31A only, increased values)
C201: 470/50V
C202, C203, C204: 47/50V

This is extremely helpful! I did think 35mfd, but couldn't find a 35/50V - those alternate (available) values help a lot, thank you.

 

[mjrippe]

Those 4 output caps are really only 2, doubled up due to size constraints at the time. No need to stuff 4 caps.

31B value was 100mfd. 72A value, 1500mfd. Translating to 31A that's 750mfd x 2. Go 470 or 680 at least.
Same output transformer relationship.

True, but less confusing to fill all the holes. I've only been in a couple "A" versions, so I defer to Doug here. I don't know if the germaniums respond differently to capacitance. Of all the other variants I have seen 100, 150, 500, and 1500 fitted from the factory. Generally, if you have 470 or more on the output you should be in good shape.

 

[madshamanstudios]

Those 4 output caps are really only 2, doubled up due to size constraints at the time. No need to stuff 4 caps.

31B value was 100mfd. 72A value, 1500mfd. Translating to 31A that's 750mfd x 2. Go 470 or 680 at least.
Same output transformer relationship.

470 or 680... per cap (C11-C14) at to apply well over 1500 mfd at 25V?
Or do you mean just use 2 caps and run up to around 1360 mfd (680 x2) at 25V?
pm'ed you also!

 

[madshamanstudios]

That would be 35mfd, not 3.5, and 50V. Those tall black cylinders are the caps, so a modern radial cap will fit nicely. As for the better low end question, increasing the value of caps in the audio path can help with this. Mainly the output caps (C11, C12, C13, C14) could be 150mfd or even 220mfd. Some of the original values like the 70mfd will not be available these days anyhow. For those, go up to the next common value (100mfd). The exceptions would be that 50mfd can be replaced with 47mfd, and 25 with 22. Voltage ratings should be the same or higher.

Here is a list of the cap values I would use. They are all commonly available and I increased the output and PSU cap sizes.

RCA BA-31A or BA-71A Cap List (6 transistor germanium boards)

C1, C3, C4: 100/16V
C2, C8: 47/16V
C7: 150/6V
C9: 10/25V
C10: 47/25V
C11, C12, C13, C14: 150 or 220/25V (increased values)
C15, C16: 100/16V

Power Supply (BA-31A only, increased values)
C201: 470/50V
C202, C203, C204: 47/50V

Just double checking C7....is that 6V supposed to be 16V, or is 6V correct?

 

[madshamanstudios]

True, but less confusing to fill all the holes. I've only been in a couple "A" versions, so I defer to Doug here. I don't know if the germaniums respond differently to capacitance. Of all the other variants I have seen 100, 150, 500, and 1500 fitted from the factory. Generally, if you have 470 or more on the output you should be in good shape.

Why the increased values in power supply, nearly doubling C1 mfd? Is that just just based on availability of radial caps or is that also improved performance?

 

[mjrippe]

I'll try to get all those in one reply

C11/12 and C13/14 are pairs. In order to use fewer caps you could just double the value and insert one cap for each. Remove C11&C12, replace with a single 470 or 680mfd. Do the same for C13&C14.

C7 is shown as 6V on the schematic (attached, if you don't already have it). A 16V would work fine and may be more common.

The higher value PSU caps are for better filtering of AC hum. Modern caps are smaller and will fit just fine.

 

[madshamanstudios]

I'll try to get all those in one reply

C11/12 and C13/14 are pairs. In order to use fewer caps you could just double the value and insert one cap for each. Remove C11&C12, replace with a single 470 or 680mfd. Do the same for C13&C14.

C7 is shown as 6V on the schematic (attached, if you don't already have it). A 16V would work fine and may be more common.

The higher value PSU caps are for better filtering of AC hum. Modern caps are smaller and will fit just fine.

Man you guys are so kind and so good. Thank you, this helps immensely.

 

[madshamanstudios]

Anyone able to identify the material make-up of these capacitors? Polymer?

For recapping, I'm going for some increased low-end with updated mfd and V, but skipping out on the approach of vying for increased fidelity (by applying say ELNA Silmic IIs), and instead looking to see if the original material is available new. Feel free to also tell me why this is a terrible or perhaps impossible idea!

 

[mjrippe]

They are electrolytics, generic. I use Nichicon just because I always have and they always work.

 

[madshamanstudios]

They are electrolytics, generic. I use Nichicon just because I always have and they always work.

Gotcha, ok ordering up the replacement caps...but hey - any thoughts on C5 and C6? They weren't in your list but they're on schematic at what looks like 470pF, and I wonder what V rating. I'm also having trouble physically finding them visually.

 

[Walrus]

C5/6 are not electrolytic caps, and are very unlikely to need changing.

 

[madshamanstudios]

C5/6 are not electrolytic caps, and are very unlikely to need changing.

Thanks you that makes sense!

 

[mjrippe]

Gotcha, ok ordering up the replacement caps...but hey - any thoughts on C5 and C6? They weren't in your list but they're on schematic at what looks like 470pF, and I wonder what V rating. I'm also having trouble physically finding them visually.

As Walrus said, no need to replace them. Finding them visually on the PCB? Use the component diagram I posted

 

[madshamanstudios]

As Walrus said, no need to replace them. Finding them visually on the PCB? Use the component diagram I posted

Def using that schematic, hence my wondering about those caps (thinking they were radial electrolytic caps). Every day I learn a little more, thanks to you and others on this thread. The BA-31A is in the mail to me, so I'm using the really limited photos to reference the schematic as I prep the BOM. (I actually bought a pair of them, both looking the same condition and life story, but I only plan to keep one.) Any thoughts on a good chassis to rack a single module?

 

[madshamanstudios]

Hello BA-31ers!
Can anyone answer this BA-31 schematic question?
The schematic says "arrows...indicate signal polarity" See attached - Does this mean C is negative and E is positive / F is negative and D is positive?
Don't want to get positive and negative mixed up in soldering!

 

[ruffrecords]

Presumably it means the hot input or output of a balanced circuit.

Cheers

Ian