Question for RCA BA-31A experts

[madshamanstudios]

Hi all!
Happily, I have 1 BA-31A module working and sounding great! I'd like to thank everyone for their kind and helpful responses, and in particular I'd like to thank Mike for his encouragement and feedback.

Unhappily, the 2nd module I bought is not functional.
It powers on and puts out a low hum.
I used a multi-meter to check the I/O that I built. And the IEC/power is working fine.

Can anyone direct me on the proper way to test this device's signal path?
How can I use the multi-meter to check the signal path?

Attached is a photo of the happy healthy module which sounds super!
Hope to figure this one out - halp!

 

[mjrippe]

Congrats! Re-cap the 2nd one and I can 98% guarantee it will work

 

[madshamanstudios]

Hey belated Happy New Year! I did a complete re-cap of the BA-31 that wasn't working. It sounds really similar tonally to the working one. However, its output is over 20db lower.

- Is it possible I melted a capacitor? It was a few caps until I realized I was making them quite hot and I needed to solder them in faster. Wonder if I should re-cap again. I do have extra capacitors just in case.

- I see one resistor that is looking pretty melted. Should I try to replace that?

Any other thoughts on how this could be figured out? It sounds exactly the same tonally, but just quiet. I re-soldered the I/O just to confirm I had that right, so there are no outstanding connectivity issues.

 

[substitute]

I'd check the gain strap on the quieter unit first, then make sure all the test voltages around the transistors are right. You can compare voltages to the schematic but it might be more meaningful to compare unit to unit.
If that's not the issue feed a test tone into both units and scope the signal at each gain stage comparing the good unit to the quiet one. That should zero in on your problem area.

 

[mjrippe]

Also check the transformer wiring. These can be 150 or 600 ohm in or out and different combinations will give you different overall gain.

You can measure the resistor in circuit compared to the good one also.

 

[madshamanstudios]

Hey Mike! Thanks for your reply. It was a good exercise. I went through the resistors and transistors. All seemed to have a little less resistance than the working model, but only by about 5 ohms, nothing jumping out. I replaced the early re-caps I did when I didn't understand how hot I was letting them get. Eureka! I have normal output volume. BUT another issue arises from this module: a constant crackle that jumps up and down, between -40db and -30db in the DAW. Any thoughts on this sir? I've uploaded the crackle noise.

 

[madshamanstudios]

I did notice that R9 is getting quite hot. I see it's connected to a recapped C10. I think I'll redo the rest of the recaps to start.

 

[madshamanstudios]

I'd check the gain strap on the quieter unit first, then make sure all the test voltages around the transistors are right. You can compare voltages to the schematic but it might be more meaningful to compare unit to unit.
If that's not the issue feed a test tone into both units and scope the signal at each gain stage comparing the good unit to the quiet one. That should zero in on your problem area.

Thank you! This was an extremely helpful suggestion as I learned to compare between the units. The transistors did show slightly different values, but replacing them with ones from my working model showed no change. I think its transistors are not the issue.

I completed the re-do of the recap, and still in this new situation - plenty of volume, but a crackle between -40db and -30db on the fader in the DAW.

Should I start looking at the resistors? R9 was getting unusually hot. I know it connects to the power source so I wonder if the problem is in the power section.