Confusion Probing RCA Ba-31, also confusion probing in general.

[substitute]

Hello, I recently recapped 4 rca BA-31's for some one, when I gave them back they were working fine but now two are behaving poorly. I'm getting confused trying to trace the signal with a scope, on the good units I can see the my test tone going in at the base of Q1 and coming out at C12. I can even see it at the base of Q3 but I loose it at C6. It's obvious the signal is there, but I don't understand why I can't see it on my scope in that path between Q1 and Q2. Any explanations? It'd be helpful if I could trace the entire signal paths to help diagnose the problematic units.

http://www.waltzingbear.com/Schematics/RCA/BA-31A.htm

 

[emrr]

Ba-31A?  Or B/C?  Very different. 

 

[mjrippe]

As Doug mentioned, you posted the schematic for the 31A but these are pretty rare and many were converted to B spec back in the day.  If you have six transistors on the main pcb then it is a genuine A revision.  If there are four then it is a B or C.

Also useful to note, unless modded these modules use a negative 30v supply so ground is positive.

 

[substitute]

Hey guys, they're BA-31B's (4 transistors). I know that link to the schematic says "A" but the link must be mislabeled or something, the schematic is accurate, it shows four transistors.

 

[abbey road d enfer]

Hey guys, they're BA-31B's (4 transistors). I know that link to the schematic says "A" but the link must be mislabeled or something, the schematic is accurate, it shows four transistors.

then you say "I can even see it at the base of Q3 but I loose it at C6."
Something wrong there. C6 is before Q3 in the signal path, there's a gain stage between them. No wonder signal at C6 is small.

 

[substitute]

What I'm asking is really an oscilloscope question. So that bit of path around c6, why wouldn't I be able to see signal there? I don't mean what is wrong with the RCA (it works fine), but what am I not understanding about how my scope works? I've tried it set to both AC and DC coupling.

 

[emrr]

you should certainly be able to see it there, I've no idea. 

 

[abbey road d enfer]

What I'm asking is really an oscilloscope question. So that bit of path around c6, why wouldn't I be able to see signal there? I don't mean what is wrong with the RCA (it works fine), but what am I not understanding about how my scope works? I've tried it set to both AC and DC coupling.

You don't see it because it's very small. You must increase the sensitivity and they you may see it, but probably with some noise.

 

[substitute]

Interesting, I was not expecting the voltage to be lower coming out of that first transistor. Setting my scope as sensitive as possible and increasing the output of my function generator got me a visible signal, a little clipped, but good enough. From there I was able to determine that on the non-working unit that first transistor is bad. I checked the voltage measurements on it before all this, they're different than the working units but pretty close to the values in the schematic so I wasn't certain it was bad.

Thanks

 

[waltzingbear]

its old, but it should be answered.
Because that is a current node, not a voltage node. These are transistors which are inherently current devices, not FETs or valves which are inherently voltage devices. I recommend any standard basic electronics textbook.

I don't know about what version is what, but that schematic is from my original RCA documentation.

 

[substitute]

Thank you wb for the excellent explanation. I suspected it was something like that, even after all these years as a hobbyist I still don't understand current in a very real way like I understand voltage.