Capacitor function in BX20

[BGroen]

Hi guys,

We just bought a BX20 and noticed one capacitor is missing (C49). The unit still works so I don't think it is essential.
On google I can see more units have this one missing.

C49 should be a styroflex type. I've read those are more fragile to maybe that's why it's missing.

In the snippet below you can see part of the circuit. IC3 according to the manual is a uA 709C. Is this equivilant as a LM709? If so, pin 5 is the output frequency compensation.

C3 = 68pF
R31 = 10k
C49 = 15 pF

My question is more general than this specific example (but I can ofcourse apply the awnser immediately ):

What is happening in this little R/C filter circuit with C49, C3 and R31? Do the values really matter?
I've got ceramic capactitors 10 and 22 pF laying around and I am thinking of replacing C49 with these values. Should I better use the smaller or bigger one?

 

[ruffrecords]

It looks like stability compensation for the op amp. You should find details in its data sheet.
Cheers

Ian

 

[BGroen]

Hi Ian,

Looking at the BX20 schematic I guess it is an amplifier as it sits just before the output transformer and both inputs are used and both inputs are connected to each other via a trimmer.

The 709 datasheet shows the capacitor total should be 200 pF. Note that this is a 'modern' datasheet, I don't know if it corresponds to the actual component in the BX20.

68 pF and 15 pF in series (as in BX20) makes approx. 12 pF.
68 pF and 22 pF in series would make approx 16 pF.

Now the 15 pF capacitor is simply omitted. I do not know the purpose of R31 in the BX20 schematic.

All of this makes me to think that putting in the 22 pF it is at least an improvement as to it is now.

 

[zamproject]

noticed one capacitor is missing (C49)

It's usual that designers let extra component and footprint options on a pcb.

Probably they thought a second C in parallel to R might be necessary for the compensation, or it depending of opamp used, but finally not, so they don't populate this on on pcb.

I have no motivation to open my BX20 right now, but there is chance this capacitor is not populated either.

If everything is OK on your BX20, just let it as it is.

 

[BGroen]

That would also explain why there are so many examples online without the capacitor.
Weirdly channel 1 is populated with one, but later cut off. It still 'hangs' there with only 1 leg connected.

For now I'll leave it like it is. If any one else has additional info that is much appreciated!

Thanks Ian and Zam!

 

[Philbo King]

If you use a ceramic one make sure it is a zero temp coefficient type. NP0

 

[BGroen]

I found some additional info in the manual.

 

[zamproject]

In doubt, just check with scope for oscillations.