[Al Dente]

I'm replacing a bunch of electrolytic caps on an active speaker board and I found a discrepancy. The schematic calls for a 4.7uf 50v cap but in it's place is a 2.2uf 50v cap. This board is in original condition and was never recapped. The other speaker also has the same situation.

The speakers worked fine this way for over twenty years until one recently failed. So what should I go by for cap replacement? Is this difference in rating significant and what could be the effect if either one is used?

[FullRangeMan]

I'm replacing a bunch of electrolytic caps on an active speaker board and I found a discrepancy. The schematic calls for a 4.7uf 50v cap but in it's place is a 2.2uf 50v cap. This board is in original condition and was never recapped. The other speaker also has the same situation.

The speakers worked fine this way for over twenty years until one recently failed. So what should I go by for cap replacement? Is this difference in rating significant and what could be the effect if either one is used?

If it worked fine 20 years I would use the 2.2uF value.
The circuit may be modified at the factory after the PCB printed.
For an analysis of the merit of the question is necessary study the effort of this component among other facts.

[DannyBadorine]

I would definitely replace with the same value.  I'm guessing that they changed the driver and it might have a different impedance than what the schematic says.  If you used the 4.7uf capacitor then you could be drastically changing the crossover point depending on where that cap is in the circuit. 
If you tell me more about the speaker and show the schematic then I could try to tell you what would change if you use the 4.7uf cap.

[richidoo]

It only matters if the capacitor is acting as a filter in the audio band, like crossover filter, or feedback phase compensation.

If it's part of high freq noise filter, or a coupling capacitor or part of power supply then larger value will work fine. I would go with the value in the schematic.

More likely explanation is they ran out of 4.7s or problem with them, so engineer substituted 2.2s for temporary. Who knows why.

It wont hurt to try 4.7. If it sounds weird go back to 2.2. They are very cheap. Nichicon low leakage sounds excellent. Make sure new parts will physically fit before ordering, and that the temperature rating and voltage rating of new caps are equal or higher value than the old parts.

[Mike B.]

I would replace the cap with the same value (2.2). Use good quality film caps.

[S Clark]

I've also seen factory caps that didn't match the schematic on a Victor turntable TT-81.  I used the value of the cap on the board rather than the value on the schematic.  I'd bet a change was made in production and it didn't get changed back in the files.   

[Al Dente]

Thanks for the very valid opinions in choosing either cap, it puts me back flipping a coin.

If you used the 4.7uf capacitor then you could be drastically changing the crossover point depending on where that cap is in the circuit. 
If you tell me more about the speaker and show the schematic then I could try to tell you what would change if you use the 4.7uf cap.

The Speaker is an active Beolab 4000. I uploaded image of the whole schematic but somehow the resolution gets reduced to it being illegible so i uploaded 4 sections. I hope it's not too confusing. Is there any way to upload images here without loosing resolution? Is the image resolution being limited on the posts?












 

[FullRangeMan]

Perhaps the value of this cap was modified by the tech guy at the time of assembly because of an adjustment due to the sensitivity of the speaker, as Snell used to do.

[richidoo]

It’s not xo filter nor feedback phase compensation.

The pics are only 640x490 which limits the detail. There’s enough visible to see that it doesnt matter which cap.

EDIT: According to the datasheet for chip amp ic, the cap in question is part of a ripple filter to damp power dissipation when muting circuit is activated. The circuit is similar to but more complex than the mfg sample circuit in the data sheet. Like I said, it’s not a signal filter so the size is not critical, it just prevents oscillation on muting.

[Al Dente]

Thanks again all for sharing your knowledge.

I decided to use the 2.2uf, right or wrong, because it's been there sounding good all these years having a known result. The 4.7uf even though specified has an unknown result.

Another thought is that the board has (7) other 2.2uf specified on it and this is the only (1) 4.7uf called for. Maybe they decided to consolidate and use 8  2.2uf.

Anyway the speaker is back together with (22) new caps and sounds great.

The schematic pic was 2653 x 1869 when I selected it to upload. I think pics are automatically reduced by this site when they are uploaded to a post.

[richidoo]