Jamo AVR-793 Ceramic Caps for Outputs - diyAudio
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Old 26th March 2015, 07:17 AM   #1
nzst205 is offline nzst205  New Zealand
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Join Date: Sep 2013
Location: New Zealand
Question Jamo AVR-793 Ceramic Caps for Outputs

A newbie question here, I am trouble shooting my Jamo AVR-793 Amp and found that the small ceramic caps across the the speaker outputs are shot.
When removed from the board they measure between 35nf and 10nf (Fluke 289), I assume they should be 15nf as they are marked 153K (Green ceramic caps).
The schematic I have for the Jamo AVR-790 (cannot locate a circuit for the AVR-793) doesn't show any such caps at the speaker output.
My first question is I assume these just remove DC from the output, is that correct.
Secondly Are these small Ceramic Caps necessary, given they don't appear on the circuit diagram. Interesting they were glued to the board, presumable to stop vibration.
Any and all help appreciated
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Old 26th March 2015, 08:06 AM   #2
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Join Date: Apr 2010
Location: Coffs Harbour
First, I don't pretend to be familiar with this amplifier (Presumably its a home theatre multichannel affair?)

15 nF is quite small in the output stage and these caps will be for shorting out RF or other HF instability arising from the speakers and leads etc. The actual value of the capacitor won't be too critical and is unlikely to affect HF response of the speakers. 35 nF is certainly high relative to a marked value of 15nF but not critical, nor is 10nF a problem. Remove the speaker leads or preferably remove and test these parts out of circuit if you want to be accurate. Replace with similar grade ceramics of ≥ the marked voltage rating (not all ceramic caps are the same type) and if in doubt, use MKP (polypropylene) or MKS (polystyrene) film caps if space permits.

Be wary using a DMM for small capacitors as long leads and surfaces they may cross affect measurements. The glue used is probably an assembly aid as the caps may not have been available in SMD or possibly an afterthought for a design that proved to be susceptible to RF.
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