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What sort of cap is this ?
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Arfa Daily
Jun 18, 2015, 9:31:32 PM6/18/15
to
I currently have a Peavey PVX-P12 amp chassis on the bench. There was no
audio at all from either the HF or LF digital output ICs. The IC in question
has a 'go / mute / standby pin and this pin on both ICs is connected to a
little control circuit. That in turn is driven by the "AC Detect" circuit on
the power supply. This bit of circuitry is very straightforward, and
comprises a bridge fed with AC from the line input, via a 0.47 uF 275 v ~
x-class cap. Across the output of the bridge, are two caps in parallel,
shown on the schematic as being 22uF at 25 v working. But here's the thing.
They are not shown as being polarised, nor is there any marking that I can
see on the caps themselves, that indicate any polarity. They are surface
mount and small - approx 3.6 x 2.6 x 1.8 mm - and look just like a typical
sm ceramic cap. Pink-y glazed body with conventional 'end caps'. One of
these two capacitors is short circuit. With it removed, everything returns
to normal, and the amps both un-mute. The circuit only produces a few volts,
and this is used to drive the LED in an opto via a 47 ohm R, the transistor
side of the opto being the "AC Detect" signal that connects to the mute
control circuit on the amp board.
So what type of caps are these ? They seem awfully small for any kind of
solid dielectric non-polarised cap of that value, and that sort of voltage
rating. I've had a look around at a few component supplier's offerings, and
can't find anything that seems to match. I checked the capacitance of the
one that isn't short, and it came up at 18 uF, so a bit low, but in the
ballpark of what it says on the schematic. Given that these caps only seem
to serve as the filter for the bridge output, and that there is only a few
volts across them in normal operation, can anyone see any potential problems
with replacing them with a pair of 'conventional' 22 uF 16 v polarised
tants, obviously taking care to put them the right way round ?
Arfa
audio at all from either the HF or LF digital output ICs. The IC in question
has a 'go / mute / standby pin and this pin on both ICs is connected to a
little control circuit. That in turn is driven by the "AC Detect" circuit on
the power supply. This bit of circuitry is very straightforward, and
comprises a bridge fed with AC from the line input, via a 0.47 uF 275 v ~
x-class cap. Across the output of the bridge, are two caps in parallel,
shown on the schematic as being 22uF at 25 v working. But here's the thing.
They are not shown as being polarised, nor is there any marking that I can
see on the caps themselves, that indicate any polarity. They are surface
mount and small - approx 3.6 x 2.6 x 1.8 mm - and look just like a typical
sm ceramic cap. Pink-y glazed body with conventional 'end caps'. One of
these two capacitors is short circuit. With it removed, everything returns
to normal, and the amps both un-mute. The circuit only produces a few volts,
and this is used to drive the LED in an opto via a 47 ohm R, the transistor
side of the opto being the "AC Detect" signal that connects to the mute
control circuit on the amp board.
So what type of caps are these ? They seem awfully small for any kind of
solid dielectric non-polarised cap of that value, and that sort of voltage
rating. I've had a look around at a few component supplier's offerings, and
can't find anything that seems to match. I checked the capacitance of the
one that isn't short, and it came up at 18 uF, so a bit low, but in the
ballpark of what it says on the schematic. Given that these caps only seem
to serve as the filter for the bridge output, and that there is only a few
volts across them in normal operation, can anyone see any potential problems
with replacing them with a pair of 'conventional' 22 uF 16 v polarised
tants, obviously taking care to put them the right way round ?
Arfa
Phil Allison
Jun 18, 2015, 10:56:06 PM6/18/15
to
Arfa Daily wrote:
>
> I currently have a Peavey PVX-P12 amp chassis on the bench. There was no
> audio at all from either the HF or LF digital output ICs. The IC in question
> has a 'go / mute / standby pin and this pin on both ICs is connected to a
> little control circuit. That in turn is driven by the "AC Detect" circuit on
> the power supply. This bit of circuitry is very straightforward, and
> comprises a bridge fed with AC from the line input, via a 0.47 uF 275 v ~
> x-class cap. Across the output of the bridge, are two caps in parallel,
> shown on the schematic as being 22uF at 25 v working. But here's the thing.
> They are not shown as being polarised, nor is there any marking that I can
> see on the caps themselves, that indicate any polarity. They are surface
> mount and small - approx 3.6 x 2.6 x 1.8 mm - and look just like a typical
> sm ceramic cap. Pink-y glazed body with conventional 'end caps'. One of
> these two capacitors is short circuit. With it removed, everything returns
> to normal, and the amps both un-mute. The circuit only produces a few volts,
> and this is used to drive the LED in an opto via a 47 ohm R, the transistor
> side of the opto being the "AC Detect" signal that connects to the mute
> control circuit on the amp board.
>
> So what type of caps are these ? They seem awfully small for any kind of
> solid dielectric non-polarised cap of that value, and that sort of voltage
> rating. I've had a look around at a few component supplier's offerings, and
> can't find anything that seems to match.
** I found lots - eg:
>
> I currently have a Peavey PVX-P12 amp chassis on the bench. There was no
> audio at all from either the HF or LF digital output ICs. The IC in question
> has a 'go / mute / standby pin and this pin on both ICs is connected to a
> little control circuit. That in turn is driven by the "AC Detect" circuit on
> the power supply. This bit of circuitry is very straightforward, and
> comprises a bridge fed with AC from the line input, via a 0.47 uF 275 v ~
> x-class cap. Across the output of the bridge, are two caps in parallel,
> shown on the schematic as being 22uF at 25 v working. But here's the thing.
> They are not shown as being polarised, nor is there any marking that I can
> see on the caps themselves, that indicate any polarity. They are surface
> mount and small - approx 3.6 x 2.6 x 1.8 mm - and look just like a typical
> sm ceramic cap. Pink-y glazed body with conventional 'end caps'. One of
> these two capacitors is short circuit. With it removed, everything returns
> to normal, and the amps both un-mute. The circuit only produces a few volts,
> and this is used to drive the LED in an opto via a 47 ohm R, the transistor
> side of the opto being the "AC Detect" signal that connects to the mute
> control circuit on the amp board.
>
> So what type of caps are these ? They seem awfully small for any kind of
> solid dielectric non-polarised cap of that value, and that sort of voltage
> rating. I've had a look around at a few component supplier's offerings, and
> can't find anything that seems to match.
http://www.newark.com/murata/grm32er61e226me15l/ceramic-capacitor-22uf-25v-x5r/dp/24R6350
> I checked the capacitance of the
> one that isn't short, and it came up at 18 uF, so a bit low, but in the
> ballpark of what it says on the schematic. Given that these caps only seem
> to serve as the filter for the bridge output, and that there is only a few
> volts across them in normal operation, can anyone see any potential problems
> with replacing them with a pair of 'conventional' 22 uF 16 v polarised
> tants, obviously taking care to put them the right way round ?
>
The SMD caps are cheaper than tants of the same ratings.
.... Phil
N_Cook
Jun 19, 2015, 2:49:33 AM6/19/15
to
the required capacitance. Any polarising mark on the SM carrier strip
perhaps
Arfa Daily
Jun 19, 2015, 3:49:29 AM6/19/15
to
"Phil Allison" <palli...@gmail.com> wrote in message
news:d1c37f77-0764-47d7...@googlegroups.com...
company as Newark, and these caps did not appear. But sticking the Murata
part number into the Farnell search engine - there they are. Just goes to
show that what I have always said - that Farnell's product searcher is
useless - has been borne out again ...
>
>
>
>> I checked the capacitance of the
>> one that isn't short, and it came up at 18 uF, so a bit low, but in the
>> ballpark of what it says on the schematic. Given that these caps only
>> seem
>> to serve as the filter for the bridge output, and that there is only a
>> few
>> volts across them in normal operation, can anyone see any potential
>> problems
>> with replacing them with a pair of 'conventional' 22 uF 16 v polarised
>> tants, obviously taking care to put them the right way round ?
>>
>
> ** Should work fine.
Yeah, I thought so. The guy is hoping to use it tonight so I think I will
>
>
>> I checked the capacitance of the
>> one that isn't short, and it came up at 18 uF, so a bit low, but in the
>> ballpark of what it says on the schematic. Given that these caps only
>> seem
>> to serve as the filter for the bridge output, and that there is only a
>> few
>> volts across them in normal operation, can anyone see any potential
>> problems
>> with replacing them with a pair of 'conventional' 22 uF 16 v polarised
>> tants, obviously taking care to put them the right way round ?
>>
>
> ** Should work fine.
just go with the tants
Arfa
Arfa Daily
Jun 19, 2015, 4:04:09 AM6/19/15
to
>
> Why would they have to be unpolarised? Just 2 paralleled together to get
> the required capacitance. Any polarising mark on the SM carrier strip
> perhaps
ones that Phil has found. I think it's just because they *are*, if you see
what I mean. To be honest, I never knew that you could get multilayer
ceramics in such high values. Presumably, they don't go up as high as 47 uF
which the designer felt he needed, so he just put 2 x 22 uF in parallel
instead. As I said, the circuit is absolutely un-critical in that it only
needs to produce a small amount of DC from the incoming mains to power the
LED in the opto that produces the muting signal. Because of time constraints
with ordering-in those ceramics, I think I'm just going to go with a pair of
small tants, fitted to observe their polarity.
Arfa
John-Del
Jun 19, 2015, 6:12:50 AM6/19/15
to
On Friday, June 19, 2015 at 4:04:09 AM UTC-4, Arfa Daily wrote:
> >
> I don't think that they *do* have to be un-polarised, as such, given the
> ones that Phil has found. I think it's just because they *are*, if you see
> what I mean.
That's correct. They are not polarized because that's the technology. Electrolytics are generally polarized because that's the tradeoff in the technology at the time. You can always use a non polarized cap in place of a polarized.
> >
> I don't think that they *do* have to be un-polarised, as such, given the
> ones that Phil has found. I think it's just because they *are*, if you see
> what I mean.
>To be honest, I never knew that you could get multilayer
> ceramics in such high values.
Because of time constraints
> with ordering-in those ceramics, I think I'm just going to go with a pair of
> small tants, fitted to observe their polarity.
N_Cook
Jun 19, 2015, 6:36:34 AM6/19/15
to
MLCC. Was it actually dead short or very low resistance. I suspect the
usual metal migration failure mode of MLCC , then paralling-up just
increases the chance of failure , for the circuit. Someone mentioned
paralling ten , asking for trouble.
N_Cook
Jun 19, 2015, 7:01:15 AM6/19/15
to
On 19/06/2015 09:04, Arfa Daily wrote:
>
>
>
>>
>
>
>
>>
Arfa Daily
Jun 19, 2015, 9:10:33 PM6/19/15
to
>
> So to get that capacity in that package , they must be seriously multi
> MLCC. Was it actually dead short or very low resistance.
consider it to be 'dead short', anyway ...
Arfa
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