Question about SX202 for techies

[Dan]

Hi all,

I've got a bad capacitor in my SX202 (C4 in the schematic, if you have
it), but I don't recognize what type of capacitor it is. I'm pretty
new to this stuff. I've been Googling for many hours, and have yet to
get a really good answer. Anyway, it looks like a smallish minty
green resistor. The band pattern is gold-green-reddish
brown-silver-silver. Everything I've been able to find suggests that
gold should not be at the beginning of the banding pattern, but I'm
quite sure that it's really gold at one end and silver at the other.
The schematic shows a .001uF capacitor there, which is completely out
of whack with the banding pattern anyway. My questions are:

1. What type of capacitor is this thing? What would be the best new
one to replace it with?

2. What the heck is going on with the banding pattern? There are
four identical capacitors (C1 - C4), all of which have the same
pattern.

3. Should I go with the value from the schematic, assuming that I
can't figure out what the actual caps say? Question 4 might change
your answer to this one.

4. Does anyone have a schematic for Revision G of the SX202? I've
got a Rev. G. box, but my shematic is for Rev. A. I have a feeling
that the CORRECT schematic might help a lot here.

Just to head off the inevitable suggestion...I'm using this as a bit
of practice as well as trying to get it to work, therefore taking it
in to a shop would defeat my main purpose in tinkering with it. It's
my $150, and I'm quite prepared for the possibility of screwing it up
worse. That channel is unusable now anyway, so I really don't have
that much to lose.

I would really appreciate any suggestions!

Thanks,

Dan
http://www.mp3.com/danwarren
http://www.stationmp3.com/danwarren

[Justin Ulysses Morse]

Dan,

The whole problem with modifying SX202s is finding a schematic that
matches the unit in your hands. I've got piles of units and piles of
schematics, but I don't have ANYTHING (including Monte's great
suggestions) where the part numbers match what's printed on my
circuitboards.

The part you're describing sounds like something other than a
capacitor. A resistor or inductor. But I'll take your word for it
since I don't have an open box in front of me right now. My suggestion
would be to replace ALL of the identical parts you're dealing with, so
the one you NEED to replace will match the rest of them.

Looking at the schematic (Rev A) I see there is an inductor on each
input leg (L1-L4), right next to the caps you're talking about (C1-C4).
Normally I would expect the inductor to be just a ferrite bead on a
wire, but it could also be a wound inductor which would probably be a
minty green package that looks like a resistor with colored bands upon
it. Any chance that's what you're looking at? The best way to figure
it out is by the fact that the inductors are in series while the caps
shunt to ground. They both are there to reduce RF entry into the
preamp. A better anti-RF design in the first place (like an input
transformer) would have rendered these parts unnecessary, but as it
sits you need them there because the preamp is prone to some
RFI-related problems.

How do you know you have a bad capacitor? What is it doing?

ulysses


Dan > wrote:

> I've got a bad capacitor in my SX202 (C4 in the schematic, if you have
> it), but I don't recognize what type of capacitor it is. I'm pretty
> new to this stuff. I've been Googling for many hours, and have yet to
> get a really good answer. Anyway, it looks like a smallish minty
> green resistor. The band pattern is gold-green-reddish
> brown-silver-silver. Everything I've been able to find suggests that
> gold should not be at the beginning of the banding pattern, but I'm
> quite sure that it's really gold at one end and silver at the other.
> The schematic shows a .001uF capacitor there, which is completely out
> of whack with the banding pattern anyway. My questions are:
>
> 1. What type of capacitor is this thing? What would be the best new
> one to replace it with?
>
> 2. What the heck is going on with the banding pattern? There are
> four identical capacitors (C1 - C4), all of which have the same
> pattern.
>
> 3. Should I go with the value from the schematic, assuming that I
> can't figure out what the actual caps say? Question 4 might change
> your answer to this one.
>
> 4. Does anyone have a schematic for Revision G of the SX202? I've
> got a Rev. G. box, but my shematic is for Rev. A. I have a feeling
> that the CORRECT schematic might help a lot here.
>
> Just to head off the inevitable suggestion...I'm using this as a bit
> of practice as well as trying to get it to work, therefore taking it
> in to a shop would defeat my main purpose in tinkering with it. It's
> my $150, and I'm quite prepared for the possibility of screwing it up
> worse. That channel is unusable now anyway, so I really don't have
> that much to lose.
>
> I would really appreciate any suggestions!
>
> Thanks,
>
> Dan
> http://www.mp3.com/danwarren
> http://www.stationmp3.com/danwarren

[Dan]

Ulysses,


It's quite definitely at the spot marked C4. I see the inductors,
they're black, somewhat bigger than a resistor, and are marked with an
L rather than a C. As for how I know it's a bad cap...well, I don't
for sure. My wife is the one doing the actual work here, it was just
easier to tell the whole story in the first person. We're both
interested newbies in electronics, but she's the one wielding the
actual weapons in this case. We suspected a capacitor because it seems
like the channel works well for the first 10 seconds or so, and then
immediately starts going haywire. Anyway, she went through and tested
all of the capacitors in the unit, and found that C4 didn't behave
like its supposedly identical counterparts (C1-C3). We're just going
on the temporary assumption that C4 is fried. As I said before, this
is as much of an educational foray as a repair job, so we're going to
try out her diagnosis. We had pretty much decided to replace them
all, as soon as we can figure out what to replace them with.

As for the behavior...as I mentioned before, it works fine for about
10 seconds. After that point, the mic signal drops by a large amount,
noise comes up, radio signals start coming in, and I hear infrequent
loud crackles. I find your statement that these particular caps are
there to reduce RF particularly heartening; it sounds like exactly the
spot to cause the malfunction I've been hearing.

Thanks for your help,

Dan


On Sun, 27 Jul 2003 07:02:59 -0500, Justin Ulysses Morse
> wrote:

>Dan,
>
>The whole problem with modifying SX202s is finding a schematic that
>matches the unit in your hands. I've got piles of units and piles of
>schematics, but I don't have ANYTHING (including Monte's great
>suggestions) where the part numbers match what's printed on my
>circuitboards.
>
>The part you're describing sounds like something other than a
>capacitor. A resistor or inductor. But I'll take your word for it
>since I don't have an open box in front of me right now. My suggestion
>would be to replace ALL of the identical parts you're dealing with, so
>the one you NEED to replace will match the rest of them.
>
>Looking at the schematic (Rev A) I see there is an inductor on each
>input leg (L1-L4), right next to the caps you're talking about (C1-C4).
>Normally I would expect the inductor to be just a ferrite bead on a
>wire, but it could also be a wound inductor which would probably be a
>minty green package that looks like a resistor with colored bands upon
>it. Any chance that's what you're looking at? The best way to figure
>it out is by the fact that the inductors are in series while the caps
>shunt to ground. They both are there to reduce RF entry into the
>preamp. A better anti-RF design in the first place (like an input
>transformer) would have rendered these parts unnecessary, but as it
>sits you need them there because the preamp is prone to some
>RFI-related problems.
>
>How do you know you have a bad capacitor? What is it doing?
>
>ulysses
>

http://www.mp3.com/danwarren
http://www.stationmp3.com/danwarren

[Justin Ulysses Morse]

Dan,
Those caps only shunt RF frequencies to ground. In theory, if you
simply removed them, the preamp would still work just fine except for
that you'd get more RFI into the SSM2015 preamp chip. But clipping out
all 4 caps would be a reasonable trouble-shooting excercise, since
you're going to replace them anyway.

The cap could fail in a limited number of ways. It could short out,
which would ground that side of the input and give you a dramatic
reduction in audio level and a whole lot of hum and noise. This
failure could be intermittent, or it could happen a few seconds after
you turn on the phantom power.
Other failuires would be an open circuit or a dramatic change in
capacitance, but frankly you probably wouldn't notice that. A
differing amount of shunt capacitance (and therefore high-frequency
impedance) on the two sides of the balanced input would mess up the
RF-rejection, and it sounds like that's what's happening to you, but it
wouldn't cause a reduction in signal. So if the cap is bad, it's
probably shorted out.
If you're measuring the parts in-circuit, then you have no way of
knowing whether the defect you measure is in the cap itself or in a
nearby component. Still, it makes sense to start where you're
suspicious. These parts are cheap and easy to replace, so go ahead and
replace them. Any capacitor that will physically fit in the space
alotted, whose wires reach the circuitboard holes, will work. The caps
need to be rated for at least 50V but you ought to use 100V parts
anyway. Go to www.digikey.com and start searching for .001µF
capacitors. The stock part is a ceramic cap, but you could use a film
or mica cap if you wanted. Ideally you want all 4 caps to be closely
matched in value, so you should buy parts with 5% or 2% tolerance, buy
10 of them, and then hand-match them with whatever meter you have
access to. You don't need to hit .001µF exactly, but you need the two
caps on each channel to be identical.

I don't have an open SX202 in front of me, so I can't see the pin
spacing on these capacitors. The pin spacing is probably 0.3 or 0.4
inches. If you can let me know, I'll give you a specific Digikey part
number.

ulysses

Dan > wrote:

> It's quite definitely at the spot marked C4. I see the inductors,
> they're black, somewhat bigger than a resistor, and are marked with an
> L rather than a C. As for how I know it's a bad cap...well, I don't
> for sure. My wife is the one doing the actual work here, it was just
> easier to tell the whole story in the first person. We're both
> interested newbies in electronics, but she's the one wielding the
> actual weapons in this case. We suspected a capacitor because it seems
> like the channel works well for the first 10 seconds or so, and then
> immediately starts going haywire. Anyway, she went through and tested
> all of the capacitors in the unit, and found that C4 didn't behave
> like its supposedly identical counterparts (C1-C3). We're just going
> on the temporary assumption that C4 is fried. As I said before, this
> is as much of an educational foray as a repair job, so we're going to
> try out her diagnosis. We had pretty much decided to replace them
> all, as soon as we can figure out what to replace them with.
>
> As for the behavior...as I mentioned before, it works fine for about
> 10 seconds. After that point, the mic signal drops by a large amount,
> noise comes up, radio signals start coming in, and I hear infrequent
> loud crackles. I find your statement that these particular caps are
> there to reduce RF particularly heartening; it sounds like exactly the
> spot to cause the malfunction I've been hearing.
>
> Thanks for your help,
>
> Dan
>
>
> On Sun, 27 Jul 2003 07:02:59 -0500, Justin Ulysses Morse
> > wrote:
>
> >Dan,
> >
> >The whole problem with modifying SX202s is finding a schematic that
> >matches the unit in your hands. I've got piles of units and piles of
> >schematics, but I don't have ANYTHING (including Monte's great
> >suggestions) where the part numbers match what's printed on my
> >circuitboards.
> >
> >The part you're describing sounds like something other than a
> >capacitor. A resistor or inductor. But I'll take your word for it
> >since I don't have an open box in front of me right now. My suggestion
> >would be to replace ALL of the identical parts you're dealing with, so
> >the one you NEED to replace will match the rest of them.
> >
> >Looking at the schematic (Rev A) I see there is an inductor on each
> >input leg (L1-L4), right next to the caps you're talking about (C1-C4).
> >Normally I would expect the inductor to be just a ferrite bead on a
> >wire, but it could also be a wound inductor which would probably be a
> >minty green package that looks like a resistor with colored bands upon
> >it. Any chance that's what you're looking at? The best way to figure
> >it out is by the fact that the inductors are in series while the caps
> >shunt to ground. They both are there to reduce RF entry into the
> >preamp. A better anti-RF design in the first place (like an input
> >transformer) would have rendered these parts unnecessary, but as it
> >sits you need them there because the preamp is prone to some
> >RFI-related problems.
> >
> >How do you know you have a bad capacitor? What is it doing?
> >
> >ulysses
> >
>
> http://www.mp3.com/danwarren
> http://www.stationmp3.com/danwarren

[Scott Dorsey]

Dan > wrote:
>
>It's quite definitely at the spot marked C4. I see the inductors,
>they're black, somewhat bigger than a resistor, and are marked with an
>L rather than a C. As for how I know it's a bad cap...well, I don't
>for sure. My wife is the one doing the actual work here, it was just
>easier to tell the whole story in the first person. We're both
>interested newbies in electronics, but she's the one wielding the
>actual weapons in this case. We suspected a capacitor because it seems
>like the channel works well for the first 10 seconds or so, and then
>immediately starts going haywire. Anyway, she went through and tested
>all of the capacitors in the unit, and found that C4 didn't behave
>like its supposedly identical counterparts (C1-C3). We're just going
>on the temporary assumption that C4 is fried. As I said before, this
>is as much of an educational foray as a repair job, so we're going to
>try out her diagnosis. We had pretty much decided to replace them
>all, as soon as we can figure out what to replace them with.

Okay, this sounds like a tantalum (epoxy drop) capacitor. What is the
value?

You can get a schematic from Symetrix (maybe even on their web site)
or you can use the resistor color code to figure out the value and voltage.

You can replace it with a Panasonic tantalum from Digi-Key.

>As for the behavior...as I mentioned before, it works fine for about
>10 seconds. After that point, the mic signal drops by a large amount,
>noise comes up, radio signals start coming in, and I hear infrequent
>loud crackles.

That sounds like a coupling cap.

> I find your statement that these particular caps are
>there to reduce RF particularly heartening; it sounds like exactly the
>spot to cause the malfunction I've been hearing.

If it's an RF decoupling cap, it probably won't do this, and it won't
be a tantalum either. Is it a tantalum?
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Justin Ulysses Morse]

Scott Dorsey > wrote:

> Okay, this sounds like a tantalum (epoxy drop) capacitor. What is the
> value?

> If it's an RF decoupling cap, it probably won't do this, and it won't
> be a tantalum either. Is it a tantalum?


No, he's talking about the .001µF ceramic caps that shunt RFI to
chassis ground right at the inputs. He's going to replace them with
polypropylenes, which makes sense to me. I still don't think these are
causing his problem, but he'll find out soon enough, and he's only
spending three bucks on replacement caps.

ulysses

[Scott Dorsey]

Justin Ulysses Morse > wrote:
>Scott Dorsey > wrote:
>
>> Okay, this sounds like a tantalum (epoxy drop) capacitor. What is the
>> value?
>
>> If it's an RF decoupling cap, it probably won't do this, and it won't
>> be a tantalum either. Is it a tantalum?
>
>No, he's talking about the .001µF ceramic caps that shunt RFI to
>chassis ground right at the inputs. He's going to replace them with
>polypropylenes, which makes sense to me. I still don't think these are
>causing his problem, but he'll find out soon enough, and he's only
>spending three bucks on replacement caps.

I'd go with an NPO ceramic on those. Better at RF than any film cap,
and transparent at audio.

If it IS one of these capacitors, removing the capacitor will fix the
problem for diagnostic purposes. It's a decoupling cap, so it's in
parallel with the source and load. As long as it's not a dead short,
it won't hurt anything other than RFI rejection if it fails. If it's
a dead short, it'll be easy to find.

But ceramic caps don't fail very often, and they very seldom fail into
shorts. I'd start looking elsewhere.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Dan]

Thanks all for the advice. We're going to try just disconnecting them
to see if that will fix the problem. I'll let you know how it goes.

Dan

[Dan]

>>While you're at it, I hope you're going to upgrade the output chips
and
>>replace the output caps with a single interstage cap a la Monte.


We've replaced the opamps with the LT1469CN8, but haven't changed any
capacitors yet. Since I know the question will arise, I'll tell you
that the problems we're having predate the modifications we've done.

We were planning on doing the mods listed under "SSM 2015
Compensation" when we replaced the (hopefully) malfunctioning
capactior. This is the one that consists of replacing three
capacitors per channel with lower valued capacitors. We hadn't really
planned on doing anything else right now, but I'm not averse to the
idea. I'm assuming that you're referring to something different from
the mod I just mentioned. Could you tell me slowly in newbie-speak
which of the other mods you'd recommend (or are recommending)?


Thanks,

Dan

[Justin Ulysses Morse]

Well, I'm not actually recommending them because I haven't gotten
around to doing most of them myself. But one of the upgrades Monte
McGuire recommends is that after you replace the crappy 4560 op-amps in
the output section with something good, like you've done, then you
don't have to worry about DC offset generated in that output section
anymore. So you can replace the big electrolytic coupling caps on the
outputs with jumper wires, and instead use a single film cap between
the SSM2015 and the output chip to block any first-stage offset. The
whole low-down is in Monte's little recipe which presumably you've
found through Google or something. He posted it here a year or so ago
I think. Getting unbiased polarized electrolytics out of the signal
path is a good idea.

ulysses


In article >, Dan
> wrote:

> >>While you're at it, I hope you're going to upgrade the output chips
> and
> >>replace the output caps with a single interstage cap a la Monte.
>
>
> We've replaced the opamps with the LT1469CN8, but haven't changed any
> capacitors yet. Since I know the question will arise, I'll tell you
> that the problems we're having predate the modifications we've done.
>
> We were planning on doing the mods listed under "SSM 2015
> Compensation" when we replaced the (hopefully) malfunctioning
> capactior. This is the one that consists of replacing three
> capacitors per channel with lower valued capacitors. We hadn't really
> planned on doing anything else right now, but I'm not averse to the
> idea. I'm assuming that you're referring to something different from
> the mod I just mentioned. Could you tell me slowly in newbie-speak
> which of the other mods you'd recommend (or are recommending)?
>
>
> Thanks,
>
> Dan

[Dan]

Removed the capacitors, and voila! That seems to have been the
problem. I'm very happy to say that I now have a preamp with 2
functioning channels. Thanks loads for your help, everyone.

Now on to those mods...
http://www.mp3.com/danwarren
http://www.stationmp3.com/danwarren