A Stereo Receiver Question

[Ken]

I have an Insignia IS-HC040917 Stereo Receiver, a 6 channel 5.1 vintage.
I use it primarily for FM, playing CD's, and input from the TV via the
optical input. It does NOT have HDMI.

Recently I noticed what sounded like AC hum from all the speakers when
there was a pause in the input. The hum was not there when no signal
was input, and only after the receiver had been on for about 30 minutes.
I thought since it is an old receiver that perhaps the filter caps
(10000 uf 63v) in the bridge rectifier section for the +&- 50v were
perhaps weak. I changed them and that did not solve the problem. After
a more thorough visual examination of the receiver and some research on
the Internet, I discovered a couple of .1 uf caps (placed across each of
the bridge diodes) that had opened. My research said this was commonly
done to suppress noise on a system by smoothing out the transition of
the diodes in the bridge. I was looking for ripple on the 50v, but it
appears the noise was being picked up further down the line in the
latter stages of amplification.

I am confident I have solved my problem, but the question is this: Why
did the hum not show up immediately upon turning on the receiver rather
than after 1/2 hour? I enjoy solving a problem, but I always like to
fully understand it. Thanks.

[Dave Platt]

In article <osvcvn$uv4$1...@dont-email.me>, Ken <K...@invalid.com> wrote:

> After
>a more thorough visual examination of the receiver and some research on
>the Internet, I discovered a couple of .1 uf caps (placed across each of
>the bridge diodes) that had opened. My research said this was commonly
>done to suppress noise on a system by smoothing out the transition of
>the diodes in the bridge. I was looking for ripple on the 50v, but it
>appears the noise was being picked up further down the line in the
>latter stages of amplification.
>
>I am confident I have solved my problem, but the question is this: Why
>did the hum not show up immediately upon turning on the receiver rather
>than after 1/2 hour? I enjoy solving a problem, but I always like to
>fully understand it. Thanks.

At a guess, the caps probably undergo some amount of thermal cycling
as the receiver warms up (when turned on) and cools down (when turned
off). The temperature changes cause the material of the caps (and the
leads) to expand and contract, creating a temperature-dependent
physical stress on the cap body (the leads are soldered down and
their ends can't move, and so stress is created where the leads enter
the body and where the leads are soldered to the board).

Over time, this repeated stress ended up weakening and cracking
something in the caps - possibly the dielectric itself, possibly the
dielectric-to-wire bonds. It's also common for the component itself
to be OK, but for a stress crack to occur in the solder joint where
its leads connect to the PC board or solder terminals.

Apparently, when the receiver warms up, the thermal expansion is enough to
cause this crack to open up. When the receiver down, the cracked
junction is pressed back together enough to "close the circuit" again.

Replacing the parts with known-good subsitutes, after cleaning off the
older solder from the junction, and re-soldering carefully with the
right type of solder and (often useful) some extra liquid solder flux,
is the right fix.

Problems of this sort can sometimes be found by tapping various
components gently with a wooden chopstick... when you find the area of
the cracked component or solder joint, a test speaker hooked to the
receiver will start making dreadful popping and banging noises, or the
problem will appear and disappear. Don't do this with a speaker you
care about :-)

[Ken]

I figured it must be heat related, but your explanation was precise and
probably correct. Thanks.

[jurb...@gmail.com]

Actually lytics are worse when cold, the opposite of your situation.

[Ken]

jurb...@gmail.com wrote:
> Actually lytics are worse when cold, the opposite of your situation.
>

The caps I was thinking of were disc caps across the diodes, not
electrolytics elsewhere in the circuit. What puzzled me about the 30
minute delay was that electrolytic DO behave that way. If the problem
was the caps across the diodes, that would make sense since they seem to
be the source of the noise.

[olds...@tubes.com]

One of my latest additions to the shop tools is a digital infared
thermometer. They are made to measure the temperature of anything, but
are sold for automotive use, such as finding a hot brake drum, or hot
spot on an engine. But I have found this thing is pretty accurate on
electronic components too. It has a built in laser pointer to highlight
the thing it's pointed at. So it will tell the temp of a heat sink, or
CPU, or even an opamp or other chip or for that matter, any component.

If you have one of them, or get one, try to get the temperature of
different parts on the chassis or PCB when you first turn it on, then do
the same after a half hour. Certain things will get warmer like
heatsinks, power resistors, etc but if there are passive components or
semiconductors getting quite warm or hot, you may want to check them
further. They used to sell some sort of stuff in a can to cool parts. It
might have been freon??? I used it many years ago, I have not even
looked to see if they still sell it, but if it's still sold, that could
help see if the sound changes when a suspected part is cooled.

I did not read this whole thread, but I think you said those disc caps
across the diodes were open. I assume you mean the power rectifier
diodes. I cant understand why those would open, unless you had a
lightning surge in your power lines. That makes me wonder if those
diodes could be a little flaky. Usually diodes are either good or bad,
but lightning surges can do strange things and nothing in electronics
has a 100% definite solution. Regardless what the books say, sometimes
strange things happen that have no real explanation.

[pf...@aol.com]

A few things:

Laser-pointer digital infra-red thermometers are interesting tool. I have used one for many years to determine air discharge temperature, true temperatures of heating elements water temperature and the like. Whereas they are reasonably accurate, the do lack precision when it comes to small parts. For a transformer, large capacitor or even a power transistor, and at a very close range, they would be fine. Or in finding a warm area on a board, sure. But the have far to broad a view, even at a few inches to be any more accurate than that.

https://db1736767dbd5e7094bb-d61bbc5d0b342a54145a236e2d5d1ebf.ssl.cf4.rackcdn.com/Product-800x800/6dc5243f-87d8-4820-9868-667cabd4f907.jpg

Freeze spray is still with us. It is useful for tracing intermittents and components that fail when warmed up. But, be very careful with it. Cold spray hitting an overheated part or board can cause fractures - which can cause trace failure.

Whereas disc caps tend to be fairly reliable, they do fail. And given that the typical second-tier Pacific rim receiver uses parts that are at the absolute minimum edge of tolerance, that they failed is not *necessarily* an indication of an external cause. Replace them, of course. And I would suggest going to a higher voltage as well.

On the hum-when-no-input and after-some-time things. There are two things going on here. First, the failed low-value caps stopped protecting the electrolytics from the initial power surge at start-up. Again, these are marginal parts probably operating very close to their voltage rating. If you were to put an ESR meter across them it would be revealing. What is happening is that as they heat up - my guess anyway having seen it often enough before - internal perforations in the foil open up and the cap very slowly starts to short, and capacitance drops severely. The hum is at a very low level, masked as soon as a signal is present. And, does the receiver have an AVC function? There are some Surround Sound TV-types that do, such that when input signal level drops, the pre-amp function raises the volume. Which would amplify any residual hum.

You are on-target, caps and caps.

Peter Wieck
Melrose Park, PA

[et...@whidbey.com]

IR thermometers are pretty handy but can give wildly false readings.
This is because of the emissivity of the surface being measured. For
example, bare aluminum will measure muich cooler than it really is
when the temp is measured with an IR thermometer while black anodized
aluminum will measure pretty accurately when the IR thermometer is
used.
Eric

[whit3rd]

On Sunday, October 29, 2017 at 5:03:43 AM UTC-7, pf...@aol.com wrote:

> Freeze spray is still with us. It is useful for tracing intermittents and components that fail when warmed up. But, be very careful with it. Cold spray hitting an overheated part or board can cause fractures - which can cause trace failure.

But, it has other uses. A little freeze spray onto one of dozens of DC/DC converters
identified the source of an annoying crosstalk situation, and we gained about 6 dB of
extra signal/noise, once I'd isolated the ripple source. You can also (roughly) identify hot
components, by observing how fast the frost dissipates.

[tabb...@gmail.com]

I suspect an IR thermometer is about the same price as a can of freeze spray nowadays.


NT

[pf...@aol.com]

> I suspect an IR thermometer is about the same price as a can of freeze spray nowadays.

About. But, here is the biggest issue with these devices. I have a reasonably good one, with a reasonably fine pitch. The target diameter at 12" is 1.5". For those using the new currency: 38mm at 300mm. For an idea of target-creep, 3" at 36", 75mm at 900mm, and so forth.

Which must be kept in mind when in use. The little laser light is merely a visual guide, not the size of the target.

[Tim R]

We have a FLIR camera at work.

It was $3500 when we bought it but well worth it for diagnosing hot components.

I see they're only $5-600 on Amazon now. There's even an iPhone version for $200.

[~misfit~]

Agreed. I have one and find that side of it really annoying. Sometimes you
can stick a bit of black insulation tape to the object being tested but most
often not.

If only I could afford a FLIR...
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)

[tom]

"~misfit~" <shaun.at...@gmail.com> wrote in message
news:ou2tdd$pcn$1...@dont-email.me...

Kapton tape is very black to LW IR.

[bruce2...@gmail.com]

Any hum results from vibration or oscillation between wires, but usually within a wire (with too small of an AWG) or between wires. Re-connecting a load side wire or replacing it with a larger size AWG could help.