A Stereo Receiver Question

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 :-)

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

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.

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.

A few things:

Laser-pointer digital infra-red thermometers are interesting tool. I have u sed one for many years to determine air discharge temperature, true tempera tures of heating elements water temperature and the like. Whereas they are reasonably accurate, the do lack precision when it comes to small parts. Fo r 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 acc urate than that.

Freeze spray is still with us. It is useful for tracing intermittents and c omponents that fail when warmed up. But, be very careful with it. Cold spra y 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 abs olute minimum edge of tolerance, that they failed is not *necessarily* an i ndication of an external cause. Replace them, of course. And I would sugges t going to a higher voltage as well.

On the hum-when-no-input and after-some-time things. There are two things g oing on here. First, the failed low-value caps stopped protecting the elect rolytics from the initial power surge at start-up. Again, these are margina l 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, m asked as soon as a signal is present. And, does the receiver have an AVC fu nction? There are some Surround Sound TV-types that do, such that when inpu t signal level drops, the pre-amp function raises the volume. Which would a mplify any residual hum.

You are on-target, caps and caps.

Peter Wieck Melrose Park, PA

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

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.

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

NT

ray nowadays.

About. But, here is the biggest issue with these devices. I have a reasonab ly good one, with a reasonably fine pitch. The target diameter at 12" is 1.

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

Peter Wieck Melrose Park, PA

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.

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...

Kapton tape is very black to LW IR.

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.