Chinese-Made Stereo Has a Meltdown

DN Staff

October 15, 2009

6 Min Read
Chinese-Made Stereo Has a Meltdown

Engineer Andy Morris recently discovered an unwelcome feature among several Chinese-made stereo sets he owns: Shoddy work, leading to scorched pinkies.

Andy bought this Emerson model ES50 stereo new and to his dismay after 9 months of fairly light CD use (no teenagers in the house, presumably), the CD player literally melted.

Although the resulting product is likely the result of a series of bad-tradeoffs, Andy describes the key design flaw that resulted in the player’s quick demise and some scorched pinkies (his) : “The voltage regulator output transistor was mounted on the copper side of the PC board with a small amount of copper (almost none) used as the heat sink. It was required to drop 10 volts at 500mA, dissipating 5 watts of power. It failed and shorted out, applying 18 volts instead of 8 volts to the CD circuit, essentially frying the CD player. The voltage regulator circuit is similar to the one shown here except for no Q1, D1, D2 and a different zener diode.”

Click here to see Andy’s redesign of the circuit and transformer, though his warning to the wise: Stick with quality brands like Sony and Panasonic. He says:

“First, I replaced the transistor and put it on a decent heat sink, and I replaced the CD mechanism with an identical one from a junk CD player I had on hand for parts. That got it working, but a lot of heat would still build up inside the set. After about one hour of use, the CD player would start skipping and then stop playing altogether.

I figure that the CD servo chip was slightly damaged, by the 18 volts, making it more sensitive to temperature than it originally was. I’m amazed that it didn’t blow out completely. In any case, the stereo set got quite warm after an hour or so of use. I nearly scorched my finger on the voltage regulator heat sink, so I decided to do something about it.

In this set with a similar problem, I replaced a large dropping resistor and a 7808 voltage regulator with the switch-mode down converter circuit from a cell-phone car charger, readjusted to 8 volts output. Since it has a completely self-contained edgewise CD player, with a built-in 5 volt regulator, this fix worked well in that set. It did not interfere with the radio, because the circuit is turned off when the radio is in use. I tried this fix in the Emerson ES50 set, and got an error message when starting the CD player.

I thought of using an SCR pre-regulator that I have already published on the web but I felt that the high in-rush current when the SCR fires would interfere with the sensitive CD player circuit. I had to come up with something better. I thought of a circuit that Dave Johnson had created , where the output capacitor was charged on the rising slope of the rectified sine wave, instead of the falling side as in the SCR circuit. I designed a switch mode pre-regulator , using Dave’s idea, and that fixed the problem. The set still gets pretty warm, but now I can play the CD player for hours without problems.

The pre-regulator circuit is an after-the-fact fix. In a good design, the power transformer should have had a tap or an extra winding for the CD player.

Although the stereo set no longer stops playing, it still gets quite warm, due to the fact that the CD player uses 4 watts of power and the power transformer is poorly designed. All this is in a very compact package with very little ventilation.

Now, let’s talk about the power transformer — take a close look at the figures above. The compact design called for a “low profile” power transformer. Most people might miss it, but a transformer designer should have known better. To implement the low profile design, the designer just squashed the transformer down and spread it out sideways to get the required wattage. Note that this doubled the length of the wire needed in the transformer windings, doubling the IR loss in the transformer. Every low profile power transformer I have seen has the windings rotated 90 degrees from what you see here, so that the size of the coils is kept small, minimizing IR loss.

I have another compact stereo, the Emerson model MS3110 shown above that had little use, but that I wanted to see how well it was designed. It had the same overheating problem and so I installed the interference-free switch-mode pre-regulator circuit into it. The circuit shown shown above in this post is the voltage regulator from this stereo set. By the way, when installing the pre-regulator circuit into sets with this type of voltage regulator (which also acts as a power switch for the CD player), the best way is to disconnect the collector of Q2 and connect it to the 10 volt output of the pre-regulator circuit. You should leave the zener diode circuit alone, unless that circuit is also poorly designed as the circuit was in this case. View circuit
R1 and R2 are both tiny SMT resistors that can’t be more than ¼ watt parts, yet they’re dissipating almost twice that. You don’t need all this current through those resistors, anyway. I changed R1 to 4K7 and R2 to 1K.

Another thing; what the hell are diodes D1 and D2 for? They just waste power, seriously lowering the gain of the Darlington amplifier (Q1 and Q2). This requires one to push much more current through R2. I disconnected D1 and D2 with no negative performance effects.

In this set, I measured about 19 volts on the main power supply. The data sheet for the audio amp IC sets the absolute maximum allowable voltage at 20 volts. The recommended max is 15 volts. When I get my hands on a 7815 or an LM317, I’ll fix that problem.

Note that all these stereo sets were made in China and they all have about 10 watts of audio power. I have seen similar stereo sets at flea markets and garage sales with different brand names and slightly different styling. I would recommend sticking with quality brands like Sony or Panasonic. Please note however, that I saw a small stereo set very similar to the Emerson ES50 at a flea market with a JVC name on it. It also had a faulty CD player. JVC wouldn’t make a piece of crap like that, but they obviously resold it. They probably didn’t know at the time about the poor quality.

Many Japanese companies (like Sony, for example) are having products made in China for cheap labor, but they’re designed in Japan and subject to Japanese quality control.”

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