Good ideas with bad execution, or good execution of what should be bad ideas - an analysis of inferior, off-beat or malfunctioning products, and how other people's failures can help us design better stuff.
Emerson Stereo Made in China Has a Melt-Down
Don’t Get Soaked by Your Maytag Neptune washer If you own a Maytag Neptune washing machine you need to be aware that there is a failure waiting to happen. The affected models include the Stacker 2000, MAH3000, MAH4000 and the MAH5500A models. Engineer Jeff Hartman has deviced There is a small solenoid like device that locks the door before the spin cycle located in the door latch assembly. This device is called a wax motor, Maytag part # 12002535 is the redesigned part. It works by heating a ball of wax up which causes the actuator pin to push out and lock the door. It takes around 45 seconds to extend so it is rather slow compared to a typical electromechanical solenoid. The older wax motors have a brown actuator pin while the newer wax motor has a black actuator pin. The failure is caused by moisture penetrating the wax motor (older design) over time, which causes the electrical resistance to drop to such a value that the small Q6 triac located on the upper control board overheats and shorts out internally. R11 burns because the failed Q6 is allowing 120 volts on the triac’s center control pin (triac gate). Sometimes Q6 still looks normal and other times the plastic case ruptures after the failure. However, a burnt R11 is the telltale sign that you have had this failure and the Neptune washer should be left off to prevent further damage to your upper control board. To inspect your control board you need to do the following: 1. Unplug Neptune’s power cord 2. Remove three philips screws along top of washer 3. Pull top of control console towards you while pushing bottom of console away from you. 4. Control board is located behind timer assembly on back panel 5. Inspect the R11 resistor located near the left control board mounting screw If your R11 is burnt, you will need to replace the wax motor and repair or replace the control board. If you don’t have the no-spin failure yet you need to inspect your wax motor a.s.a.p. If you verify that you have the older brown pin wax motor you need to replace it a.s.a.p. so you don’t experience the R11/Q6 failure and risk damaging your upper control board. This wax motor and control board replacement averages $350 if you bother the Maytag repairman. Leave him be and do it yourself! See website for easy Neptune teardown photos. www.neptunewaxmotor.com Engineer Andy Morris recently discovered an unwelcome feature among several Chinese-made stereo sets he owns: Shoddy design work. 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. Where to begin, really, in our recitation of the design flaws? Starting with the miserable execution of a compact footprint [...the designer just squashed the transformer down and spread it out sideways to fit the envelope and get the desired wattage] to what can only be described as thermal mismanagement, to the inexplicable use of certain components, the list of egregious design errors goes on and on. My these monkeys were busy. 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. View circuit Click on the link below to see Andy’s redesign of the circuit and transformer, though his warning to the wise: Stick with quality brands like Sony and Panasonic.
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 and at left. 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 in figure 5 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.Tags: door latch assembly, maytag neptune washer, resistor, ruptures, screws