ark (09/09/82)
As I write this, one power amplifier board is on its way to Benton Harbor for diagnosis and repair. While I think the events that led to this are so unusual as to border on the freakish, I did say I would report my experiences.... First, the construction. Most of the circuitry for the amplifier is on three boards. Each channel has an amplifier board, mounted on its own (large) heat sink, and there is a separate board for protection circuits. To give you an idea of the complexity of this amplifier, each amplifier board is about 4 by 11 inches and contains 27 transistors and many, many other components (including four fuses). The protection board is smaller, but still contains a dozen transistors, four LEDs, an integrated circuit (four exclusive-or gates), a relay, and other components. Chassis wiring includes a huge (25 pounds or so) power transformers, two 13,000 microfarad (125V) capacitors, two relays, a bridge rectifier, level controls, a fuse, switches, and sockets, all interconnected by two pre-cut wiring harnesses. The thermistor serves an interesting purpose. Its cold resistance is rated at 3.3 ohms, and it is in series with the power transformer primary. When the amplifier is first switched on, the thermistor limits the current to the primary so that charging the filter capacitors will not draw enough current to blow the household wiring. As the capacitors are charged, the thermistor heats up and (presumably) decreases its resistance to let the capacitors charge more. When things have gotten close enough to full charge, a voltage taken off a different secondary transformer winding closes a relay that shorts the thermistor and couples the primary directly to the wall socket. The four LEDs on the protection board are power, protection, and left and right clipping. The clipping indicators are just estimates, based on the output voltage, but it seems safe to say that if they do not light, the amplifier is not being overdriven. The protection LED comes on whenever the output terminals have been disconnected from the amplifier boards. This happens during the first eight seconds after power-on, and if the protection circuitry detects excessive DC, evidence of oscillation, or excessive heat-sink temperatures. The mounting of the LEDs is the worst part of the physical design of the kit. They protrude from, and are soldered to, the foil side of the protection circuit board. The directions say to make sure that the LEDs are exactly a specific distance from the board, and when they say "exactly" they mean it. When the board is finally mounted, behind the front panel, any small variations in how high the LEDs sit shows up as a variation in how far the LEDs peek out of their little holes in the front panel. Even a single millimeter of variation is unsightly. I reheated and slightly shifted the LEDs so many times that I began to damage the circuit board foils. These LEDs should be in sockets. When I first turned on the power supply circuits, the power LED did not come on. This turned out to be due to a dead power LED. Maybe it was defective and maybe I resoldered it too many times. At any rate, neither of the Heath Eletronic Centers in my neighborhood have these LEDs in stock, so I obtained a replacement locally. Before I did that, though, I tried to check out the protection circuits. The instructions tell you to connect the thermal circuit breaker from one of the boards into the protection circuit and to short the other circuit breaker socket with a wire jumper, and then the protection LED should go off after eight seconds of operation. It didn't. Two hours later, I discovered that if the power LED is broken, the protection LED will never turn off. Sigh. After fixing that problem, I started testing the amplifier boards. The first board was uneventful. The second time I applied power to the second board, the result was a shower of sparks and smoke. After I disconnected everything and went trouble-shooting, I found that one of the power transistors had a leaky insulating gasket. It wasn't a dead short, so the resistance tests I had previously performed on the board didn't turn up any problem. There was enough leakage, though, to blow out two resistors that ordinarily should have almost no current through them. I unsoldered and tested every transistor on the board after that, and found one leaky power transistor. I replaced that and the two fried resistors, and everything was almost normal, but that "almost" has me worried. The present symptom is that the output level wanders, apparently randomly, with an amplitude of about 10 mV and a frequency of about 2 Hz. A friend who knows more about these things than I do has said that these symptoms look very much like a transistor that is about to give up the ghost, and I sure don't want that to happen with the amplifier connected to my loudspeakers. Thus I have dispatched the ailing board to Heath in Benton Harbot. They tell me it will be about three weeks in their shop. When it returns, I will tell you what I have learned. Meanwhile, I'm working on the Heath preamp. I'll let you know how it goes.