what are the audio design goofs and downright disasters you have seen? this is a continuation of the rabbit trail started in the power supply section with a discussion a bit OT about "10 watt" zeners made from 5 watt zeners and a soldered on fin.
i once rebuilt (3 times before giving up) an Acoustic 360 amp..... completely rebuilt except for the power transformer, bridge, caps and chassis. would bring it up very carefully on a variac, but as soon as i got to 60V.... BAM!!! everything would go back up in smoke. found out later from another tech that that's what happened with those amps. once they blew, they were scrap.
i once rebuilt (3 times before giving up) an Acoustic 360 amp..... completely rebuilt except for the power transformer, bridge, caps and chassis. would bring it up very carefully on a variac, but as soon as i got to 60V.... BAM!!! everything would go back up in smoke. found out later from another tech that that's what happened with those amps. once they blew, they were scrap.
A Philips mini sound system (don't remember the model#) with an NTC that turns on a fan should the amplifier heat sink get too hot. Well, that NTC was first of all covered in heat shrink tubing, secondly no thermal compound applied to the NTC, and lastly, lack of any mechanism to ensure the NTC makes contact with the heat sink. In order words, the amplifier heat sink could reach up and over 90 degrees without the fan even turning on. Maybe not enough to cause immediate failure, but that would certainly reduce the life span. Doesn't that also negate the purpose of having a fan in the first place?
Not related to audio, but I saw electric garage door openers with plastic (yes, PLASTIC) gears. Soon as something puts force on the door (such as a spring failure), the gears get shredded to bits.
Not related to audio, but I saw electric garage door openers with plastic (yes, PLASTIC) gears. Soon as something puts force on the door (such as a spring failure), the gears get shredded to bits.
i once worked on a YORX cheapie sound system that had a tension pulley for the dial string attached to the rear panel with a spring anchored to one of the screws holding the RCA jacks for the speakers. while disassembling the unit, i unscrewed the RCA jack panel and heard the spring go TINK!!! fortunately i didn't have to figure out how they attached the pulley after reassembly because the amp outputs were a charred ruin and the unit was not economically repairable.
WYSE computer terminals:
the WYSE 50, and 60 RS232 terminals had almost ALL of the electrolytic caps mounted either next to heat sinks or next to 5 and 10 watt resistors. when i was repairing these, i made up a complete recap kit for them. the neg terminal of the caps was identified by a half-moon symbol on the board silkscreen. one particular revision of the board had all of the half moons reversed!!!! guess what happened when the terminal was powered up the first time i recapped on of those.... yep that's right, every cap either went up in smoke, or got it's can lodged in the suspended ceiling of the shop....
WYSE computer terminals:
the WYSE 50, and 60 RS232 terminals had almost ALL of the electrolytic caps mounted either next to heat sinks or next to 5 and 10 watt resistors. when i was repairing these, i made up a complete recap kit for them. the neg terminal of the caps was identified by a half-moon symbol on the board silkscreen. one particular revision of the board had all of the half moons reversed!!!! guess what happened when the terminal was powered up the first time i recapped on of those.... yep that's right, every cap either went up in smoke, or got it's can lodged in the suspended ceiling of the shop....
also recently saw a pick-and-place goof on a Denon receiver main board. another tech in the shop ordered a main board and installed it. he found that the receiver was now drawing excessive current. he asked me to look at it. i soon found that EVERY channel was running high current, and the bias pots had very little effect. the voltage between output bases was 2.5 to 3 volts (running the test with a lamp in series with the receiver AC line). this problem was somewhat baffling, since it was a new main board from the factory. as i began looking for problems in the bias circuits, i found that the factory had populated 680 ohm resistors in the VBE multiplier circuit instead of the 2k7 resistors shown in the schematic..... in EVERY CHANNEL......
I used to be involved in a lot of work for Comecon (soviet) based electronic industry.
They copied lots of ICs like 7400 series. Quite good copies but based on 2.5mm lead spacing rather than 2.54 (Imperial? Them!?)
Our first big job was a large PCB, A3 size, containing hundreds of chip positions. We designed the boards and manufatured the first thousand PCBs for assembly in Russia using their components.
A great Joint-venture, you may think!
However, we designed using an imperial library of components ....
8 pin DILs will go in. 16 pin, just, with a wiggle. 32 pin - forget it!
Complete disaster - never saw what finally happened and who got the chop .....
They copied lots of ICs like 7400 series. Quite good copies but based on 2.5mm lead spacing rather than 2.54 (Imperial? Them!?)
Our first big job was a large PCB, A3 size, containing hundreds of chip positions. We designed the boards and manufatured the first thousand PCBs for assembly in Russia using their components.
A great Joint-venture, you may think!
However, we designed using an imperial library of components ....
8 pin DILs will go in. 16 pin, just, with a wiggle. 32 pin - forget it!
Complete disaster - never saw what finally happened and who got the chop .....
i worked for one of Carver's competitors (APT) at the time...... we thought the cube amp was a nice concept, but too complex for what it was made for..... actually APT wasn't without it's "disasters".... not exactly a design problem (although there was a design flaw in the APT-1), but a temporary parts procurement disaster. after Tom Holman went to work for LucasFilm, APT hired a new parts manager. he thought he could save-a-buck by bypassing the OEM process we had in place with Motorola, and buying output transistors from a distributor. sure, the transistors were 1/3 of the cost, but they weren't pre-screened for a beta of 75 or less. this caused some problems on the production line, and being the final test tech, i had to hold a bunch of amps that wouldn't pass final test. the amps were blowing fuses during the common mode conduction test. that was a test where we ran the amp at 100khz. if the AC current draw was over a certain amount, the amp was failed. these amps were actually blowing the line fuse when tested. after a visual comparison (remember boys and girls, your first troubleshooting step is always a visual), i discovered that the failed amps had a mix of output transistors (the OEM contract transistors had APT as part of the Mot factory stamp, the ones bought through distributors did not) or no OEM parts at all. as it turns out, the OEM transistors were screened for a beta of 75 or less, and some of the "new" transistors had betas as high as 150. this meant that the Miller capacitance could be multiplied by a higher beta, and the base charge took longer to bleed off. what this did at 100khz is caused both sides of the amp to remain in conduction longer, resulting in massive current draw between the + and - rails. it cost us more to screen the transistors in house than it cost for the pre-screened devices.....
COMPAQ 461 SVGA monitors. one of the early microprocessor controlled video monitors. one slight problem however, the processor was mounted on an unshielded daughterboard within 1/2" of the CRT, and the static charge from the CRT powering up would eventually fry the processor chip. that model monitor also had a high voltage shut-down circuit that operated off of a low voltage winding of the flyback. if this particular winding was not loaded down, it would produce 400-500V spikes. the circuit fed a zener and a comparator through a resistor string. when loaded by the resistor string and zener, there was a maximum of 12V on the zener. any more than this and the zener would conduct, feeding a voltage to the comparator and shutting down the horizontal drive. if the zener shorted, one of the series resistors would burn open, and the winding would be unloaded, and the 500V spikes would appear and arc over to, you guessed it, one of the control lines to the microprocessor. if you were fortunate enough to get one of these beasts working again, there was one challenge remaining. there were no physical adjustments for this monitor, all of the horizontal, vertical and video adjustments were handled by the processor, which had to have a set of initialization parameters loaded through a TTL serial port.
this monitor was made by ADI, and ADI's monitor (3515 model IIRC) was identical.
this monitor was made by ADI, and ADI's monitor (3515 model IIRC) was identical.
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