Join Date: Dec 2007
Location: Sunnyvale, CA (Silicon Valley)
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Before I say how my meter just went berserk, let me ask, does anybody remember these things that Radio Shack sold in the early to mid-eighties? The APM-300 Audio Power Meter is the all-LED version which displayed power output in a fashion similar in appearance to the grill of "Kit," the car in Knight Rider. The center LED is green, which is both the "on" indicator and equates to zero watts output. Then as amp power output increases, red LED's sequentially "blossom" out from the center green LED to the edges of the meter, the left and right channels blossoming in their respective directions on the front panel. It's oh-so-eighties, somewhat informative, and hugely entertaining! So, what could possibly go wrong with an LED power meter?
I have two of these APM-300's and one just went on strike. The red LED's should come on sequentially as voltage from the amp increases. Each LED should come on discreetly and quickly, with next to no "overlap" between one LED and the next. On the one that just went flaky, the LED's don't turn on discreetly one at a time. They sort of "smear" across the display, with the low-power LED's brightest, and subsequent LED's getting dimmer as they reach the end of the display. A power level that should only light up to, say, the 10 watt LED instead "bleeds" all the way to the 200 watt LED at the end, each subsequent LED beyond the 10 watt LED getting slightly dimmer towards the end. It acts like a drunk that slurs his speech, or just plain got lazy. It affects both left and right channels. When I disconnect one channel, there is no "bleed" to any of the LED's on that channel.
I still have one good working APM-300, so at least I am able to test components and compare values between the good unit and the bad. My question is directed toward someone who might be familiar with a simple LED power meter circuit and, from my description, be able to point me toward which components might be most suspect. What might cause the LED's to not turn on discreetly, but instead "smeared?" Some voltage is leaking somewhere. A diode? Capacitor? Or might it be more difficult to pinpoint, such as a driver IC or such? Having popped the hood, there are four identical IC's, two per channel. Since the problem is in both channels, and no LED's glow on a cahnnel that has been disconnected, I am thinking it's not likely an IC, but a simple component common to both channels.
I have pictures of the unit and even the schematic from the manual. Having never used a image hosting website, I need recommendations for an easy-to-use image hosting site so I can link pictures of the meter and the schematic to this post. I'm taking a gander at ImageShack right now. Looks like it might be pretty easy. Please be patient for the images and my replies to be posted, as I can only access AVS Forums from work (three days per week), since my browser at home crashes every time I try to access AVS Forums from home.
Thank God my too-cool APM-200 power meter is still going strong, the one with two softly glowing analog meters, each crowned with five red LED's at the top.