I was visiting a friend who sent me home with a broken amplifier out of a pair of powered column speakers. It's an Acoustic Research Hi-Res Series AR1 and the Intertubes and the current website for the manufacturer haven't been forthcoming with a schematic. I can't even get a user manual to understand just what I'm dealing with (it's not as simple as line in and speaker out).
Initial complaint was, unit wouldn't power on. Internal 5A mains fuse was blown. Checked toroidal power transformer secondaries and they're all fine.
There are three boards; one is a power supply board but all it really does is rectify and filter the +-130V supply for the power amp board and route the other PT secondaries to the other two boards. There's a preamp board that gets an 18VAC secondary and the power amp board gets two other 18VAC secondaries that seem to be turned into +-15VDC or so on the power amp board. I think the power amp board is a biamp. The power amp board seems to have some sort of standby circuit on it. There are four pairs of TO-220-case transistors bolted to the metal back panel and then there are two pairs of big ol' honkers in what I think are TO-264 or TO-3P cases. I ohmed the big ones out in-circuit and discovered that one of the two NPNs, a Toshiba 2SC3281, had dropped the "semi" from "semiconductor" and was acting as a pair of jumpers.
I removed both the NPNs, established for certain which was the bad one, left it out, and soldered the good one back in. When I applied power, the mains fuse did not blow but what was weird was that the amp itself - not speakers; no speakers were connected - was making a very pointy-sounding 60Hz noise. I turned the variac down to supply about 50VAC, just enough to have the onset of the noise. I have tried but so far failed to determine what component(s) are vibrating to make the sound, but I got out the scope and saw nasty waveforms on the power transistors.
This leads me to suspect the +-18 power supply for the power amp board and sure enough, if I disconnect those PT secondaries the noise stops. But before I go any further I want to ask if the noise could have anything to do with running the amp with one of those two paralleled NPNs missing. I'm not giving the amp any audio input and as I said, no speakers are connected as of yet. I'd like to avoid making one purchase of a 2SC3281 just to turn around and order more parts separately.
Initial complaint was, unit wouldn't power on. Internal 5A mains fuse was blown. Checked toroidal power transformer secondaries and they're all fine.
There are three boards; one is a power supply board but all it really does is rectify and filter the +-130V supply for the power amp board and route the other PT secondaries to the other two boards. There's a preamp board that gets an 18VAC secondary and the power amp board gets two other 18VAC secondaries that seem to be turned into +-15VDC or so on the power amp board. I think the power amp board is a biamp. The power amp board seems to have some sort of standby circuit on it. There are four pairs of TO-220-case transistors bolted to the metal back panel and then there are two pairs of big ol' honkers in what I think are TO-264 or TO-3P cases. I ohmed the big ones out in-circuit and discovered that one of the two NPNs, a Toshiba 2SC3281, had dropped the "semi" from "semiconductor" and was acting as a pair of jumpers.
I removed both the NPNs, established for certain which was the bad one, left it out, and soldered the good one back in. When I applied power, the mains fuse did not blow but what was weird was that the amp itself - not speakers; no speakers were connected - was making a very pointy-sounding 60Hz noise. I turned the variac down to supply about 50VAC, just enough to have the onset of the noise. I have tried but so far failed to determine what component(s) are vibrating to make the sound, but I got out the scope and saw nasty waveforms on the power transistors.
This leads me to suspect the +-18 power supply for the power amp board and sure enough, if I disconnect those PT secondaries the noise stops. But before I go any further I want to ask if the noise could have anything to do with running the amp with one of those two paralleled NPNs missing. I'm not giving the amp any audio input and as I said, no speakers are connected as of yet. I'd like to avoid making one purchase of a 2SC3281 just to turn around and order more parts separately.
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Turns out that the regulators on the +-15VDC supply are not working so I need to address that before continuing.
I already dealt with these things a few years ago.
take particular care in checking all welds and particularly on the components with the most overhang.
take particular care in checking all welds and particularly on the components with the most overhang.
Hi WattPusher. I've fought with several of these - mostly with little luck. Two or three just as you described - one 'output' shorted. You might also find that one or more of the FETs in the 'tracking down converter' have trouble. May have stumbled onto something just today. If you are still trying / have access perhaps you might please check these connections and let me know should you find them bad.
Found two with bad connections today - in the burned unit and another that's now working for the moment. (All outputs, drivers, FETs, optocouplers and much, much more replaced / repeated failures.) Hope this helps. Cause or not, nothing should burn this badly with the correct fuse types / sizes in place.
And as you may have already discovered; a significant -80VDC remains on the units long after power is removed. I'm revealing an anti-Carver bias. Worked in the industry most of my life and after wasting countless amount of time and other resources have only successfully repaired one or two Carver units or their ilk. Good luck! Here are some hints I found previously on this forum.
Thanks to Jack Hidley - clues have been an immense help.
Thanks to Jack Hidley - clues have been an immense help.