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Old 26th August 2006, 10:59 PM   #1
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Default Rythmik Audio 250 basic plate amp help needed please!

hi, my sub amp made a strange noise (hard to explain) but, then quit producing music and just had a medium level hum wasn't real low like a 60hz hum but higher. i quickly, shut it down and looked it over real well and saw nothing obvious or burnt looking, and no smell. the amp appears to use 2sc5198/2sa1941 outputs. there is a pair of 5198s on one side, and a pair of the 1941s, on the other side. in between the two pairs is a MPSA13 Darlington. i checked all transistors on power board plus, the outputs 5198/1941. they all checked fine not quite sure how to check the darlington! now, when i plug unit in nothing gets hot, or smells but, i have almost full negative rail voltage of -68.8v at speaker wire outputs? any help would sure be appreaciated! could it maybe be the MPSA13? since collector feeds to one side and the emmiter feeds the other side? thanks i also checked all diodes and bridge rect. as well
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Old 27th August 2006, 05:23 PM   #2
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-68.8DC is not normal.Something is shorted ,probably transistor outputs.Fuse blown?Positive/negative supply OK?Recheck again.
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Old 27th August 2006, 05:30 PM   #3
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The MPSA13 will probably be temperature sensing for the bias current or thermal protection. Take a look if any fuses are blown. Measure the rail voltages.
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Old 27th August 2006, 06:13 PM   #4
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hi fellas and thanks for your help! i checked at the bridge rect. and the ac in is around 100v. the pos. out of bridge is around +67-68 vdc in respect to ground and the negative volts out in respect to ground are also, -67-68v. dc i removed all output transistors and, drivers and checked for shorts and opens with diode check on my fluke and all tested fine? if the bridge rect. has correct output voltages and all output transistors have around 67-68v. dc +/- going to their collectors, i wouldn't have a blown fuse would i? or is it still possible. i've looked and still, haven't been able to find any fuse yet. when the amp ouit, it had a light buzz comming out sub so, i thought maybe a cap, diode or , rect. problem but so far, i can't seem to find anything. could an output be shorting under load when powered up? i've always thought either they were permanent shorted, or open when faulty. thanks very much for your help and any more, will sure be appreaciated! can't seem to find schematic sorry!
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Old 27th August 2006, 07:00 PM   #5
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A dodgy output device can test OK with a diode check. I'm surprised there is no fuse anywhere. Yes as you say if there really is voltage at the output devices that part must be OK.

A blown cap could cause hum, but I would expect it to be bulged a bit so should be possible to spot with the naked eye. Try putting the drivers back in and just one pair of output devices and see if anything's different, just don't crank it with only one pair.
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Old 27th August 2006, 07:12 PM   #6
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just found out i have around -67 v. dc on every base, and emmiter of all the outputs? nothing even warm? but the collectors, on the npn devices have their proper positive, 68v. dc?
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Old 27th August 2006, 07:16 PM   #7
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Without the schematic it hard to diagnose, but I suspect that the VAS transistor might be open circuit.

edit: just seen your edit. It looks like it uses a CFP/Sziklai output configuration then. Trace back to the drivers and their bases should give you more what voltages you are expecting to see.
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Old 27th August 2006, 07:20 PM   #8
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yeah, i know what you mean. i tried to get schematic from rythmik but, no luck.
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Old 27th August 2006, 07:27 PM   #9
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the positive speaker out red wire, feeds off the outer pnp emmiter which has around -68v. so some how, that emmiter is getting the same collector voltage? what a mess LOL!
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Old 27th August 2006, 07:45 PM   #10
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I'm confused,are you taking measurements with trasistors in or out of the pcb?There should be a fuse just follow the AC input to power switch and somewhere along you should see it,either a glass fuse or something like a resistor green or yellow marked with XA /110V.You are wrong to say red wire feeds off the emitters of pnp when conventional complementary circuit ,the collecters are the oitputs unless it's a different topology.
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