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No input signal yet still getting output? and clipping
No input signal yet still getting output? and clipping
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Old 28th December 2015, 12:42 PM   #1
singaporerock is offline singaporerock  United States
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Default No input signal yet still getting output? and clipping

Hello you great people,

I have been on my first repair of a crown xs1200. I tore it all apart and discovered immediately two dead output devices on one of the two channels. these were both pnp devices. EACH channel has 4 pnp devices and 4 npn devices. amp works fine with the two duds removed. no dc offset. no ac voltage on outputs. i replaced two of the pnp devices, checked bias voltage (good at 3mV) and was ready to check for balancing but then! the amp turns itself off, turns back on again, and now the bias is at 1.0v (on both channels, thats the weird part) , the bias resistor doesnt do anything to change it no matter what direction i turned it. used an infrared temp device and all the devices are heating up fast up to 120F. ok so i turn the amp off and back on again manually. all better normal bias great, however the input light is on, with no signal input (both channels) i check the output, No dc. But when you turn the volume pot up, it goes into clipping (again no input signal) and the output is around 25v AC. and POP goes the 10amp fuse in my variac. (voltage set to 125VAC on the variac, measured with dmm) checked the temp of o/p devices and the channel i drove into clipping was around 90F while the other channel was 80F (ambient temp is around 55F) so i was frustrated and gave up for now. didnt check with an O-scope, didnt hook up the old series light bulb trick. and im going on vacation for new years so i cant mess with it until january 8th. but just wanted to throw this out there.

I think i will try removing my two new pnp devices, see if the amp is stable at that point. i performed a diode test on ALL the pnp output devices (new and old) and they are all around .5v but did not check the gain. not sure if this would cause the other channel to have issues and warm up as well though. Also Voltage between heatsinks when running normally is ~140v. when the bias goes nuts it dips down to ~120v, only AFTER the bias increases.
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Old 30th December 2015, 02:42 AM   #2
kct is offline kct  Canada
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No input signal yet still getting output? and clipping
Check for oscillation. To me it sounds that perhaps some nasty oscillation occurs causing over current to be drawn and perhaps kill devices. I highly suggest you utilize the old light bulb...

Also, I would look into the BIAS circuit, perhaps a transistor is on the way out leaving you with uncontrolled biasing.

Last edited by kct; 30th December 2015 at 02:49 AM.
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Old 30th December 2015, 10:13 AM   #3
singaporerock is offline singaporerock  United States
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Hello,
Thanks for the reply.
what would cause it? Failed op amps? Or miss matched OP devices? Or some mystery component failure :O
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Old 31st December 2015, 01:55 PM   #4
djoffe is offline djoffe  United States
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I agree that oscillation is a really good guess. There are many things that can cause it, so it will be a matter of eliminating the many possibilities...amongst the many...an open Zobel network at the output node. I haven't looked at a schematic, but 99.4% of amps have the Zobel network as an important of their stability.

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Old 31st December 2015, 02:03 PM   #5
kct is offline kct  Canada
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No input signal yet still getting output? and clipping
Quote:
Originally Posted by singaporerock View Post
Hello,
Thanks for the reply.
what would cause it? Failed op amps? Or miss matched OP devices? Or some mystery component failure :O
If I had to guess again, I would not guess a faulty OpAmp, more so a failing Transistor, burned or cracked Resistor or perhaps a cracked or broken ceramic capacitor.

Happy New Year
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Old 5th January 2016, 03:15 PM   #6
singaporerock is offline singaporerock  United States
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Hello,
bought new OP amps, a full set of electrol. Caps and will replace them all this weekend when i get time.

thanks
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Old 6th January 2016, 12:53 AM   #7
kct is offline kct  Canada
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No input signal yet still getting output? and clipping
Good luck.
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