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Old 28th September 2009, 03:32 PM   #1
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Default RF 250a2 1.9Vdc on output

The right channel seems to have dc on it. Measuring with a meter I see about 1.9 volts across the output terminals.
For troubleshooting purposes I have 1khz sine wave input and followed it through the input section, looks the same as the working channel. I notice the waveform distorts once it gets past C223 and R264. At that point the wave form seems to clip off the upper part, I'm not seeing the amplitude I do on the working channel either.
Would the output fet cause this, it seems the drive circuitry would be the failure.

thanks,

cory
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Old 28th September 2009, 03:38 PM   #2
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Lift D202 and D204. Does the signal remain clipped?

Be careful. This will disable the protection circuit so you should have the transistors clamped and have a 10 amp fuse (or a current limiter) in the B+ line.
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Old 28th September 2009, 04:05 PM   #3
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My waveform looks good now. With the protection circuit eliminated my output won't function. That would make sense to me.
If I place one diode back in will that allow me only high side or low side switching?
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Old 28th September 2009, 04:11 PM   #4
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If having D202 in the circuit causes clipping, check Q206/207/208.

If having D204 in the circuit causes clipping, check Q216/217/218.

Also confirm that the 0.1 ohm source resistors are within tolerance.
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Old 28th September 2009, 04:51 PM   #5
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I don't what happened after soldering D202 waveform looked good, then I soldered 204 and it still looked good. Right now I have it running on my bench to see if the problem comes back. Maybe a solder joint issue, poking around the transistors may have secured it for the time being.

Thanks,

Cory
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Old 2nd October 2009, 01:53 AM   #6
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The problem has returned. The left channel works nice the right has clipping/distorted wave forms.
The attached waveform is what I see at the base of q203, the left channel I see a nice clean 1khz sine.
The left channel ties to the base of q102 and the right to the base of q203. What is being accomplished in that part of the circuit.

Thanks,
cory

250a2 schematic is located here:http://sites.google.com/site/brandescm/home/projects
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File Type: jpg DSCN0741.JPG (930.6 KB, 54 views)
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Old 2nd October 2009, 02:21 AM   #7
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brandes,

quick question, on your 250a2, can you see the any numbers that are just under the transformer? should be something like pc-xxxxx-xx.

are you sure you are looking at the right schematics because i have a 250a2 also, but the versions of the 250a2 are different, and don't trust that the heat sink positively i.d.'s it because my 250a2 should be in the heat sink that has the "r" molded in it, but it actually has the older style.

if you see pc-3080-xx anywhere on the circuit board, then you are using the wrong schematics as the pc-3080 calls for the IRF540H, not the IRF540, not that that would make a difference, but according to what i have seen so far, the fosgates tend to be picky at times.
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Old 2nd October 2009, 02:33 AM   #8
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Did you try lifting those diodes again?

The left channel goes into the non-inverting input of the differential amp. The right channel goes into the inverting input of the differential amp. This makes the right channel inverted so that the amp is bridgeable.
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Old 2nd October 2009, 03:01 AM   #9
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toobnoob, the silk screen shows pc-2035-D. This amp apparently has been worked on. the right channel fets are irf9540 H849 L and irf540 H9 01. The left channel show irf9540 H905 L and irf540 H909 L. The 9540 in the right channel has been replaced, just by looking at the solder on the tab.

Perry,

Haven't lifted them again, the right channel fets and q9 get extremely hot when it isn't operating correctly. I have no load on the outputs. I shut it down and let cool for a minute and now it is running fine again.

edit:I removed d202 it was still operating correctly before I removed it though. I will have to check it when it gets back to room ambient. If it is still operating correctly when I check it later then I will test those transistors again.

Last edited by brandes.cm; 2nd October 2009 at 03:23 AM.
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Old 2nd October 2009, 05:05 AM   #10
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cool, was just curious.
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