same amp, new prob MTX 8302 LP filter - diyAudio
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Old 7th December 2008, 07:20 PM   #1
damage is offline damage  United States
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Default same amp, new prob MTX 8302 LP filter

Hi!

High-pass and Full-range filters work. Low-pass filter makes a loud thump when engaged and then there is no sound.

It pops when I change the RCA output xover.
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Old 7th December 2008, 09:30 PM   #2
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Location: Louisiana
Measure the DC voltage on the output pins of the op-amps near the RCA end of the board. Do any have more than a few millivolts of DC? If so, does the offset vary significantly when adjusting the gain?

What is the DC voltage on pins 1 and 8 of U6?

Use the RCA shield as the reference (black meter probe on RCA shield).
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Old 7th December 2008, 11:33 PM   #3
damage is offline damage  United States
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I cannot tell which one is U6 because the board is marked underneath the opamps.
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Old 7th December 2008, 11:36 PM   #4
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U6 is the 8 pin SIP IC.
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Old 8th December 2008, 01:11 AM   #5
damage is offline damage  United States
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The NE5532 right behind the gains has significant voltage change on the outputs when gains turned. The rest appear to have 3mV of offset.

U6
1: -.75V
2: 3.2V
3: .4mV
4: -2.6V
5: -57mV
6: -22mv
7: .4mV
8: -.75V
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Old 8th December 2008, 01:29 AM   #6
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Pull U6 and install a jumper from pin 1 to pin 8. Does the amp work normally (low frequency crossover working, no pop, no DC offset)?
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Old 8th December 2008, 03:02 AM   #7
damage is offline damage  United States
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Hello!

Yes, after removal it appears to work normally. Slight pop when engaging the switch, but nothing like it was before. Offset is now normal on the NE5532.
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Old 8th December 2008, 03:20 AM   #8
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Replacing the THAT 2155 will probably solve the problem The 2155 is obsolete. You'll have to replace it with a 2180 (mouser - search for THAT 2180).

The amp will work without it but the remote level controller and the compressor (if it has one) won't work.
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Old 8th December 2008, 03:30 AM   #9
damage is offline damage  United States
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Hi!

I'll get a replacement. At least the compression circuit is useful.

Thanks Again Perry!
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