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Old 18th November 2012, 02:36 PM   #31
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Location: Louisiana
The voltage still don't look right but since it's a new op-amp, it must be OK.

Look at the board. There are several jumpers and resistors that have solder on the top of the board. If those legs of those components also have solder on the other side of the board, ALL of them are suspect. When solder is used on both sides of cheap circuit boards, the result is almost always poor quality, intermittent solder joints. The best solution is to have solder on only one side of the board and use a wire jumper to make the connection to the other side of the board but this is time consuming. The alternative is to desolder the components, scrape their wire leads and then resolder. This will generally allow the amp to work normally for a while but the problem will almost always return in time. When this type of connection is used in the power supply drive circuit of an amp (as was used in the Pmiller series of bazooka amps), the bad connections causes the FETs to fail.
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Old 18th November 2012, 03:24 PM   #32
wrxer is offline wrxer  Australia
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I have a scope grab as well. These are from C412 (C312 is very similar in waveform), next to the switch. They seem to recieve a signal from the HP/Full/LPF switch. The HP/Full selection is the same and the LPF setting makes a slight change in amplitude.This grab is from the HP/Full/LPF switch side:
scope grabs 001.jpg
10mv horizontal and 10us verticals. Around 4mv peak to peak, (I think it is 0v offset, I need to recheck this. I think my DSO has a built in +15mv offset as it doesnt return to 0v when on this fine scale. even my DVM shows +20mv when its not connected to anything)

The other side of the cap shows:
scope grabs 002.jpg
10v horizontal and 0.2s verticals. Around 25v peak to peak, (and 0v offset, also need to recheck this, but I think thats right)


The caps are 10uf and 50v. I dont know, maybe they are getting the signal from somewhere else (backvoltage?), but this is the only part that i can find that has a very different signal from one side to the other, and it seems to be the point that feeds into the bass boost area.
I do not know if it is possible for a cap to produce that waveform and if it is correct.
I will look at the ch1/2 section of the board tomorrow and see if i can find the corresponding caps and look at its waveforms

Last edited by wrxer; 18th November 2012 at 03:35 PM.
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Old 18th November 2012, 04:20 PM   #33
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In my opinion, you need to clean up all of the solder connections that I previously described as problematic before you can do any further troubleshooting.
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Old 21st December 2012, 01:07 AM   #34
wrxer is offline wrxer  Australia
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Ok, thanks Perry.
I have cleaned all the solder connections. I have removed and reinstalled the surrounding components to gain access, and cleaned the joints you have suggested.
It seems to be ok now at idle. Ch4 now has 1mv bias and has the least bias of all channels.

I will bring it home with me and replace the cut output Tips with the resoldered legs, ensure that R169 is properly resoldered, and run it up on the test bench till it reaches thermal shutdown.

Thankyou very much and Merry Christmas.
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Old 10th January 2013, 01:20 PM   #35
wrxer is offline wrxer  Australia
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I have been running this amp only from bridged channels 3-4 into a 12" sony sub and box. The amp is balanced on the corner of the workbench and provides the bass reinforcement for my shed stereo.
It has been running nicely. I still havent fixed the Tips yet, but I havent driven it real hard and tested it yet.
Still it is Ok for now.

Cheers
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