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Old 10th March 2013, 03:49 PM   #21
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The 4v of DC on pin 5 of U300 is possibly being caused by a leaky Q313 or a defective U300.
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Old 11th March 2013, 01:45 PM   #22
ultra is offline ultra  Greece
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Strange things....
i replaced U300 again as well as Q313 even though the transistor measured ok.
same thing, pin 5 of U300 has -5.30V.
without U300 in, i also removed R303 and D1 and there is no voltage on pin 5 (in the pcb pad of pin 5).
Now, with D1 soldered back to the circuit, i still have no voltage on pin 5 pad.
But as soon as i solder back R303, i get around -13V on the pad of pin5...

Last edited by ultra; 11th March 2013 at 01:47 PM. Reason: spelling
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Old 15th March 2013, 11:47 AM   #23
ultra is offline ultra  Greece
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back to this amp
whenever i replace U300, i get the same

pin 2 has 0.08V
pin 3 0.19
and the output pin 1 has 4,30V
Any other ideas?
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Old 16th March 2013, 12:05 AM   #24
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If you remove R300, do you get ~0v on pin 1?
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Old 16th March 2013, 01:04 AM   #25
ultra is offline ultra  Greece
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Yes i do.
In the mean time, i did mount the output fets on to do some testing.
The amp idles fine, i do have a 105Khz square wave on pin 2 of both UC 3706 (although not perfectly clear), and no dc voltage on the output terminals.
There is no output, only a noise when i tested with a speaker. (it swings between 0.9 and 1.5VAC)
After testing the pre amp section, i found that most op amps had DC on their output pins.
I isolated them from the regulators and i only powered the pre amp with -/+12V from an external source.
It works fine, the pots are responding as well as the switches.
There is a nice clear 100Hz sine wave on pin 1 of U107, and of course no DC.

However, when i removed R300 to check whether pin 1 of U300 gets low, i noticed that C355 that i had replaced earlier, had swollen.
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Old 16th March 2013, 02:37 AM   #26
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The op-amps are mostly DC coupled so errors add up. For testing, you can sometimes switch in the subsonic filter to reduce the DC.
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Old 16th March 2013, 02:50 AM   #27
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Is it ok if i feed directly C355 to see if the amp has some output?
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Old 16th March 2013, 03:06 AM   #28
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If you feed that capacitor directly, it will drive signal back into the previous op-amp which could cause it to oscillate. If you want to do that, I'd suggest lifting the end of R300 connected to C355 and then injecting signal into the resistor.
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Old 16th March 2013, 06:26 PM   #29
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After replacing a leaky D37, i fed the amp at R300 removing C355. There is a loud mid to high frequency hum, but on the background i can hear the test signal (100Hz) getting louder as i increase the gain.

where should i check to eliminate this hum?

When i soldered back C355 and fed the amp from the RCAs, there was no 100Hz sound, only the same hum. This is not a problem, i can easilyreplace all 8 op amps.
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Old 16th March 2013, 10:07 PM   #30
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Did you have your signal source grounded to the RCA shields when you injected signal at R300?
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