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Old 12th April 2013, 08:04 AM   #1
tm1988 is offline tm1988  Australia
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Join Date: Apr 2013
Default Need help with SC480 Build

Hi all,

I've recently built 2 x SC480 kits I purchased from Dick Smith and am up to the point of adjusting the pot to meet the expected quiescent current output of 50mA. To do this, the resistors should be measuring 28V across, however mine can only get up to max 17-19V with the pot fully turned up. I resoldered all the joints on both boards and nothing has changed. Anybody have any advise on how to proceed from here?
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Old 12th April 2013, 09:11 AM   #2
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Location: The Jurassic Coast, England. GB
If you post the schematic, we should be able to help.
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Old 12th April 2013, 02:14 PM   #3
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Location: Adelaide
I built four of these from Jaycar kits. They supplied schematic drawings but they have no designations for the passive components.

What are your power supply voltages when loaded by the amplifier you are trying to adjust? Measure the voltages around Q5 and Q6. In particular the drops across the 100 Ohm emitter resistors and the supply rails to collectors.

Measure the DC voltage between the power supply 0 volts and the speaker output + side. This should be close to zero volts. Let us know what you find.

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Old 14th April 2013, 01:34 AM   #4
tm1988 is offline tm1988  Australia
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Harleyjon, please find attached.

Keith, my power supplies are 40VDC. when you mention measuring the voltages do you mean the 100 Ohm resistors in series with Q5 and Q6? If so, Q5 resistor is 1.38V (should be 1.6) and Q6 resistor is 0.53V (should be 0.9).

As for DC voltage between power supply 0V and speaker output +, this is 16.9V
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Old 14th April 2013, 07:33 AM   #5
tm1988 is offline tm1988  Australia
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Join Date: Apr 2013
Sorry Keith, just realised I measured -40VDC to output + which gave 16.9V. I just measured 0VDC to output + and this was 0V.
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