Need help with SC480 Build

tm1988

Member
2013-04-12 8:59 am
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?
 

Keith Taylor

Member
Paid Member
2007-11-04 2:06 pm
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.

Keith
 

tm1988

Member
2013-04-12 8:59 am
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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