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Old 17th July 2007, 06:55 AM   #1
bojo2 is offline bojo2  Australia
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Default SC480 woes/ need smart people :D

Hello.

I'm building/built an SC480, although i can't seem to set the queiscent current at all.

VR1 does have an effect, but not what i'm expecting.

The voltage across one of the setup resisitors (560ohm5W ones) is only ever maxed at 6v!.

This is a far cry lower than the 28v i need!.

Changing the resistor to max one way gets 6V max the other brings it down to numbers which are close to 0V.

I've compared the voltages at various points to the articles/jaycars ones, and i've found 3 that doint match up.

On the +40v side the two 100ohm resistors don't match.

The one in series with the 1N4148 diode, has a voltage of 0.445V (instead of 0.7V)

The 100ohm resistor which sits inbetween the +40V rail, and the E on Q6 has a voltage of 0.5v (instead of 0.9v).

The other mismatched voltage is the 6.8kohm 0.5w resistor in parrarel with a 12nF capacitor. I get a reading of 30V from this instead of 50.5V.

Anyone have any ideads?
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Old 17th July 2007, 11:33 AM   #2
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hi bojo2,

I thought I'd post the schematic. It may help.

regards
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Old 17th July 2007, 11:50 AM   #3
sandyK is offline sandyK  Australia
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Default SC480 woes

Ask Silicon Chip ! I don't think you would have to wait for them to publish an answer . (?)

SandyK
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Old 17th July 2007, 02:07 PM   #4
bojo2 is offline bojo2  Australia
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is there anyway i can ask silicon chip without signing up to the magazine?.
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Old 17th July 2007, 02:07 PM   #5
bojo2 is offline bojo2  Australia
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btw thanks for the schematic greg, i wasn't sure whether i would be allowed to post it or not.
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Old 17th July 2007, 02:31 PM   #6
rabbitz is offline rabbitz  Australia
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Send them an email as they do respond with questions on their projects.

Check the voltages on your power supply first.

Most of these problems are components of the wrong value or in the wrong place. Check and recheck. Go away and have a smoke / coffee and check again. It's surprising how you can overlook these errors.
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Old 17th July 2007, 03:11 PM   #7
Nordic is offline Nordic  South Africa
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Most advice doesn't include anything as drastic as takeing up smokeing, you know RabZ.
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Old 17th July 2007, 03:13 PM   #8
rabbitz is offline rabbitz  Australia
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Nothing like a bit of nicotine to get the brain going... *cough cough*... OK, some weed then.
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Old 17th July 2007, 05:04 PM   #9
Nordic is offline Nordic  South Africa
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Touché, Monsieur Pussycat
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Old 18th July 2007, 04:51 AM   #10
bojo2 is offline bojo2  Australia
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power supply is working fine.(checked and checked +39.9v and -39.9V on both rails ) i think maybe a capacitor may not be working but?. i have my doubts about the 12nF cap thats in parrallel with the 6.8k resistor, cause im missing approx 20v along that resistor, so im wondering where it all went :P.

haha and i've been checking and checking again and checking my checking . if i've done it wrong im overlooking it badly , but i still could be, ill check again tonight lol.

i'll also mention that i had the power lines switched to begin with, though the 560ohm resistors were in place. I'd wager this was the problem, and some lil component has died, but it's my first amp, and i'm not to hot with the theory, so im not 100% sure what may have bit the dust.

So what would be most likely to kill itself, with about 2-3 seconds reverse polarity? . So i can start looking at things. Short of desoldering the whole kit, checking each component, and resoldering it again :P

i don't smoke matey, filthy filthy habit :P.
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