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Old 16th January 2012, 07:40 PM   #1501
Marra is offline Marra  United Kingdom
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I'll post some tomorrow but its wired up pretty much like in post 77.
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Old 17th January 2012, 08:45 AM   #1502
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Power up again with the bulb and measure the new and probably lower voltages of the PSU.

Power down and have a think.
What have you added or changed that is drawing so much current to pull the PSU down like that?

Can you go back a stage or two, or is there really only ONE change?
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Old 17th January 2012, 12:27 PM   #1503
Marra is offline Marra  United Kingdom
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Thanks Andrew I will recheck things tomorrow when I get chance. I did'nt check the power supply voltages with the amp board connected but the bulb limited the mains voltage to about 90 volts. I will disconnect everything and recheck the pcb's with a magnifying glass.
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Old 17th January 2012, 07:13 PM   #1504
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Whoops I haven't done that in a long time It was a solder bridge between two pins on the 4780. I connected the second channel and have an offset of 260mv on both channels. This is without the input connected the pot; wires just hanging. Will the offset drop when I connect it up?
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Old 17th January 2012, 07:44 PM   #1505
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If I short the input wires together and connect a dmm to the output the voltage rises and the bulb starts to glow. Have I cooked my chips?
With nothing connected the LM 4780's are running at 40 degrees Centigrade.

Last edited by Marra; 17th January 2012 at 07:54 PM.
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Old 17th January 2012, 09:41 PM   #1506
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What is your configuration (parallel, balanced)?
When everything is done right, shorting the input will not damage the chip.

Do you have it on a heatsink?
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Old 18th January 2012, 09:30 AM   #1507
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They are wired in parallel; all resistors fitted except Ro and with a single transformer and one p/s board. The signal input wires are not yet connected to a pot. Yes they are mounted on a piece of 10mm alluminium appro 12"x4".
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Old 18th January 2012, 01:06 PM   #1508
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Test it when you have the pot connected. You will also notice that with low volume settings, the offset is much less.

When idling, the temp shouldn't go as high. Post some pics, if you have a chance.
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Old 18th January 2012, 02:16 PM   #1509
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Thanks Peter .I just connected the LDR volume control and have these dc offsets.

Right -17.8 rising to -149

Left 9.3 rising to 138.6

Are these passable?
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Old 18th January 2012, 05:00 PM   #1510
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The last time I posted a comment about variable output offset and described it as typical of a DC coupled amplifier, someone deleted my post.
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