405 - R33 and TR8 burned

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12-Feb-2020 12:46 PM

Lars

Lars

Posts: 2

Hi

I have restored and upgraded two 405's. One works perfect, but out of my hands now. The other worked for a while, then I noticed distortion in both channels.

Hooked up a signal gen and scope. The output clips the neg. at about 10V. Pos. is not clipped.

Opened up and found R33 burned. Inspected TR8, also defect. No other visual damage found. The driver tr appears ok, and I get the non-clipped output up to 10Vpp. They are 2N3773.

Changed R33 and both drivers to TIP42C. Power on and immediately R33 smokes again.

The clamp circuit is mounted, so there should be no DC on the output.

Note this problem appear on both channels.

I don't really know where to look, any guidance or ideas of what the problem could be?

Thanks and regards,

Lars



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20-Feb-2020 04:12 AM

EJP

EJP

Posts: 1742

This would indeed suggest Tr8, but because it was trying too hard to turn Tr10 on, either because Tr10 was open-circuit or disconnected somehow, or more typically because Tr9 was s/c between collector and emitter.  The clamp circuit only comes into operation at 6 volts so it is not exempt from scrutiny, and neither is the DC offset, which you should measure.

EJP

Dada Electronics Australia

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20-Feb-2020 10:41 AM

Lars

Lars

Posts: 2

Thanks, afterwards I checked TR9, TR10 and both are defective. Actually I replaced them myself, and they worked for a while. I purchased them cheap on ebay.. To investigate one of them, I carefully opened the housing and found one of the bondings got disconnected from the silicon. Now off to buy some genuine parts.

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