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CR 820 Resistor burns up when powered on

Nuklhed

Nuklhed

Active Member
Recapped the 820, including the filter caps which had degraded to about 75%. Deoxit all switches. No left channel. Swapped the output trannies made no difference. Replaced all left channel diodes and transistors 701-721 in left channel. Found fuse resistor FR701 open. Replaced it with a 68 ohm resistor. As soon as I turn power on, that 68 ohm resistor burns up. Thoughts?
 
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Nuklhed said:
Replaced all left channel diodes and transistors 701-721 in left channel. Found fuse resistor FR701 open. Replaced it with a 68 ohm resistor. As soon as I turn power on, that 68 ohm resistor burns up. Thoughts?
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Either you have a solder bridge in your solder work. Or one or more of your new transistors is now shorted or installed incorrectly.
 
Nuklhed said:
Found fuse resistor FR701 open. Replaced it with a 68 ohm resistor. As soon as I turn power on, that 68 ohm resistor burns up.
Click to expand...
Consider building a Dim Bulb Tester, a current limiting device, to reduce the carnage, numerous plans on AK.

Test TR719, TR721, TR723 and TR725 (probably 721 or 725). Resistance check R763 and R765(both 4.7) and also emitter resistors R771 and R773(both 0.22)
 
Thanks so much for the help gents. Status update: Found both TR723 and 725 shorted. Pulled and tested TR719 and 721, both good and reinstalled. Pulled and tested the 2 4.7 ohm resistors in the white sleeves, both tested good but replaced with new. Inspected back of board in detail…not seeing any solder problems. Still have to check the emitter resistors. I mentioned the dim bulb tester to the missus….she pointed at the mirror!
I have what I need to build one..thanks again!
 
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BTW…should I be testing the sugar cubes? I’ve only ever seen one fail on a 1040 and it was pretty dramatic.
 
Sorry, that’s what I’ve been calling them. A bunch of resistors packed between the transformer and the output transistor wall.
 
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