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Old 24th October 2010, 02:19 AM   #1
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Unhappy Scott LK48-B restoration

I’ve changed a bunch of parts in my Scott LK48-B: all the coupling caps, all the electrolytics and all the resistors out of spec… I made a stupid mistake when soldering the four 100uF bias caps: I forgot that the can was common positive… That cost me one 10 Ohm resistor (burnt), one 100 uF cap (blown) and the very strong and original 5AR4 – gone… Once I replaced the faulty components (33-Ohm instead of 10-Ohm brought the negative bias to -46V) I turned the power on. What I got now was very strong hum (regardless of the volume)… I quickly measured the voltage behind the rectifier: 430V DC but then it starts going down… At the same time the AC is present after the rectifier: 600-something volts and rising… I immediately shut off the power…
What’s causing the hum and the stratospheric AC voltage? Are there any more leaking/bad electrolytics caused by my mis-wiring? My Heathkit IT-11 is not working currently so I can’t check the caps; and I hate to de-solder everything – the space inside the amp is so tight, it takes forever to do any kind of work…Not to mention the fact that I don’t know where to start…
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Old 24th November 2010, 08:00 PM   #2
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I finally found the time to work on the amp some more… All the caps are fine, resistors as well… Voltages are identical to those in the schematic… But: there’s the hum… Not too loud but it’s there nevertheless… When the loudness control is at the minimum a faint sound (CD player hooked into the EXTRA input) can be heard from both speakers. As the loudness control is turned up only one channel gets louder, the other stays barely audible… Any ideas/explanations as to what might be taking place here?
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Old 24th November 2010, 09:52 PM   #3
kevinkr is offline kevinkr  United States
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Do you have a scope or can you borrow one? Most likely in the restoration you have made a wiring mistake or you have a bad capacitor or in some cases a dead tube. A scope will help you to quickly find where you are loosing the signal.
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