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Old 11th December 2005, 01:35 AM   #1
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Default powering up old amp-120hz at output?

I posted last week about using a variac to power up and old Sugden amp and so far so good.

However, probing around I see 120hz from one of the outputs. It increases as I increase the variac supply voltage and is about .2v with a supply of about 70v. The other channel has none of this.

Any thoughts? Is this a bad cap or perhaps a bad transistor?
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Old 11th December 2005, 02:07 AM   #2
poobah is offline poobah  United States
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Most likely a bad cap. Age aside. has it sat a long time without being powered? If it has might be good re-cap before you go much further. A bad cap could fry a tranny and then your scr__ed.
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Old 11th December 2005, 03:04 AM   #3
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Thanks poobah,

I did just swap the power supply caps (2900uf) and the output coupling caps (2900uf) as well as a couple of others that looked iffy. My plan is to change all electrolytic caps after she runs and now I have confirmed that the left channel is well not alive yet. The good news is that the tranny seems fine and the voltage at the PS cap is fine for that channel.

Best I can tell the preamp section is passing the 120hz and none of the caps are warm. I am wondering if a diode is belly up.
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Old 11th December 2005, 03:34 AM   #4
Leolabs is offline Leolabs  Malaysia
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Get a scope at the PSU section,check for ripple.
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Old 11th December 2005, 04:36 AM   #5
poobah is offline poobah  United States
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Just a silly thing... but make sure your inputs are shorted when you're looking for hum.

You got a good plan going... change out all the lytics anyway just cuz' if they ain't gone their goin'. You're on top of your tranny... which is what counts for now.
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