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Hello inmates,
I have a hum in the phono section of my Scott 222c integrated amp. I recently replaced all the filters in the power supply and it's been running very well and very quiet. The hum only occurs when phono is selected and it's in both channels. It occurs regardless if the TT is plugged in or not, or if the inputs are shorted.
The hum is not present when the volume is down, and this is what fooled me. It is quiet until you hit about 2 on the volume, then you can notice it and it gets successively louder as you turn up the volume. I thought it was OK until I got my turntable hooked up to it this month.
I actually thought everything was perfect. There is no hum in any of the line stage and I been playing mostly CD's, I tested the phono but only at low volume, I did not turn it up until lately, so this problem went unnoticed until now.
The hum seems to be 120hz hum to my ears. I tried grounding the filter in different locations but result is the same. I do not think it is related to the filter supply. Checked the resistor values in the phono stage, they all seem good, nothing else was touched..
A 120hz hum that increases with volume, I have dealt with hum problems before, but they are usually 60hz and do not increase with volume..
I'm totally lost, any help appreciated..
J
Follow Ups:
Well, my first mistake was downloading a schematic from the HH Scott site. There's a glaring mistake in the negative power supply (see below). No wonder it hums!!
That aside, this amplifier powers the phono and tone stage filaments with DC from the bias supply. The supply would have to be really dirty for the problem to become so obvious at the output. A serious defect in that supply would also affect bias on the output stages, but that woldn't be a phono-only issue. That leaves the B+ caps C202 and C201 (phono/tone B+ filtering) as possible culprits. Are those the caps you replaced? Also, I know you said it sounds like 120 Hz, but are all the tube shields in place?
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Buy Chinese. Bury freedom.
What is the glaring mistake? I just rebuilt a Scott 340B with the same bias circuit (different resistor values in the 340B) and I get very little hum at no signal input & maximum volume. I used a new bridge rectifier and 4x 100uF caps plus changed the first resistor in the 340B to 22 ohm to get the voltage back to stock.
Look at the rectifier diode bridge. Two diodes are drawn backwards.
H-K Citation IV schematics have the same error, just FYI.
Look at that!
ha, I double checked to make sure it doesn't follow that part of the schematic :)
yes all the tube shields are present, and all of the filters and DC caps have been replaced including 201 and 202, now a 50/50 F&T..cap. I experimented changing the ground of that cap to different locations but no change..
It is hard to tell with hum if it is 120 or 60, I could be wrong..
Is this an amplifier you purchased recently (any chance it's been modified)? Other than the hum, does the phono section sound about right from the standpoint of tonal quality? Are both channels humming?
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Buy Chinese. Bury freedom.
no, I have owned this amp for a long time now, no mods other than the usual upped capacitance from the new filters. Coupling and by pass caps were replaced earlier about 4 years ago, along with selenium rectifier..- the usual restore stuff... re-did the DC with 4 Muse 700uf 35v, new resistors in the DC and new resistors in the B+ between the caps. Voltages on the B+ stages are spot on.. I would venture to say however I just got my Turn table out and I don't recall if the phono already had this issue or not. It's possible it was always this way and I never noticed it..
Phono sounds correct other than hum..yes both channels hum..
If I really turn up the volume past 12:00 on the aux or any line stage I can get a slight hum around 1:00 2:00 but this is past the amps usable power and I would never need to turn it up that far ever..Not sure if that is unusual. I have super sensitive speakers 98db, so some hum or noise is bound to come through when that high..
If I turn the volume up past about 8:00 -9:00 on phono however, very noticeable hum occurs..
Well, at this point, I'm pretty much out of ideas for a quick fix. I think you need a scope to measure ripple on the various B+ voltages. Normally, I don't encourage cross-posting, but maybe you should post this problem on the TubeDIY forum. Some of the guys who hang out there also repair factory gear. You might find someone who's more familiar with the day-to-day idiosyncrasies of this amplifier than I am.
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Buy Chinese. Bury freedom.
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