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Rotel RC-1070

jheu02

jheu02

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No pics to post currently as I'm away for work, but I started noticing a buzzing during quiet passages of music. Initially I thought it was the power amp, however, I narrowed it to the pre. Opened it up today,and the only thing I saw that looked suspect was a couple resistors near a darker section of the board. I initially thought they burned up, but upon removal, it almost looked like, dried glue. But why just two resistors. It was definitely some substance, but the Rs tested perfectly fine. I did end up replacing three of the 4 in question and a couple caps in the PS and the buzz is gone. So, not positive about which the cause of the buzz was, but two of the caps were 17% low in value and may have some leakage around the legs. Components in question follow:

C926,927 TK brand 100uF/35V were the low value caps at IC902,902 voltage regulators. Replaced with Panasonic FM 100uF/50V
C924 TK 470uF/35V checked out fine, but still was replaced with a Nichicon HD 470uF 50V cap. 1000/50 was too big to fit the area easily.
C451,452 TK 4.7uF/25V BP were replaced with Nichicon ES 4.7/25V

The resistors that were in the "glue" were
R534 10K and R520 22K

R526 10K appeared to have a light grey spot on it, as if it had been exposed to something
R532 10K was clear, but since in the same circuit as R534.

R532, R534 and R520 were replaced with 1/4W MF resistors and the clean R532 replaced the "spotted" R526.
 
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I'm looking at doing a few other tweaks to this pre, so curious of any opinions. Local filtering of the op amps +/- rails are either 100uF/25V or 220uF/25V using Panasonic FC series. Any benefit of increasing the filtering to 220uF and 470uF respectively? How about using LL UKL series here or is low ESR better for filtering the rails. There is also a slight hiss (noise floor) that I can hear when close to the speakers. The pre already uses MF resistors, which I believe are either very small 1/4W or perhaps even as low as 1/8W (I didn't see any note about wattage ratings unless otherwise specified). I've read that using a higher wattage COULD lead to lower noise as they are less stressed by heat, etc. Any real benefit to be gained with audio path replacement? I do have a bunch of Dale RN60s I could use, but they'd have to stand vertically, so that may negate any benefit due to longer exposed lead.
 
Buzz is back and accompanied by still hearing music even with the volume fully closed. I'd think a ground issue due the volume being present with the control at minimum, but the buzz doesn't get louder as the volume knob is increased. It also doesn't sound like 60Hz tone... so not sure it is a ground issue. Will need to check the PS and the previous work...perhaps a solder joint?

Thoughts?
 
Opened the top and don't see anything obvious, so I now have it driving the amp section of my SA-9500 II trying to recreate the buzzing. Wonder if it's heat related as it warms up as there's no evidence yet of the buzzing. And it's not the NAD C270 it was plugged into, because I subbed in my Kenwood Basic C-2 and there's no buzz.
 
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