[NJARC] Hi, Guys. What am I looking at here?

[Dave Sica]

Virtual repair clinic anyone?

You may recall my post a few weeks ago about "Looking for a phonograph
for a young person" or something to that effect. I spent today working on
the record player I picked up from Darrin Hoffman at the club meeting
yesterday. He had done a wonderful electrical and mechanical
refurbishment and the thing plays like the day it was new. The girl we're
giving it to is coming over tomorrow to pick it up (she spent today at
the Princeton Record Exchange stocking up!) and I spent today sprucing up
the cosmetics.. (I hadn't ever "restored" a set with that
leatherette/painted paper covering and it was a learning experience!) It
now looks like 999,000 bucks. (I would have said it looked like a
million, but to be honest, there are at least a million paint speckles I
didn't deal with. The future owner probably has more patience than me
anyway.)

Anyway, here's my predicament. After running this thing off and on all
day and marveling at how nice it sounds, late this evening it apparently
decided to get noisy.  There was always a bit of hum, about what you'd
expect from the 35W4-powered 50C5 amp.  But now it's got more hum, I'd
say a fair amount of hum. Maybe still acceptable. Possibly even still the
same as it was this morning except that now I'm paying attention. What
caught my attention was a new noise, a buzz. Sounds a lot like
interference from a fluorescent light, but it only appeared at night and
I've got most of the lights in the house turned off. Even stranger, the
buzz is most noticeable when the volume control is down, GOES AWAY (!?!)
when you advance the volume control slightly and then RETURNS and
strengthens (along with the hum being amplified) as you advance the
volume control further. I don't recall ever hearing about that type of
symptom before.

I'm the farthest thing from a competent electronics engineer, but I can't
imagine this high frequencly damped wave buzz is coming from inside the
record player. But then again, I can't figure out where it might be
coming from. Or why it behaves the way it does when I fiddle with the
volume control And why only now after it's been running for a while?  

I've posted photos of the noise as seen on my scope at
www.njarc.org/phono_noise. Can anybody figure out what's going on here?

Thanks,
--Dave Sica
____________________________________________________________
Digital Photography - Click Now.
http://thirdpartyoffers.juno.com/TGL2141/fc/BLSrjpTDvmSDoFcYunilkg9b54YqPiYZpu5ziKnvpSHHWfEtDt62lQHWdKg/