Scott 222D Phono Stage-Intense oscillation

[sisyphus51]

I used to be able to turn up to MAX level with the tonearm lifted to guage noise/hum. Now 12:00 causes the output to burst into approx. 500 hz. heavily distorted oscillation even with input shorted. There is something wrong with the phono EQ that is generating feedback.

 

[sisyphus51]

I actually prefer the sound of my Schiit Mani into the Scott Line-Level input!
It gives me a very rich and powerful sound!

 

[Chris Hornbeck]

I used to be able to turn up to MAX level with the tonearm lifted to guage noise/hum. Now 12:00 causes the output to burst into approx. 500 hz. heavily distorted oscillation even with input shorted. There is something wrong with the phono EQ that is generating feedback.

First, clean all the switches and controls. Seriously, you'll thank me later.

Next, it's time to check all the power supply electrolytic caps. Originals are old enough to be grandparents, and about half a century past their design age.


All good fortune,
Chris

 

[sisyphus51]

bypass the RIAA/NAB switch to permanently be in the RIAA position.
NAB eq is not needed anymore as reel to reel tape is not common.
Check ground connections on 12AX7's

I needed a break from worrying about this thing so, I just bypassed the whole Phono stage with an external Phono Amp/EQ. However, I like your suggestion to by-pass the NAB/RIAA Switch! * There is a slow 'motor-boating' oscillation audible in the speakers when I switch to the Scott Phono Input before All Hell breaks loose at about 12:00 on the Volume Knob. I know something is messed-up in that area but, I need to get a scope probe in there!
I see a decent Tektronix Scope for sale locally and will look into it. And my friend, in L.A., who gifted me the Scott will send me a probe, or two - and some other tools - I don't want to "boldly go" ripping-up the original circuit without further insight into the defect. ;-)
Thanks to you, and the others, for all the interest and advice!

 

[sisyphus51]

Best option IMHO and after much hand-wringing is an external Phono EQ - in my case, the Schiit Mani - into one of the High-Level inputs.
So, what I have now is: AR-XB Turntable, Shure M97XE Cart. into the Schiit Mani, then into the 'Extra' input of the HH Scott 222D.
That is what I have running now and I like it!
I am hesitant to go back to using my Adcom 545 unless I can get a vacuum-tube pre-amp with tone controls and/or loudness-compensation.