Sunn 200S Notes

Hum and Buzz that affects EQ only and not volume.

Cleaned out spider webs. 

Was repaired in '68. Looks like it was also repaired fairly
recently. Many of the resistors have been replaced in the last 10 or
so years. 

Also found some modifications:
     - Adjustable bias installed
     - Additional power supply filtering

Checked all loose connections and swapped 12AX7, no difference.

Removed 12AX7, no difference.

Installed tube shields, no difference.

Checked/Adjusted bias on power tubes, no difference.

Removed PI tube, still slight hum, probably due to transformer, tubes,
rectifier, or filter caps. But narrows most of the hum to somewhere in
the pre-amp.

Caps are old, no leakage. Most test in their tolerance range. Filter
caps are low.

Will replace powercord to 3 prong. Recommend full cap job and re-tube.

Noticed that the ceramic caps have the wrong values based on the
codes. Could be old codes, but 470pF has a code of 471, not 470. Will
replace all of them.