Ampeg VT-22 hum in standby

[goldstache]

Got a VT-22 on the bench


This variant:
https://www.drtube.com/schematics/ampeg/vt22-v4-70.gif


When mains is energized, loud hum is present in speaker.
When standby is in operate, hum subsides, and idle it’s fine.


All cans test good. Other remedial parts that tested bad are swapped. New screen R’s (1K/5W)
Bias supply rebuilt.
Getting around 26ma idle out of a quad of sovtek 5881/wxt.


Since standby is 0v reference connection. Is that normal to have ripple noise present in Floating output section? 


Things to checks???
All chassis isolation is intact on caps/jacks that need it.
Only footswitch and verb PCB loop that ampeg baked in. Isn’t a problem in play mode, so likely unrelated.
Looking forward to any pointers.
Thanks
-joe

[goldstache]

Alrighty, after further combing. I realize that this amp is similar to later model Vt-22's. 


The version with the standby delivering 0V chassis to two points:
-Where the flyback diodes (2 in series) for each Plate groupings (2)
-The (-) of the bridge rectifier and totem filter (-)


Have not been able to locate a schemo that matches what is in front of me. (SOLVED ATTACHED) https://www.drtube.com/schematics/ampeg/vt22-v4-74.gif
I have a 6CG7, 12DW7, and 6K11 compliment. But with a different standby setup.  And the mentioned flyback diodes on the plate taps.

[goldstache]

Welp..... it was D5 diode. For Ground bus reference. Diode tested fine in circuit, and out of circuit too! Swapped. Issue resolved.
Interesting......

[Latole]

Good job !

Did the amp have a 3 prongs power cable ?

[goldstache]

Yep, stock 3 prong standard convention