Sansui G-901 Transformer Buzz – Load or Transformer Issue?

[Mecii]

If anyone has any ideas, or has dealt with a similar issue on a G-9000/G-901, I would really appreciate any additional troubleshooting suggestions that might help me narrow down the cause of this transformer buzzing.
Issue:
The unit powers up normally, comes out of protection, and plays music. However, the power transformer produces a noticeable mechanical buzzing sound. There is no hum or buzzing through the speakers.
Here are the troubleshooting steps I have taken so far:

• With all secondary feeds to the F-2807 and F-2808 boards disconnected from the transformer, the transformer is completely quiet — no buzzing at all.
  Transformer secondary - voltages measured with secondary leads disconnected.
  
  
• Secondary voltages feeding F-2807:
  • 52V winding reads 54.37V
  • 52V winding reads 54.33V
  • 32V winding reads 33.18V
  • 32V winding reads 33.29V
  • 50V winding reads 52.54V
  • 50V winding reads 52.55V
• Secondary voltages feeding F-2808:
  • 50V winding reads 52.45V
  • 50V winding reads 52.55V
  • 7.5V winding reads 7.48V
    
• Reconnected the secondary feeds to F-2808 only (50V and 7.5V), while leaving all feeds to F-2807 disconnected.
  • Result: Very slight transformer buzz.
• Reconnected all secondary feeds to F-2807 as well.
  • Result: Transformer buzz becomes loud.
• Disconnected the 52V and 32V feeds to F-2807, leaving only the 50V feeds connected. All feeds to F-2808 remained connected.
  • Result: Very slight buzzing, but nowhere near as loud as with the 52V and 32V windings connected.
• Reconnected the 52V feeds to F-2807 while leaving the 32V feeds disconnected. All feeds to F-2808 remained connected.
  • Result: Transformer buzz is at its loudest. Seems to be tied to the 52V.
• Reconnected all secondary feeds to both F-2807 and F-2808. I then disconnected the brown and blue output wires from F-2807, which supply the approximately +/-72V rails to pins 6 and 7 of F-2809.
  • Result: The loud transformer buzzing stops.
• From there, I tried disconnecting the various connectors/cards downstream of F-2809 that I could safely eliminate from the circuit.
  • So far, nothing I disconnected downstream eliminated the loud buzzing when the +/-72V supply from F-2807 was connected.

At this point, my thinking is that I may have one of two situations:

1. Something downstream is drawing excessive current and placing an abnormal load on the transformer, causing it to mechanically buzz.
2. The transformer itself has developed a mechanical/internal problem and becomes noisy once it is placed under normal load.

The fact that the transformer is completely quiet with the secondaries disconnected makes me lean toward a load-related issue, but I certainly don’t want to rule out the transformer itself. Unfortunately, I don’t have a spare G-9000 transformer available to swap with the in the unit.

If anyone has seen this before or has suggestions for additional measurements or isolation tests that would help determine whether this is excessive current draw versus a defective transformer, I would appreciate the help.

Thank you,
MecII

 

[Robisme]

Does the amp section run hot? Could be bias is way too high or oscillation.

Rob

 

[Hyperion]

Maybe check the tightness of the transformer fixing screws/bolts?

But best not overtighten, as it might damage them.