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Radio Road

Sony ST J75 tranny(?) hum

185.153.179.198

I was lucky to find this great condition and great sounding tuner on the bay, and it does not suffer from the common "bass dropout on weak signal due to muting adjustment" problem this tuner can sometimes have.

However I notice that it has a low lever - perhaps 60hz although it sounds a little higher in frequency - chassis or internal hum when powered up.

I don't see how it can be ground hum since the hum is audible even with no amp power, so no signal at all going to the speakers.

How to know if it is actually tranny hum, and if so,what should I try for a fix?

I am not trained in electronics so I would not want to attempt anything inside the unit powered on (although my ex wouldn't mind if I made such a mistake ;)

I could upload the schematic if that would help.

thanks if advance for any ideas!

Follow Ups:

RE: Sony ST J75 tranny(?) hum - 6bq5 16:17:55 08/23/23 (2)
Itman-
If the hum is present with the unit plugged into mains power, but NOT connected to an antenna, or preamp/system, then it sounds like a mechanical noise coming from the unit.

That being the case, you may be able to tighten the connection of the transformer to the chassis, and check the connection of the chassis parts to each other.

Before you do anything, unplug the unit from the mains power and let it sit for a day or two. This should provide time to allow the components to be deenergized.

Opening the unit carefully, take pictures if this is new for you, making note of orientation of the chassis parts-
once the cover / top is off, the transformer should be readily identifiable - gently try to move it to confirm it is loose.
A Toroidal transformer should have 1 connector thru the middle of the transformer to the bottom of the chassis - check to see that that fastener is tight (righty tighty). If it is an E-I or C Core- the transformer will be square in shape and may have 2-4 fasteners to secure it to the chassis.

Happy Listening
- RE: Sony ST J75 tranny(?) hum - ltman 22:36:06 08/24/23 (0)
  Mike Williams of Radio X Tuners put his ear to the case of one and heard a slight hum. Mike told us, "I did some isolation and dampened the transformer with soft-start resistors, which reduced the hum to almost nil, but it was still faintly heard. I have had several ST-J75s come through here since and all have had the same hum when you put your ear to the case.
- RE: Sony ST J75 tranny(?) hum - ltman 17:28:20 08/23/23 (0)
  10-4.
  Will have a look at let you know.
  thanks!

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