Sony STR-7065A Transformer Hum

[Ray66]

My current restoration project is a Sony STR-7065A. Aside from the usual problems (dirty contacts/pots, burnt out lamps, etc.), it has a very noticeable hum coming from the transformer. It's moderate when idling, but pushing it to max power into dummy loads gets it buzzing really loud. To be clear, this is a 60Hz vibration and not something coming through the speakers. I've read posts saying the cause is likely loose transformer laminations, but as you can see from the photo Sony has this transformer in a can. There are 4 screws holding it to the chassis. I'll try pulling it out to see if I can get at the actual transformer inside but figured I'd check if anyone else has run into this. There's lots of wiring bundles wrapped around it so expecting this job with be a royal PITA. Any advice on getting this apart would be much appreciated.

 

[rottalpha]

My current restoration project is a Sony STR-7065A. Aside from the usual problems (dirty contacts/pots, burnt out lamps, etc.), it has a very noticeable hum coming from the transformer. It's moderate when idling, but pushing it to max power into dummy loads gets it buzzing really loud. To be clear, this is a 60Hz vibration and not something coming through the speakers. I've read posts saying the cause is likely loose transformer laminations, but as you can see from the photo Sony has this transformer in a can. There are 4 screws holding it to the chassis. I'll try pulling it out to see if I can get at the actual transformer inside but figured I'd check if anyone else has run into this. There's lots of wiring bundles wrapped around it so expecting this job with be a royal PITA. Any advice on getting this apart would be much appreciated.

View attachment 3698321

Few things that come to mind:

• Current overdrawn - leaky bulk caps or other aging components could draw more current than spec.
• DC coming back to the transformer - check film cap (typically 10nf) across secondary windings.
• Noisy rectifier bridge can cause ringing - check scrubber caps
• Bad connection somewhere

 

[Ray66]

Here's an update. I was able to pull the can off of the transformer. It wasn't difficult, just screws holding it to the chassis, while the transformer itself is held down with additional screws. With the can off the hum is barely noticeable, even with the amp putting out full power. I'll wait until restoration is complete before thinking of ways to mitigate the vibration.



Thanks for the suggestions @rottalpha. Filter caps C901, C902 are original, and a bit tired. They're measuring 10-20% below spec, ESR is good. I doubt they are the cause but will likely replace them. I don't see any film caps across the secondary, but there are mylar caps across each of the bridge rectifier diodes (C801-804, C831-834). I know that the power supply section of the STR-7065 (non-A version) had some heat issues that cooked the bridge rectifiers and the associated mylar caps, however that seems to have been addressed on the A version so I would be surprised if any of those capacitors are bad. However, I will likely replace the diodes.

 

[WSideWitness]

Few things that come to mind:

• Current overdrawn - leaky bulk caps or other aging components could draw more current than spec.
• DC coming back to the transformer - check film cap (typically 10nf) across secondary windings.
• Noisy rectifier bridge can cause ringing - check scrubber caps
• Bad connection somewhere

What he said. Rebuild the power supply if not already done

 

[Ray66]

Took a little ski break but back at the bench now. I recapped the Power Supply/AMP PCB and voila, no more transformer hum. None of the capacitors replaced looked particularly bad physically. They all measured reasonably good for capacitance, however a few had high ESR. I still plan to replace the bridge diodes, trimmers, and the two 2SC1124 transistors on this board. After that I'll have a go at the Control PCB. It's buried pretty good under the dial face so if anyone has any tips on how best to work on it without removing the tuning string I'm all ears.

 

[onwardjames]

I wish I had your skills. If I did, that sweet little receiver I pretty much gave away would still be here.

Some of the sweetest sound from the era of giants. It was, and still is, my favorite vintage receiver.

 

[Ray66]

Update: I went ahead and removed the dial pointer and function lamp assembly. I saw no other option to get full access to the Control PCB. Even with that partially swung out of the way there were still several parts that were difficult to get at. I got it recapped and the controls sprayed. All looked good on the DBT. Unfortunately got no sound out. This receiver has PRE IN/MAIN OUT connections so I confirmed that the power amp section was fine. Something in the pre amp. Lots of head scratching and self doubt. I found that the voltages on Q803 and Q804 on the Control PCB were messed up. These are part of the 'Muting' section on the preamp. Not FM muting, I think it mutes the signal at start-up and shut down to avoid speaker pops since this receiver doesn't use a protection relay. Anyway, lots more head scratching. It turns out that circuit uses the 11VAC off of one of the transformer secondaries. The other side of that secondary gets tied to ground via a binding post connected to that dial pointer assembly that I had removed. Of course I didn't re-install it before testing. Duh. Connected that assembly to the chassis and all is well again. Have to admit, this Sony is not my favorite to work on, sigh.