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Old 8th Oct 2019, 10:49 pm   #1
makeitwork
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Default Sony Receiver STR 6046A

Loud thump from speakers on switch on.

Can anyone help with this receiver. When the set is switched on, the speakers emit a loud thump instantaneously and again when the set is switched off. After the set has been switched on its fine and operates as normal, any ideas what this could be or how to resolve this..?
I have just obtained the service manual for this receiver but the number of components in this set has to be seen to be believed. Any help would be appreciated.
Thanks
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Old 8th Oct 2019, 11:31 pm   #2
toshiba tony
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Default Re: Sony Receiver STR 6046A

I'd look for DC on the speakers. It's likely to be present although you say the amp settles down. If there is a speaker protection relay does it click in after a few seconds or straight away. Don't know the model even though I used to work for them but I will look at it.
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Old 9th Oct 2019, 12:05 am   #3
TrevorG3VLF
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Default Re: Sony Receiver STR 6046A

If there is an output capacitor, then this has to be charged up on swtch on and discharged on switch off.

There may be a relay to connect the seakers after the charging is done.
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Old 9th Oct 2019, 12:10 am   #4
paulsherwin
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Default Re: Sony Receiver STR 6046A

All transistor amps of that era have some switchon thump as the speaker electrolytic capacitors charge. It's difficult to say if a fault is present without being able to listen to it.

Later amps don't have the thump, partly because they have speaker protection relays and partly because they are dual power rail designs without speaker capacitors.
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Old 9th Oct 2019, 6:41 am   #5
tri-comp
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Default Re: Sony Receiver STR 6046A

This is a ~1971 receiver so all electrolytic capacitors in the output amplifiers should be replaced. Especially since this receiver doesn't feature a speaker protection relay.
The output electrolytic cap's. C608/C658 may still be OK and weren't marked for replacement on the attached schematic.
Still they should be considered for replacement because of age.


Best regards,

/Torben
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Old 9th Oct 2019, 10:44 am   #6
paulsherwin
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Default Re: Sony Receiver STR 6046A

In my experience, the electrolytic caps in these old Sony amps do last well. I looked at a TA70 a while ago expecting to change lots of them, but there were no faults present and they all seemed fine, even the speaker caps. I decided to leave the originals in place.

It's true that lots of people do recommend changing the electrolytics as preventitive maintenance though, as they are 50 years old.
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Old 9th Oct 2019, 2:27 pm   #7
Nickthedentist
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Default Re: Sony Receiver STR 6046A

...and a lot of people change all their valves in their vintage kit too. Sometimes it's justified, often it isn't

EDIT: No offence to Torben intended, I have a lot of respect for your postings.
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Old 9th Oct 2019, 5:27 pm   #8
makeitwork
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Default Re: Sony Receiver STR 6046A

Thanks guys for all the replies, I didn't expect so many. Your all talking about capacitors so for me that narrows things down a little. Tri-comps reply was useful but its not the STR 6046 I have its the 6046A which uses STK-056 IC's as the power amps for L & R channels so its a little different. Someone suggested replacing the two big caps in the power supply rated at 35V 4700mf. They didn't test short or open but when tested with a fluke capacitance meter after discharge they only measured half the capacitance value they should have had. They didn't look physically damaged in way or show any signs of leakage. So I replaced both of these, switched the set on, and it made no difference whatsoever, Sony build quality. There are no relays in the output of either channel but the user manual I have does say that the preamp and power amp stages are direct coupled and utilise balanced pos and neg power supplies.
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Old 9th Oct 2019, 6:15 pm   #9
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Default Re: Sony Receiver STR 6046A

Amplifiers of this era sometimes had Rifa capacitors across the mains switch.

The clear plastic encapsulation tends to crack, let in moisture and then go Bang!
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Old 9th Oct 2019, 6:38 pm   #10
simpsons
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Default Re: Sony Receiver STR 6046A

Is this a fault that has just occured or is this receiver new to you.

I ask this question because looking at the schematic, there is no soft start circuit for the power supply.

Whist the user manual doesn't speak of a thump on switch on, this is a 1970's design and this reciever doesn't include a relay to delay connection to the loudspeaker on switch on.

There is no "Rifa" capacitor across the mains supply as it is not a switch mode power supply.

Chris

Last edited by simpsons; 9th Oct 2019 at 7:02 pm. Reason: Additional information
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Old 9th Oct 2019, 9:57 pm   #11
tri-comp
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Default Re: Sony Receiver STR 6046A

Really sorry about the 6046/6046A confusion.
I didn't realize there are 2 very different versions of the receiver.
The good thing is that it now became a lot easier doing a re-cap of the output amplifier.
IF that's going to make a difference at all.
It seems for someone it didn't, if I read this right:
https://audiokarma.org/forums/index....-6046a.575229/
It's actually hard to see if he really re-capped the Amp-section or not.
I would, as defective electrolytic cap's may set the STK's off oscillating followed by hara-kiri.
In the above link there's a schematic diagram for the 6046A output amp section.
That's what I can find on the 6046A at all short of places that sells the complete service manual.
Unfortunately I don't have it.

Rgds,

/Torben

Last edited by tri-comp; 9th Oct 2019 at 10:02 pm.
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