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Sony STR-6036A Hum and Filter Caps

geebjen

geebjen

AK Subscriber
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I'm working on a Sony STR-6036A receiver which had a hum when I bought it. The hum is 60hz (pic), it's in both speakers, and it doesn't change much with volume. It's there at zero volume and approx doubles with the volume all the way up. I put my mini-scope on the filter caps and one looks much worse than the other (more pics), but it's 120hz not 60hz. Power supply portion of the schematic attached also.

So a couple of questions:
1) Where is the most likely place the hum is coming from? Bad filter caps? Or elsewhere?

2) Assuming I should replace the filter caps, the near exact match Kemet caps with the 4 pin snap-in aren't available anywhere. For these fairly small 3300uF 35V caps, can they be replaced with regular radial lead caps? (3900uF, 35V, 105C are cheap and readily available).

Thanks
 

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I would replace the filter caps. The 420mv of ripple on cap2 seems like a lot. Two leg caps should be fine, usually the extra pins on 4 pin caps are just for support, but check and see if there are any connections to the extra pins.
 
Thanks. I've ordered filter caps and also the 1000uF cap that's in the rectifier circuit. Hopefully that solves the hum. Either way, I'll come back and post the results here.
 
I replaced the 2 filter caps and the 1000uF cap that's also in the power supply circuit. Still have 60hz hum.

I checked at the input to the power amp and there is no 60hz at all. So no 60hz noise at terminal 8 to the STK-054 but it has 60hz at the output (terminal 2) and at terminal 4. Of course the +/- 25V supplies have about 150mV-200mV of peak-to-peak ripple but I assume that's pretty normal.

Not sure where to look next.
 

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Do you have audio with hum or just hum from the speakers? Do you have a amp to use as a signal tracer? If so, see what the audio sounds like at the 3.3uf input cap.
 
There is hum and audio. There is hum when there is no music playing, volume down, and input selection set to something with nothing connected to it. But music plays fine through all inputs and mostly masks the hum at any reasonable volume.

I don't have a separate power amp at the moment, but I should have a receiver with pre-out/power-in connections next week. I assume you're suggesting I cut one end of an RCA cable and connect remaining RCA end to a power amp and the bare wire end to the signal at the 3.3uf input cap and see if the hum shows up through the other power amp.

When I connected my scope at the 3.3uf input cap I see no 60hz noise, but I doubt my scope has enough sensitivity to pick it up since it's only about 50mV after the power amp so I'm sure if it's on the input it would be 1mV or less.
 
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I took an alternate approach. I disconnected the left channel wire coming from the control board to the power amp board at the power amp board end and the hum went away in the left channel.

That's progress I guess.

Still thinking about what to try next....
 
That would suggest the noise is coming from the preamp section. is there some filter caps in the preamp power supply that could be bad?
 
The 25V and 30V go from the power board into the control board into a section described as "ripple filter" and "regulator" to supply power to the rest of the control board. Picture attached. I'll have to start digging into that section. There is one 1000uF cap in there and some 330's, and 220 in there.
 

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So I'm ordering the 1000, 330, & 220 caps for that section. I figured I should order the transistors for that section as long as I'm placing an order. Does anyone have any idea what the best substitutes are for:
2SC1060
2SC632A

I spent over an hour scouring the internet and AudioKarma and I think that these will work, but I'm not 100% sure. The specs can be confusing when you're new at this.
2SC1060 - KSD526Y, D44H11, or MJE15032G
2SC632A - KSC1815YTA or KSC1845FTA
 
So a little more digging found that the voltages on Q803, which are supposed to be 25.0, 30, and 25.6 (E-C-B) were actually 29.8, 30, and 29.6. So I pulled Q803 and Q802 (for comparison). Turns out both are actually 2SC1364 and not 2SC632A as the schematic shows but I guess they're equivalent anyway. But also and more importantly Q803 is shot. I won't measure gain and it measures nearly dead short across all terminals. Considering Q803 is the "ripple filter" and it's bad, hopefully that's the main issue. I'll order all the caps for this section too as long as I'm placing the order, along with a Q801 replacement so I'll have them on hand.

I'll check back after I get my new parts.
 
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