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Hum in Pioneer SA5300

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dieterboels

Member
Hi,

I have a considerable hum (or buzz?) coming from the SA5300. Somewhere in the 100 Hz range. In both speakers, also in the headphone. The hum is present when no inputs are connected and when volume is down. Doesn't turn up when turning up volume. So all these aspects points at the filter in the power amp failing. I changed all the elco's and the transistors. (Schematic of the power supply stage added)

- What else could it be?
- A bad grounding? How do I easily troubleshoot this?
- What is a way to find the source of this hum?

All help is welcome.
 

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Oh sorry, that message was confusing, I meant;
A hum is present when no INPUTS are connected (it was late when I wrote the message). And it's present in both left and right.

I actually replaced all the elco's in the amp. (I used rubycon and nichicon caps, but I don't think the cap brand would be the cause).
 
Well here is the joke, I actually didn't listen to it.
I picked it up very cheap second hand and according to the man it was supposed to be working, but I don't know a 100%. I'm giving the amp to a friend soon, so I just wanted to give it some new life before giving it to her and I was too hurried to recap it, so I didn't actually listen to it. I know this is very, very stupid of me . So now I've got this problem and I don't know if it was already there. Nonetheless that it should be reparable .
 
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It all looks perfectly fine and with a diode meter I have tested if new soldering points are making a connection (testing in they are shorted with intended other points)

So except for visually checking and this diode testing, is there another way to test bad solder connections and wiring?
 
Could you explain the procedure more detailed? I have DMM at hand, what is the purpose of the dc blocking capacitor, how do I ?
 
you connect the low value cap in series with your red meter lead then measure voltages across the power supply caps .this will tell you how much AC ripple there is .
 
So I went through this procedure. I used a 3.3 uF cap as a DC blocking cap.

There is only one capacitor where I measured some AC across and that is capacitor C64. Where I measure an AC of 0.5 V, could this be the source of the hum? Just for the speed I did replace the cap already and I'm still getting 0.5V AC across.

Looking forward how to proceed.
 
I changed the rectifier diodes, problem persist. (took me a while, I'm a bit busy with too many things at the same time as many people).

The amp sound really great, so I would be so happy to fix this, any other suggestions?
 
It's an interesting issue.... you didn't happen to see an empty space near the main filter cap where a capacitor should be did you? Then see a capacitor tacked across two points on the underside of the board?

Did you remove this and put a cap jn the board where there wasn't one? It's a design flaw pioneer corrected. The result of undoing it is hum.
 
Young46 said:
Just a stab in the dark. Is the cover on it?
Click to expand...
Yes ! Good call, I thought about it too ! But even with cover on it, still happening...

QSilver said:
It's an interesting issue.... you didn't happen to see an empty space near the main filter cap where a capacitor should be did you? Then see a capacitor tacked across two points on the underside of the board?

Did you remove this and put a cap jn the board where there wasn't one? It's a design flaw pioneer corrected. The result of undoing it is hum.
Click to expand...

Hmm, I might have done just that. Do you know what this correction should be? Or how I can find what to do?

Attached circuit of power amp and rectifier circuit.
I'd be soo happy to fix this one, it sounds soo good except for that little hum. Thank you !
 

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I've got one near the bench at the moment. I'll take a photograph when I get to it again. The other thing that I've found causes hum is when the amplifier's centre voltage is not exactly... centred! For example, measure the voltage at the collector of Q12 - it should be approximately 38V. Then check the voltage at the collector of Q14. It should be about half of that - 19V. The schematic says 18.7V

When working on mine, I found that this half voltage was around 16V. Adjusting the circuit to lift this back to the centre point of 19V helped stabilise my bias and remove more hum.
 
Ah interesting.

Would be great to see on the picture where I have to put the cap to solve this problem.

Otherwise: I'll test the voltages. How do I lift this back up in this amp?
 
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