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Inconsistent voltage after rectifier
Inconsistent voltage after rectifier
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Old 26th May 2021, 03:11 AM   #1
philschl is offline philschl  Australia
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Default Inconsistent voltage after rectifier

Hello Friends,

I've got a problem with an amp on my desk right now in that the power supply measures inconsisten voltage after rectification stage, see diagram.

The yellow highlighted caps are bulged and need to be replaced, though before I go to that expense, are the caps causing the voltage issue or must I look elsewhere?

As always, your help is hugely appreciated!
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Old 26th May 2021, 03:15 AM   #2
NareshBrd is offline NareshBrd  India
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Obviously...bad capacitors will cause that kind of problem.
And you need to replace them, before they do something bad, like exploding.
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Old 26th May 2021, 03:57 AM   #3
philschl is offline philschl  Australia
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Yes, given the state of this thing I'm pretty astonished it's not blown a gasket already. If nothing else is amiss, I'll go and replace the caps, which will cost about $100. I'm just not wanting to throw more money at this thing, in case there's a different fault at play.
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Old 26th May 2021, 04:23 AM   #4
NareshBrd is offline NareshBrd  India
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Disconnect from circuit.
Remove caps
See AC voltage
Put small caps, see DC voltage.
If they are consistent, then only the caps are at fault.
If not, another fault besides the caps.

Small caps may be handy, and you need to check the no load voltage, to find the fault. Try 330 or even 2200, whatever is at hand, only one if more are in parallel, just need to check rectifier and transformer.

Check out the price for 4700, they may be much cheaper, put two in parallel.. or one each 4700 and 3300, that adds up to 8000, close enough.

Last edited by NareshBrd; 26th May 2021 at 04:33 AM.
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Old 26th May 2021, 05:20 AM   #5
philschl is offline philschl  Australia
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Great ideas! I think I have some dead amps I can scavenge some smaller value caps from.
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Old 26th May 2021, 05:22 AM   #6
philschl is offline philschl  Australia
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Oh, and I put the circuit into a simulator, and it appears the DC voltages in theory would be 124 and 54. So looks like only the 121V is in the ballpark
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Old 26th May 2021, 05:31 AM   #7
NareshBrd is offline NareshBrd  India
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125-0-125 is 250v across the transformer...
The other winding is 55-0-55, or 110 volts.
Check in the manual, could be the 55 v line also has faulty caps.

110 * 1.4 is 154, you are seeing 121 after faulty caps, it will be less than 154 due to capacitor smoothing.

Before diving in, get hold of the service manual...see what is expected, and also it could be a fault in the later regulation section.

And Denon use too many voltages, not satisfied like the others with only two, one about 12 for the pre amp, and a higher one for the power amp in Kenwood and Philips for example.
Their design seems to need more different voltages, making it more complicated than those of the others.

Anyway, see also why it came to you, the supplies you checked because of some fault, which you have not mentioned.
I mean, what made you open it to check?

Last edited by NareshBrd; 26th May 2021 at 05:40 AM.
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Old 26th May 2021, 03:43 PM   #8
NareshBrd is offline NareshBrd  India
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The lower voltage and higher voltage capacitors are in series...high voltage rail to low voltage rail then to ground.

Peculiar
Please verify in manual and actual condition.
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Old 26th May 2021, 03:53 PM   #9
Salas is offline Salas  Greece
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Inconsistent voltage after rectifier
They put them between rails to save on their voltage rating and use all same caps
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Old 27th May 2021, 12:58 AM   #10
philschl is offline philschl  Australia
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I got the amp as not working, my interest is to learn about them by pulling them apart and fixing them. The symptom is clipping lights are on and a loud 50hz tone through the speakers even with amp on minimum gain. Both stereo channels affected but less so in bridge mode, which may indicate phase reversal of the channels.

Pulled the bulged caps off and itís not pretty. They were siliconed onto the board and some had leaked. It appears some of the connections were fouled through or at least the pins stayed on the board not on the caps. The caps also rattle when shaken. Is that normal?
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