Sony TA-N55ES DC voltage on left channel when volume turned up

squonk666

Member
2016-02-10 5:37 pm
Hello all,

I'm having another problem with the Sony TA-N55ES i'm trying to restore.

I set the dc offset (bias) to the 7mv for both channels.

When i turn the volume up to just over halfway (1 to 2 o'clock position) with no load connected to both channels i get a sudden dc voltage increase to 46.55mv (i think), on the left channel which keeps increasing until in turn the volume pot down to halfway.

The right channel has 18mv at the same volume position, which i would think is normal as you turn the volume up.

The voltage is measured from the dc offset test points on the main board.

I have changed the front panel pcb board and main output transistors with genuine Sanken replacements, and swapped the 'A class board' for a known working board.

My dim bulb tester i use has a 150w lamp which lights fairly bright when increasing the volume past 1 or 2 o'clock position.

Can anybody help with some pointers on what is happening?

Thanks in advance.
 
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rayma

Member
2011-04-29 8:37 pm
Replace C302/C352 for possible leakage. While you are in there, also replace C304/C354.
The electrolytic capacitors are much, much, more likely than the semis to be bad.
Don't ever replace semis unless they actually test as defective.
 
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squonk666

Member
2016-02-10 5:37 pm
Thanks for the reply's .

I'm not sure if it counts but i have already replaced the whole 'A Class' board with a known working board as mentioned in the original post.. I still get the dc problem with the known good replacement 'A Class' board.

Could it be component on 'main' board ? Filter caps or main output transistor? (even though the output transistors are new )

I did test the filter caps and they appear in serviceable shape, (did not remove them from main board to test)

Maybe worth running the soldering iron over solder points on left channel ?
 
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The sudden increase at a particular volume point strongly suggests oscillation - decoupling caps may have dried out. Oscillation can have only a small effect on output audio, but always causes jumps in offset and power consumption. Undiagnosed oscillation can destroy an amp if there's a load connected, or even if not due to cross-conduction.
 

squonk666

Member
2016-02-10 5:37 pm
Do you mean the decoupling caps on the main board or the ones after the volume control on the 'a class' board?

The volume point that the dc increase happens appears to vary (never affects less than the 12 o'clock position though) most times when the amp is turned off and on or when cold.
 
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squonk666

Member
2016-02-10 5:37 pm
Thanks for all your reply's.

I have replaced capacitors c352 and c302 and the 38v dc (which slowly rises) now appears at the 4 o'clock position on the volume dial.

I have been checking the capacitors on the 'a class' board, i found that if i ground one probe of my multi meter to the amp ground and probe the top casings of the capacitors with the other prob on the 'a class' board i get significant DC from the following capacitors.

C307 - 51v

C408 - 26v

C410 - 9v

The rest of the capacitors have measured with very low DC in MV from their casings.

I'm suspecting that these capacitors are the leaky ones ?
 

rayma

Member
2011-04-29 8:37 pm
No, these should have DC voltages on them, since they are bypass capacitors, not coupling capacitors.
However, the R402 may be defective as it seems to be dropping too much voltage and not allowing
the voltage regulator to function. Check its resistance value after lifting one end, it should be 3.3k.
If you need to replace it, use a 3 watt part.
 
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squonk666

Member
2016-02-10 5:37 pm
Connecting source input leads to the input seems to stop the dc. When i disconnect the source input leads i get the dc back??

Connecting a speaker and turning the volume up slowly to max (still with source input leads connected, no source) i get loud hum.

Connecting a phone to the source input the hum gets quieter but is still there.
 
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Connecting source input leads to the input seems to stop the dc. When i disconnect the source input leads i get the dc back??

Connecting a speaker and turning the volume up slowly to max (still with source input leads connected, no source) i get loud hum.

Connecting a phone to the source input the hum gets quieter but is still there.


ANY amplifier with NO source/load connected will naturally HUM because it's operating UNloaded - aka WIDE OPEN.
Shorting the inputs with shorting plugs or clip leads is how testing is done for noise levels and/or voltage checks - turning UP the volume should only result in a minimal noise floor, or silence, from the speakers.