Sansui AU-5500

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I wouldn't pull D01, D02, might be an issue with incircuit measurement.

I have made an edit of my post, with another tester, inplace all 4 diodes seem ok ...

Is there any point in removing these diodes from the circuit to test them ?

 

[mbz]

Is there any point in removing these diodes from the circuit to test them ?

No they seem fine, need to work out a possible cause for the offset.
I vaguely recall all electrolytics in power amp replaced?

 

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No they seem fine, need to work out a possible cause for the offset.
I vaguely recall all electrolytics in power amp replaced?

Yes all electrolytics were replaced by the previous owner in mars 2019 (by a professional, the jobs look for me good)

 

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Try the following, connect oscilloscope probe to D05 cathode, other channel to D06 cathode then power up and compare traces. Hopefully identical.
Maybe D05 is not ok or one of the series resistors (R23, R21 or R11) have drifted high, measure in circuit.

 

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No doubt possible, you see clearly the problem, big respect

Channel 2 is the black probe, one pics for the start-up and one for the stop

 

[mbz]

D05,6 act like a simple voltage regulator, conducting at about 8.5Vdc. Channel 1 does look sick. Possible causes are a series resistor has drifted high or D05 is faulty, conducting at about 1Vdc.

Resistance check R23(180), R21(2.2K). If ok then replace D05 (or consider replacing with D06?)

edit Need you to confirm that CH1 trace was for the faulty left channel.

 

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D05,6 act like a simple voltage regulator, conducting at about 8.5Vdc. Channel 1 does look sick. Possible causes are a series resistor has drifted high or D05 is faulty, conducting at about 1Vdc.

Resistance check R23(180), R21(2.2K). If ok then replace D05 (or consider replacing with D06?)

In place R21 and R23 seem ok, I will switch D5 and D6 to confirm the diagnostic !

There is a little dismantling to do, I will do that tomorrow during the day and I will let you know

 

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Hello, i made the job and the result was the same !

the diodes have the same characteristics with the testers.

On the pics 2V / div on chanel 1, 5V / div on chanel 2

210031 is the start-up, 210057 is the stop.

 

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Sorry, I made a mistake which explains a lot.
I realized while searching that one of my probes had inadvertently switched to x10...

there is no problem with D5, D6, R23 24 25 and 26, the tensions are all the same on both sides

 

[mbz]

There is some doubt about results of previous tests. Could you please redo the power amp output and pre-out/main in tests, ie,
- test output as per blue dots as per post #12, only power on case is needed.
- test offset at pre-out as per blue dots in post #16. The preamp test showed about 200mV at relay click, which "should" be ok, you may get a pop depending on speaker efficiency.

Possibly there is no underlying problem, so issue may be caused by a tired mute timing cap, suggest replace C23 470uf/6.3V with a low esr type (UPW?) 470uf/16 or 25V

 

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Thanks you for your help.

For the tests, does I need some output dummy charge ?

 

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For the tests, does I need some output dummy charge ?

No dummy load is required, not an output coupling capacitor design.

 

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test output as per blue dots as per post #12, only power on case is needed.

With the good progress setting no problem...

 

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test offset at pre-out as per blue dots in post #16. The preamp test showed about 200mV at relay click, which "should" be ok, you may get a pop depending on speaker efficiency.

Relay click is on blue dot, about 2.5 / 3V DC..
After some test relay click after a 2.4 / 2.8 seconds

I made the test, in separated mode (no preamp) there is no start up pop sound !

 

[mbz]

Replace C23(470uf/16V)

The preamp out trace (3V) is high however it's the offset at the spkrs that causes the pop. Output waveform looks ok too.

 

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Replace C23(470uf/16V)

The preamp out trace (3V) is high however it's the offset at the spkrs that causes the pop. Output waveform looks ok too.

I made the job, the tempo is more longer but always a start-up plop...
The plop seem more powerful on left channel.

Old C23 is a nichicon fine gold who seem very good !

 

[mbz]

I re-read post #35, it is helpful.
I need to take a look at the preamp circuit for possible causes, it appears that both channels have the same trace, certainly coupling capacitors like C01,2(power amp) and C13, 14(pre) should suppress the effect. Have these caps been replaced?