Fender Passport 500 Pro SMPS question

[JP-Stereo]

Probing D638 at Power On the anode and cathode voltage are the same.
They both start at 0 and climb to 9.75vdc in about 1 second and stay there while power is on.
At Power Off they stay there 1/2 second then get down to zero.

[x-pro]

Simple rules for understanding the operation of a bipolar transistor in the key mode.
An n-p-n (p-n-p) transistor turns on if its base has a positive (negative) potential relative to its emitter.

To see how this circuit works, you can temporarily connect two parts (a 560 ohm resistor and an LED of any color in series with 20 mA current, anode to resistor, cathode to common) to the emitter of Q608. Increase the rating of capacitors C678 by a factor of two, as well as C677 to 0.1 µf, possibly also C679 to 470-1000 µf.

[x-pro]

Probing D638 at Power On the anode and cathode voltage are the same.

Naturally. Q610 is switched off.
The response of the voltage level at the cathodes of D636 and D637 is better to observe on an oscilloscope with an open input. It is preferable to use a two-beam device or a simple one, counting the time

[x-pro]

Those pop noise mute circuits are more designed for turn on than turn off. They rely on the charging time constant of the electrolytics to mute the amp while they are charging up.

What you said above about C678. But there is also C677 and C679.
And they are just as important.

[x-pro]

"At power on at R662 i have about a second of positive 10v noise then a clean -14vdc is appearing in a few seconds and stay there.
At power down it's swinging up to 10vdc and instantly back at 0vdc."

proper circuit operation.

They both start at 0 and climb to 9.75vdc in about 1 second and stay there while power is on.
At Power Off they stay there 1/2 second then get down to zero.

Confirmation!

[JP-Stereo]

OK, so you must have made a typo in message #128, the discharge diode must then be D638 instead of D636...

C677 (18vdc) is the first up and the first down,
C679 (10vdc) follow C677 about 1#2 second behing it.

Lets get rid of terminology here please...
For me an OPEN switch is not conducting, a circuit must be CLOSED to conduct.
If i want to OPEN the light, i have to CLOSE the switch... Are we on the same wavelenght ?

[x-pro]

OK, so you must have made a typo in message #128, the discharge diode must then be D638 instead of D636...

Oh, yes.

[x-pro]

If i want to OPEN the light, i have to CLOSE the switch... Are we on the same wavelenght ?

OK, but they're transistors, not toggle switches. And the structure of the transistors determines their state.

[Helmholtz]

There might be a translation problem.
To me a transistor is either on or off.
A switch is either closed or open.
Using the term "closed" to decribe the off-state of a transistor seems ambiguous/unclear.

[x-pro]

Sorry, sometimes I use Russian slang. But I try very hard to apply technical words when translating crookedly. First I write in my own language, then I do an auto-translation program, then I translate back and see if the meaning is correct.

To me a transistor is either on or off.

Thank you. I'll take note.
Slightly corrected the posts

[Helmholtz]

But I try very hard to apply technical words when translating crookedly. First I write in my own language, then I do an auto-translation program, then I translate back and see if the meaning is correct.

Obviously that works very well most of the time .
But technical terms can be tricky.
I understood what you meant above, but only because in German we use the term "gesperrt" for a transistor being "off".
Now, the direct translation from my dictionary of "gesperrt" actually is closed (or blocked), but doesn't work here.

[JP-Stereo]

The most precise description of the state may be "conducting" or "not conducting".
This is only an opinion.

Ok so, since the Power On MUTE seem OK, the most selective correction may be to insert a resistor between D638 and Q610.
This will slow the C678 discharge at Power Off without interfering with the Power On sequence wich is ok now.
It shouldn't hurt the Power On sequence ?

As long as these Pop are not caused by something else...

[x-pro]

the most selective correction may be to insert a resistor between D638 and Q610.
It shouldn't hurt the Power On sequence ?
As long as these Pop are not caused by something else...

(resistor between D638 and Q610) Yeah, you could try.
(As long as these Pop are not caused by something else...) There may well be a different defect.

[JP-Stereo]

I've tried a 2k resistor between R663 and Q610 emitter
No more positive MUTE ON signal at R662 and the loud pop change to a jet like sound...
I suppose Q610 need to be as direct as possible to work with Q608.

I've tried the resistor between C678 and D638 anode instead.
Now i have a MUTE ON of clean 10vdc that stay there and did not return to 0 or negative after Power On.
And at Power Off the Mute line slowly go to 0 vdc, no pop or other noise.

The mixer detect the input (green led on) but no sound, look like the MUTE stay on...
At least no more loud Pop at Power Off, it really look as if this Pop was linked to this Mute line...

[x-pro]

... the most selective correction may be to insert a resistor between D638 and Q610.

you wanted.

Is C679 good quality? To increase the mute time when turning off, you can try to increase C679 to 470µf.