Fender Passport 500 Pro SMPS question

[JP-Stereo]

Maybe a small glitch..
Once per 2 or 3 shotdowns the unit emit a click noise even if the volume is down...

[JP-Stereo]

In fact it's at each power off.
The snapping sound is loud enough to be anoying.

I figure i will have to verify the Pop Noise circuit:



Any other suspect to verify ?

[g1]

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.

[JP-Stereo]

To be safe, i replace Q608 and Q610.
D636, D637 and D638 are testing OK out of circuit.
C678 and C679 are new and tested.

Still have that cracking/pop sound at shot off but seem a little weaker.

The MUTE line is a steady pulse train, is this normal.
I was thinking that the MUTE was used at power on and power off.
Otherwise it's something in the amp again ?

[x-pro]

When the device is on and AC voltage appears across diodes D636 and D637, transistor Q610 remains closed (off). Some positive component acts on its base. C679 is being charged. While C678 is in the discharged state, a MUTE signal appears on the collector of Q608. After a while, C678 is charged through R661 and R663, and Q608 closes. MUTE turns off.
When the device turns off, the voltage across the base of Q610 disappears almost immediately, but the potential of the charged C678 remains on its emitter, and Q610 opens. It turns Q608 on (MUTE turns on again) and rapidly discharges C678 through D636. The circuit is ready to re-enable the device in MUTE mode.

[JP-Stereo]

How can i confirm that the power off sequence is really ok ?
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.

The Mute or Mute_C line also interact with the SD line at page 3 OVP circuit and with the Protect circuit.

[x-pro]

You can change the mute time by increasing the value of C678

[g1]

You can change the mute time by increasing the value of C678

This would be for turn on? I think his main concern is turn off. It was not clear to me in your explanation from post #128, if the mute engages fast enough on turn off to prevent a pop?

[x-pro]

C678 works with both on-delay and off-delay.

[x-pro]

The mute delay when the device is turned off is also affected by the C677 and R644 chain. And even more than C678.
The loss of positive pulses from diode D637 leads to an instant discharge of capacitor C677 through R664 due to its low capacity. Low potential on C677 opens Q610

[JP-Stereo]

So if we want a slower/faster response on power off it's C677 and R664.
But for a longer Mute On period, it's C678 that must be increase.
That should also affect the Mute ON period at Power On.
One full second of Mute On in both case should ne fine.

But i suspect that i do not currently have a good Mute On signal present at shot down.

"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."

Is the Mute signal is this positive 10v noise ?
If so there is currently nothing like that at Power Off.

[JP-Stereo]

It turns Q608 on (MUTE turns on again) and rapidly discharges C678 through D636. The circuit is ready to re-enable the device in MUTE mode.

Question: C678 once charged is positive, how can it discharge via D636, since this one will be reverse biased ?

The mute signal is in fact the AC8V that is passing through Q608 (10v pulse/noise at Power ON), so the - 14dc must be the NOT MUTED state of this line.
At Power Off it look like i have only one of these pulse, like 1/10 of a second or less before going to 0v, so the Mute is not there or too short.

At Power Off Q610 stop conducting so C678 is then making Q608 conduct while discharging, but the Mute signal must then be provided by C679.
If this circuits work well as intended, wich i am not convinced, some timing values must then be readjust to compensate for aging component oer else.

I want a Mute signal at Power Off

[x-pro]

Question: C678 once charged is positive, how can it discharge via D636, since this one will be reverse biased ?

All according to Ohm's law

[x-pro]

The mute signal is in fact the AC8V .

Which voltage will drop faster when AC8V and AC18V are missing : voltage on C679 or voltage on C677 ?

[x-pro]

At Power Off Q610 stop conducting

Wrong.
The loss of positive pulses from diode D637 leads to an instant discharge of capacitor C677 through R664 due to its low capacity. Low potential on C677 includes Q610