Power Off and popping

[trem]

Is this ever "normal" with some amps.?
A friend of my son bought this Fender Deluxe 112 (solid state) at a garage sale for 5 bux. It would cut in and out all the time. I pulled the board and found "ring cracks" around four of the big power supply caps. I fixed that and the amp worked as it should.....except for a fairly loud pop when you turn it off.
And that is my question.....Is it possible that this is normal for this amp.?
I went ahead and installed a new switch.

Is there somewhere i could put a drain on the power supply (if that is where it comes from) so the caps drain slower.?
In this situation, would one of those X2 Caps alleviate this noise.?
Thank You

[bob p]

turn off thumps are fairly common with SS gear.

[trem]

turn off thumps are fairly common with SS gear.

Is there a somewhat simple way to "fix" this.?
Is it better to just leave it alone.?

[bob p]

you don't have to leave it alone if it bothers you, but it can be a PITB to design something to silence it automatically.

there are a million-six ways to deal with power-off thumps. you can design just about any kind of circuit to do this. a lot of SS audio amps use relays to connect/disconnect the speakers. my concerns are that an AC powered relay can couple hum, and a DC powered relay needs to react faster than the occurrence of the thump. you have to win a race, so the protection circuit can get complicated. I think the HiFi guys sell boards to do this sort of thing.

The simplest fix would be a switch on the speaker.

[Enzo]

Do not assume it is caps discharging. A loud pop is more likely a spark inside the power switch. Since the amp is fully powered at the time of turn-off, that spark gets amplified. It is normal, not a switch defect. HAving said that, sometimes a small cap across the power switch helps. Thumps are from the circuit going unstable as the voltages go down. Caps discharge at their rate. Finding a way to slow that down will also lengthen the time of instability, so a thump stretches out into a moan moreso than a kick drum sound.

The noise from the speaker will be no louder than the ampo full on, so it won't hurt the speaker.

In short, there is nothing to fix. Being aware of something doesn't make it a problem.

[trem]

10-4.....Thanks Guys

[loudthud]

There is a Mute circuit in this amp. Q7 shorts the input of the power amp to ground momentarily at power up. Not sure if it has any effect at power off, depends how fast the -16V collapses. If the power amp is working as it should, it will balance the output as the power supplies go up and down and not produce a click or thump (as long as there is no input). A worn power switch still could cause a click as Enzo says.

[Jazz P Bass]

To prove the power switch, remove the plug from the outlet with the amp on.

If it does not 'pop' then it may indeed be the switch contacts.

[Enzo]

Q7 is the power up mute, and mutes until C53 charges up to -16v.

The MUTE line running off to the left is through switches on the input jacks and power amp in jack. All those are in series with a 1 meg to +16. Plugging into any of those three jacks disconnects that +16v. Only when that happens will CR32 charge C53.

If you pull your cord from the jacks, then the amp mutes, and nothing ought to come out the speaker when you power down. Hopefully.

If you leave your guitar plugged in, I don't see Q7 helping a switch pop, and as to preamp noises or thumps, I doubt C53 can discharge fast enough to help.

[trem]

Q7 is the power up mute, and mutes until C53 charges up to -16v.

The MUTE line running off to the left is through switches on the input jacks and power amp in jack. All those are in series with a 1 meg to +16. Plugging into any of those three jacks disconnects that +16v. Only when that happens will CR32 charge C53.

If you pull your cord from the jacks, then the amp mutes, and nothing ought to come out the speaker when you power down. Hopefully.

If you leave your guitar plugged in, I don't see Q7 helping a switch pop, and as to preamp noises or thumps, I doubt C53 can discharge fast enough to help.

Great Info...looking at the schem now.
Thank You So Much

[J M Fahey]

I think you have a single section power switch, as in it opens only the hot wire.
In that case put a .047uF or .022uF cap across the opening switch legs, so when you open the switch yje cap is across contacts and quenches the arc.
Even better, add a 50 or 100 ohm 1/4 or 1/2 W resistor in eries with cap.

****IMPORTANT**** : capacitor must be "250VAC" rated, usually UL or CSA or TÜV or UR approved type, it is not a 250V DC plain vanilla cap by any means.
Search for them in the Mouser page.

Inexpensive and useful so order 5 or 10 of them to justify postage.
You will definitely use more later.

You can also wire another one across transformer primary taps.

If **absolutely** unavailable (or to try the concept) you could use a 1000V rated cap , or just for testing a 600V DC one, but it won´t last, simply because inductive kicks can go beyond 600V DC; it will survive a few but not forever.

[trem]

I will have to check and see what stays closed.
There is a Hot and Neutral from the power cord going to the switch and then a Hot and Neutral going from the switch to their destinations.
Are you saying to put the cap across just the two Hot connections, or to go from the Hot to the Neutral.?
Thank You

[g1]

Between the 2 hots on the switch.

[trem]

OK.....Thanks.

[Mick Bailey]

I've had good success with placing a cap across the transformer primary with SS amps that pop when turned off, though different amps need different approaches depending where the noise is coming from. YMMV, as they say.