Marshall AS50R with a loud popping sound on switch on

[blindboybenton]

Hi

Marshall AS50R with a loud popping sound on switch on. but not on switch off. Amp works fine no issues
Checked the pcb for djs and tired components. I have tired a new s/w still the same.
This is not something iv'e ever seen before.

Help me anyone

Bassman

[nickb]

A power on mute circuit is formed by C208 charging thru R205 and ZD3. Check that is working properly.

[blindboybenton]

Many thanks. Ive checked the ZD and cap . still the same. changed them too be sure still popping. I dont have any of the output chips. i may change that

Anybody have anymore idea's

Bassman

[nickb]

Many thanks. Ive checked the ZD and cap . still the same. changed them too be sure still popping. I dont have any of the output chips. i may change that

Anybody have anymore idea's

Bassman

Bad LM3886 seems very unlikely IMHO.

Some more ideas:

Check there is no DC across R202 - a leaky C204 could do that.
Short out R202. If that does kills the pop then it's before the power amp and you'll have to look upstream
Shorting C208 should force it into mute always and so 'stop the pop' - check it does that.
Also, measure the DC across the speaker.

[blindboybenton]

Thanks again.

If i short R202 it does kill the pop I can see a quick pulse on the scope. I have removed C204 and no pop. so im working up though the op amps and see what i get
i wish the op amps were on Ic sockets

Bassman

[blindboybenton]

I have found that the chorus s/w ( SW1 ) stops the popping if both s/ws are off..
AS50R__Main_Circuit_Diagrams.pdf

Lifted C75 no popping with switch on for both channels
Lifted C73 no popping with switch on for both channels
Lifting IC5 made no change
Lifted R133 no popping with switch on for both channels
Lifted C69 no popping with switch on for both channels

If i lift R116 popping is still there but quiter... If i lift R119/120 as well popping is gone.

Any ideas

Bassman

[nickb]

I have found that the chorus s/w ( SW1 ) stops the popping if both s/ws are off..
[ATTACH]34284[/ATTACH]

Lifted C75 no popping with switch on for both channels
Lifted C73 no popping with switch on for both channels
Lifting IC5 made no change
Lifted R133 no popping with switch on for both channels
Lifted C69 no popping with switch on for both channels

If i lift R116 popping is still there but quiter... If i lift R119/120 as well popping is gone.

Any ideas

Bassman

I think a transient on power up is pretty much a given for the BBD. It's going to take 1024 clock pulses for it even starts to get DC stable.

It's a race between the BBD output and the power on mute. If the power on mute is too short or if the BBD is too slow then you'll get a pop.

The BBD being slow would be due to it's clocks being slower than normal. I have no idea what the original center frequency for the VCO is and so I have no reference to guide you. Does the chorus work normally? Check the (clock) power supplies as they would throw it off as could the timing components C76 and R142 and just about everything else in that area.


On the mute, since you have a scope, I hope it's digital , I would take a look at what is happening on pin the LM3886 pin 8. The idea is that it doesn't get to -2.3V until all transients on the LM3886 input are done with. I estimate that at ~100ms after switch on. That doesn't seem like very much margin to me.

If the chorus does seem to be work OK I'd be very tempted just to increase the time constant of the mute but that is avoiding the issue. After all it did work once upon a time - didn't it?

I can't think of much more to help you - good luck!

[blindboybenton]

Thanks Nick..
I only fix amps for fun.... This one aint fun
I will investergate tommorow