Acoustic 320 last issue : nasty pop when "power switch" depressed

[pierrefrancois]

Hello everyone.
After successfully fixing a lot of issues on every parts of an Acoustic 320 there's one left : without any input signal depressing the "boost switch" (from Normal to Boost) produced a loud nasty pop in the speaker.
This switch grounds two inputs on the amp module :

1. one on the differential pair (increasing the gain i guess)
2. one before the first stage transistor modifying the pre-filter

Removing the second connection remove the noise so i know the filter is involved.

Has someone already faced this problem ? any clue ?
Is this noise quite usual ?

Thanks for any future help

pierre-françois

 

[Zooberwerx]

Hello everyone.
After successfully fixing a lot of issues on every parts of an Acoustic 320 there's one left : without any input signal depressing the "boost switch" (from Normal to Boost) produced a loud nasty pop in the speaker.
This switch grounds two inputs on the amp module :

1. one on the differential pair (increasing the gain i guess)
2. one before the first stage transistor modifying the pre-filter

Removing the second connection remove the noise so i know the filter is involved.

Has someone already faced this problem ? any clue ?
Is this noise quite usual ?

Thanks for any future help

pierre-françois

May want to move this "Amps". It's a head-scratcher and you'll get a better, well-informed response(s).

Riis

 

[pierrefrancois]

May want to move this "Amps". It's a head-scratcher and you'll get a better, well-informed response(s).

Riis

Thanks for this advice ; i didn' find any mean to move a post from a forum to another one ; should i simply duplicate my post in the "Amps & Cabs" forum ?
I fully confirm the "head-scratcher" nature of this amp _

 

[Zooberwerx]

Thanks for this advice ; i didn' find any mean to move a post from a forum to another one ; should i simply duplicate my post in the "Amps & Cabs" forum ?
I fully confirm the "head-scratcher" nature of this amp _

Yeah, go ahead but you may want to ask the Moderators to move it for you. These things happen. BTW, welcome to TB and fill-out your profile.

Riis

 

[StereoPlayer]

Welcome to TalkBass!
My 370 head does the same thing. It may be a normal thing….I seem to remember being told.
Ask your question here too.
The Acoustic Club

 

[pierrefrancois]

Thanks for your testimony and advice ; i've asked a moderator the permission to duplicate my question in the Acoustic Club

 

[pierrefrancois]

Thanks Riis for your kind welcome

 

[Stylee Mousse]

I've had a number of amps from the 70s that do things like that. My 70's ampeg ss head comes on with a big thump. It causes No problems at all.

 

[J-Mags]

I've never played a 70s Acoustic amp that didn't have some weirdness in the power switch, usually a thump, but it can involve other noises. I've owned three, and they've all done something. I'm no tech, but two of the three were otherwise flawless in their performance, so it may not be a problem.

 

[Stylee Mousse]

Not just Acoustic, lots of that era's amps do that including PA power amps. Best plan: don't worry about it and play bass!

 

[Jim C]

I've owned an Acoustic 370 and 450.
Both had the power on thump but neither had a thump when turning on the bass boost.

 

[pierrefrancois]

I've owned an Acoustic 370 and 450.
Both had the power on thump but neither had a thump when turning on the bass boost.

Hello Jim. thanks for this account ; i'm a little confused since i don't see any "power boost" button on 370 or 450 ; maybe you were talking about the "bright switch" ?

 

[pierrefrancois]

Not just Acoustic, lots of that era's amps do that including PA power amps. Best plan: don't worry about it and play bass!

Hi Steely Mouse
I was'nt talking about the big thump when amp is switched on (this issue has been dealt with) but a special feature called "power boost" which modify the sound spectrum and the gain

I'm just realising that the title of my post is wrong or confusing ! it should be : "nasty pop when "Power BOOST" switch is depressed

 

[Jim C]

Hello Jim. thanks for this account ; i'm a little confused since i don't see any "power boost" button on 370 or 450 ; maybe you were talking about the "bright switch" ?

My mistake. I thought it had a boost switch but haven't owned one in 25 years.

 

[chadds]

SWRs had this. Steve who was at Acoustic said to add a resistor to stop this in SWRs would have changed the overall sound.

If you think about it there is no more of a pulse than a good pizzicato funk finger style pluck.

Day one owning a 360/361 that turn on coming from 200w through an 18" in that folded horn and your heart stopped.

Bill Sheehan said, "I had a loud pop. My mom divorced him."

 

[seamonkey]

This switch grounds two inputs on the amp module :

1. one on the differential pair (increasing the gain i guess)
2. one before the first stage transistor modifying the pre-filter

Maybe the switch needs some cleaning, or it's old springs, and it's not making the connections as smooth and synchronous as it should? You could temp wire in another switch to see.